monroe/hw/monroe_oc8820_r3.00.asm

; ================================================================================
; Monroe OC8820 Boot ROM Disassembly - Version R3.00C
; ================================================================================
; Original file: monroe_oc8820_boot_prom_r3_00C_tms2516_ea751e22.bin
; ROM Type: TMS2516 (2KB EPROM)
; Disassembler: z80dasm 1.1.6
; 
; This is the boot ROM for the Monroe OC8820 computer system.
; The system uses a Z80 CPU with bank-switched memory architecture.
;
; Memory Map:
; 0x0000-0x07FF : Boot ROM (this code)
; 0x3000-0x3FFF : Display memory
; 0xC000-0xFFFF : Banked RAM (controlled by I/O ports)
; 0xFF80-0xFFFF : System variables area
;
; I/O Port Map:
; 0x78-0x7B     : Hard disk controller (DMA-based)
; 0xA0          : DART A (Dual Async Receiver/Transmitter) - Channel A data (printer)
; 0xA1          : DART A (Dual Async Receiver/Transmitter) - Channel A command
; 0xA2          : DART B (Dual Async Receiver/Transmitter) - Channel B data (keyboard)
; 0xA3          : DART B (Dual Async Receiver/Transmitter) - Channel B command  
; 0xA4          : SIO A (Serial I/O) - Channel A data (comm)
; 0xA5          : SIO A (Serial I/O) - Channel A command
; 0xA6          : SIO B (Serial I/O) - Channel B data (RS-232C)
; 0xA7          : SIO B (Serial I/O) - Channel B command
; 0xA8          : CTC (Counter Timer Circuit) - Channel 0 (Communications baud)
; 0xA9          : CTC Channel 1 (AUX RS-232C baud)
; 0xAA          : CTC Channel 2 (Printer baud)
; 0xAB          : CTC Channel 3
; 0xB0-0xB3     : Floppy disk controller (WD179x family)
;                  0xB0: Status/Command, 0xB2: Sector, 0xB3: Track
; 0xB4-0xB5     : Z80 PIO A data and control ports
;                  0xB4: PIO A data (floppy INTRQ signal, DMA control), 0xB5: PIO A control
; 0xB8-0xB9     : CRT controller (6845 CRTC)
;                  0xB8: Register select, 0xB9: Data
; 0xC0          : Floppy drive selection and control
; 0xC4          : Program map A base address (memory banking)
; 0xC5          : Program map B base address (memory banking)
; 0xC6          : DMA map A base address (floppy DMA control)
; 0xC7          : DMA map B base address (hard disk DMA control)
; 0xC8          : Map and system control register (banking enable/disable)
; 0xD4-0xD8     : Graphics and color control
; 0xFF          : Hardware detection port
;
; Boot Device Selection System:
; The Monroe OC8820 features sophisticated multi-device boot capability:
;
; 1. HARDWARE AUTO-DETECTION (Port 0xFF):
;    - Reading 0 = Floppy-only configuration
;    - Reading 1 = Hard disk + floppy configuration  
;    - Reading 2+ = Advanced hard disk configuration
;
; 2. BOOT PRIORITY ORDER:
;    - HARD DISK FIRST (when detected): Uses DMA controller at ports 0x78-0x7B
;    - FLOPPY FALLBACK: Uses WD179x controller at ports 0xB0-0xB3 (PIO A at 0xB4-0xB5)
;    - Systematic retry: Tests floppy drives 0-3 with 8 total attempts
;    - Final recovery: Special commands 0x10/0x11 on Drive 0 only
;
; 3. DUAL OPERATING SYSTEM SUPPORT:
;    - Monroe OS8 (proprietary system)
;    - CP/M (industry standard) 
;    - Boot path selection via calculated jumps at 0x05D0
;
; Communication Specifications:
; DART A & B    : High-speed serial (baud rate depends on system clock)
; SIO A & B     : High-speed serial (same rate as DART ports)
; Printer Port  : Lower-speed serial/parallel (baud rate depends on system clock)
; CTC Timing    : Ch0/1: ÷16 prescaler, time const 3; Ch2: ÷256 prescaler, time const 78
        ; Number of memory banks to test
; CRT Display   : 80 characters/line text mode (184 total horizontal timing)
; ================================================================================
; Constants
displaymem: equ 0x3000		; Start of display memory
nomemchecks: equ 9		; Number of memory banks to test (1-9)

	org	00000h

; ================================================================================
; RESET VECTOR AND BOOT SEQUENCE
; ================================================================================
; Z80 starts execution here after reset

l0000h:
	di			;0000  Disable interrupts during boot
	ld sp,0ffa6h		;0001  Initialize stack pointer (top of RAM minus workspace)
	xor a			;0004  Clear A register (A=0)
	jp l_init		;0005  Jump to main initialization routine

; ================================================================================
; DISPLAY MESSAGE ROUTINE ENTRY POINT
; ================================================================================
; This routine is called to display text messages on screen
; Entry: DE = screen position, HL = message address
; Message format: first byte = length, followed by ASCII text

display_entry:
	ld de,0303eh		;0008  Default screen position (row 3, col 62?)
	call cleartopline	;000b  Clear the top line of display
	nop			;000e  Padding
	nop			;000f  Padding
	pop hl			;0010  Get return address (points to message)

displaymsg:
	ld c,(hl)		;0011  Load message length from first byte
	inc hl			;0012  Point to message text
	ld b,000h		;0013  Clear B (BC now = message length)
	jp printroutine		;0015  Jump to print routine
; ================================================================================
; RST VECTOR TABLE (0x0018-0x003F)
; ================================================================================
; All RST vectors point to RST 38h (0x0038) - likely a common error handler
; The Z80 RST instructions provide fast calls to these addresses

	rst 38h			;0018  RST 18h -> RST 38h
	rst 38h			;0019
	rst 38h			;001a
	rst 38h			;001b
	rst 38h			;001c
	rst 38h			;001d
	rst 38h			;001e
	rst 38h			;001f
	rst 38h			;0020  RST 20h -> RST 38h
	rst 38h			;0021
	rst 38h			;0022
	rst 38h			;0023
	rst 38h			;0024
	rst 38h			;0025
	rst 38h			;0026
	rst 38h			;0027
	rst 38h			;0028  RST 28h -> RST 38h
	rst 38h			;0029
	rst 38h			;002a
	rst 38h			;002b
	rst 38h			;002c
	rst 38h			;002d
	rst 38h			;002e
	rst 38h			;002f
	rst 38h			;0030  RST 30h -> RST 38h
	rst 38h			;0031
	rst 38h			;0032
	rst 38h			;0033
	rst 38h			;0034
	rst 38h			;0035
	rst 38h			;0036
	rst 38h			;0037
	rst 38h			;0038  RST 38h - Common error handler
	rst 38h			;0039
	rst 38h			;003a
	rst 38h			;003b
	rst 38h			;003c
	rst 38h			;003d
	rst 38h			;003e
	rst 38h			;003f

; ================================================================================
; WARM BOOT AND RESTART VECTORS
; ================================================================================

	jp l0000h		;0040  Warm boot - restart system
	jp l0000h		;0043  Another restart vector
; ================================================================================
; MAIN SYSTEM INITIALIZATION
; ================================================================================
; This routine initializes all I/O controllers and system hardware

l_init:
	out (0c0h),a		;0046  Clear floppy disk controller
	out (0c4h),a		;0048  Clear Program map A base address
	out (0c5h),a		;004a  Clear Program map B base address  
	out (0c6h),a		;004c  Clear DMA map A base address
	out (0c7h),a		;004e  Clear DMA map B base address
	out (0d4h),a		;0050  Clear Hi-res color control
	out (0d8h),a		;0052  Clear Hi-res graphics start address
	ld hl,PIOA_start	;0054  Point to I/O initialization data table
	jp l0562h		;0057  Jump to OTIR routine to initialize peripherals

; ================================================================================
; I/O CONTROLLER INITIALIZATION DATA TABLE
; ================================================================================
; This table contains initialization sequences for various Z80 peripheral chips
; Format: Port address, Count, Data bytes...
; Used by OTIR routine at 0562h

; BLOCK 'PIOA' (start 0x005a end 0x008b)
PIOA_start:
	; CRT Controller initialization (6845 compatible)
	defb 0b5h		;005a  I/O port: CRT register select
	defb 004h		;005b  Count: 4 bytes to follow
	defb 0cfh		;005c  Reg 0: Horizontal Total = 207+1=208 chars/line
	defb 0b8h		;005d  Reg 1: Horizontal Displayed = 184 (likely 80 char mode with wide timing)
	defb 037h		;005e  Reg 2: Horizontal Sync Position = 55 (after visible)
	defb 0efh		;005f  Reg 3: Sync Width = V:14 lines, H:15 chars
	
	; PIO A Data Port
	defb 0b4h		;0060  I/O port: PIO A data
	defb 001h		;0061  Count: 1 byte to follow
	defb 001h		;0062  Data: Enable something
	
	; DART A (Serial port A) initialization
	defb 0a1h		;0063  I/O port: DART A command
	defb 006h		;0064  Count: 6 bytes to follow
	defb 048h		;0065  Reset command
	defb 048h		;0066  Reset command (double reset)
	defb 004h		;0067  Write register 4: Clock/format control
	defb 044h		;0068  16x clock, 1 stop bit, no parity
	defb 005h		;0069  Write register 5: Transmit control  
	defb 0eah		;006a  DTR active, RTS active, Tx enable, 8 bits
	
	; DART B (Serial port B) initialization  
	defb 0a3h		;006b  I/O port: DART B command
	defb 008h		;006c  Count: 8 bytes to follow
	defb 048h		;006d  Reset command
	defb 048h		;006e  Reset command (double reset)
	defb 004h		;006f  Write register 4: Clock/format control
	defb 044h		;0070  16x clock, 1 stop bit, no parity
	defb 001h		;0071  Write register 1: Interrupt control
	defb 000h		;0072  No interrupts enabled
	defb 003h		;0073  Write register 3: Receive control
	defb 0c1h		;0074  Rx enable, 8 bits per char
	
	; SIO A (Serial I/O A) initialization
	defb 0a5h		;0075  I/O port: SIO A command
	defb 002h		;0076  Count: 2 bytes to follow
	defb 048h		;0077  Reset command
	defb 048h		;0078  Reset command (double reset)
	
	; SIO B (Serial I/O B) initialization
	defb 0a7h		;0079  I/O port: SIO B command
	defb 002h		;007a  Count: 2 bytes to follow
	defb 048h		;007b  Reset command
	defb 048h		;007c  Reset command (double reset)
	
	; CTC (Counter Timer Circuit) initialization
	; Used to generate baud rate clocks for serial communications
	
	; CTC Channel 0 - Communications baud rate generator
	defb 0a8h		;007d  I/O port: CTC CHANNEL 0
	defb 002h		;007e  Count: 2 bytes to follow
	defb 003h		;007f  Control: Timer mode, prescaler 16, auto-trigger
	defb 003h		;0080  Time constant: 3 (high-speed rate)
	
	; CTC Channel 1 - Auxiliary RS-232C baud rate generator  
	defb 0a9h		;0081  I/O port: CTC CHANNEL 1
	defb 002h		;0082  Count: 2 bytes to follow
	defb 003h		;0083  Control: Timer mode, prescaler 16, auto-trigger
	defb 003h		;0084  Time constant: 3 (same rate as channel 0)
	
	; CTC Channel 2 - Printer baud rate generator
	defb 0aah		;0085  I/O port: CTC CHANNEL 2
	defb 002h		;0086  Count: 2 bytes to follow
	defb 057h		;0087  Control: Timer mode, prescaler 256, auto-trigger
	defb 04eh		;0088  Time constant: 78 (slower rate for printer)
	
	; End of I/O initialization table
	defb 000h		;0089  Terminator: port 0 (invalid)
	defb 000h		;008a  Terminator: count 0

PIOA_end:
	ld ix,initcontinue	;008b  Set continuation address after screen setup
	jp setupclearscreen	;008f  Initialize display and clear screen

; ================================================================================
; INITIALIZATION CONTINUATION
; ================================================================================
; Returns here after display is initialized and cleared

initcontinue:
	ld a,080h		;0092  Enable hi-res graphics, disable buzzer
	out (0d4h),a		;0094  Write to hi-res color control register
	ld de,displaymem	;0096  Point to start of display memory
	ld hl,TESTING_start	;0099  Point to "Testing." message
	jp displaymsg		;009c  Display the message

; ================================================================================
; "TESTING." MESSAGE DATA
; ================================================================================

; BLOCK 'TESTING' (start 0x009f end 0x00a8)
TESTING_start:
	defb 008h		;009f  Message length: 8 characters
	defb 054h		;00a0  'T'
	defb 065h		;00a1  'e'
	defb 073h		;00a2  's'
	defb 074h		;00a3  't'
	defb 069h		;00a4  'i'
	defb 06eh		;00a5  'n'
	defb 067h		;00a6  'g'
	defb 02eh		;00a7  '.'
TESTING_end:
; ================================================================================
; MEMORY DETECTION AND TEST INITIALIZATION
; ================================================================================
; This section detects memory size (128K vs 256K) and sets up memory testing

	ld a,005h		;00a8  DART B register 5 (transmit control)
	out (0a3h),a		;00aa  Select register 5
	ld a,068h		;00ac  Enable transmit, 7 bits, no break
	out (0a3h),a		;00ae  Configure DART B transmit
	ld a,0f0h		;00b0  Memory mapping control value
	out (0c8h),a		;00b2  Set map and system control register
	ld ix,0c001h		;00b4  Point to banked memory + 1 (0xC001)
	ld bc,00c5h		;00b8  BC = 0x00C5 (memory map port in C)
	ld d,nomemchecks	;00bb  D = 9 (number of memory banks to test)

; Memory size detection routine
	ld a,0a0h		;00bd  Test value for 256K detection
	out (0c5h),a		;00bf  Set program map B base to 0xA0
	ld (ix-001h),b		;00c1  Write test value (0x00) to 0xC000
	dec b			;00c4  B = 0xFF
	inc a			;00c5  A = 0xA1
	out (0c5h),a		;00c6  Set program map B base to 0xA1  
	ld (ix-001h),b		;00c8  Write 0xFF to 0xC000 in new bank
	dec a			;00cb  A = 0xA0 (back to first bank)
	out (0c5h),a		;00cc  Switch back to first bank
	ld b,(ix-001h)		;00ce  Read back value from 0xC000
	inc b			;00d1  Increment (0x00 becomes 0x01)
	jr z,skip256		;00d2  If zero, we have 256K (0xFF+1=0x00)
	ld d,011h		;00d4  If not zero, set to test 17 banks (256K)

skip256:
	ld b,d			;00d6  B = number of banks to test
	ld hl,memtesteval	;00d7  Point to memory test bank list
	ld de,0000h		;00da  Clear error counters
; ================================================================================
; MAIN MEMORY TEST LOOP
; ================================================================================
; Tests each memory bank using rotating bit patterns

beginmemtest:
	outi			;00dd  Switch memory bank: output (HL) to port C(0xC5), HL++, B--
	jr z,memcmp		;00df  If B=0, all banks tested - check results
	exx			;00e1  Switch to alternate register set
	ld b,055h		;00e2  Start with test pattern 0x55 (01010101)
	ld e,000h		;00e4  Clear local error accumulator

; Test loop for current memory bank using rotating patterns  
memtestloop:
	ld hl,0c000h		;00e6  Start of current memory bank
	ld c,b			;00e9  Copy test pattern to C

; Write test pattern to entire 16K bank
memtestwriteloop:
	ld (hl),c		;00ea  Write pattern to memory
	rlc c			;00eb  Rotate pattern left (01010101 -> 10101010)
	inc l			;00ed  Next byte (low address)
	jr nz,memtestwriteloop	;00ee  Continue until L wraps to 0
	rlc c			;00f0  Rotate pattern again for next page
	inc h			;00f2  Next page (high address)
	jr nz,memtestwriteloop	;00f3  Continue until H wraps to 0

; Read back and verify test pattern
	ld h,0c0h		;00f5  Reset to start of bank
	ld c,b			;00f7  Restore original test pattern

memtestreadloop:
	ld a,(hl)		;00f8  Read memory location
	xor c			;00f9  Compare with expected pattern
	or e			;00fa  Accumulate errors in E
	ld e,a			;00fb  Store error accumulator
	rlc c			;00fc  Rotate expected pattern
	inc l			;00fe  Next byte
	jr nz,memtestreadloop	;00ff  Continue until L wraps
	rlc c			;0101  Rotate pattern for next page
	inc h			;0103  Next page  
	jr nz,memtestreadloop	;0104  Continue until H wraps
	rlc b			;0106  Rotate base pattern for next iteration
	jr nc,memtestloop	;0108  Continue if not back to original (8 patterns)

; Process test results for this bank
	ld a,e			;010a  Get error accumulator
	exx			;010b  Switch back to main register set
	and a			;010c  Test for errors
	jr z,memgood		;010d  No errors - bank is good
	inc a			;010f  Check if all bits failed (0xFF -> 0x00)
	jr z,memerror		;0110  Complete failure - count as bad bank
	or d			;0112  Partial failure - OR with global error flags
	ld d,a			;0113  Store combined error status
	jr memgood		;0114  Continue testing

memerror:
	inc e			;0116  Increment bad bank counter

memgood:
	ld a,b			;0117  Check remaining bank count
	cp 004h			;0118  Is this bank 4?
	jr nz,beginmemtest	;011a  If not bank 4, continue testing
	ld a,020h		;011c  FDC control: Drive B select (dual drive system)
	out (0c0h),a		;011e  Write to floppy controller control register
	jr beginmemtest		;0120  Continue memory testing
; ================================================================================
; MEMORY BANK SELECTION TABLE
; ================================================================================
; This table contains the bank values written to port 0xC5 (Program map B base)
; Each value maps a different 16K memory bank into the 0xC000-0xFFFF address space

memtesteval:		; Memory bank mapping values
	defb 0a1h		;0122  Bank count: 161 (actually used as counter)
	defb 0c1h		;0123  Memory bank 0xC1
	defb 0e1h		;0124  Memory bank 0xE1  
	defb 000h		;0125  Memory bank 0x00
	defb 020h		;0126  Memory bank 0x20
	defb 040h		;0127  Memory bank 0x40
	defb 060h		;0128  Memory bank 0x60
	defb 080h		;0129  Memory bank 0x80
	defb 0a0h		;012a  Memory bank 0xA0 (Start of 256KB test range)
	defb 0c0h		;012b  Memory bank 0xC0
	defb 0e0h		;012c  Memory bank 0xE0
	defb 001h		;012d  Memory bank 0x01
	defb 021h		;012e  Memory bank 0x21
	defb 041h		;012f  Memory bank 0x41
	defb 061h		;0130  Memory bank 0x61
	defb 081h		;0131  Memory bank 0x81
; ================================================================================
; MEMORY TEST RESULTS EVALUATION
; ================================================================================
; Check if memory test found any errors and decide next action
nop
memcmp:
	xor a			;0133  Clear A register
	or d			;0134  Check global error flags in D
	or e			;0135  Check bad bank count in E - Any errors?
	jr z,floppystart	;0136  If no errors, proceed to floppy boot
	ld ix,memcheckdisplay	;0138  Memory errors found - prepare error display
	jp setupcrt		;013c  Jump to CRT setup for error display

; ================================================================================
; MEMORY ERROR DISPLAY ROUTINE
; ================================================================================

memcheckdisplay:
	ld de,displaymem	;013f  Point to display memory
	ld hl,ERROR2_start	;0142  Point to error message
	jp displaymsg		;0145  Display "Error #2 :Call service."

; ================================================================================
; ERROR #2 MESSAGE DATA
; ================================================================================

; BLOCK 'ERROR2' (start 0x0148 end 0x0160)
ERROR2_start:
	defb 017h		;0148  Message length: 23 characters
	defb 045h		;0149  'E' - "Error #2 :Call service."
	defb 072h		;014a  'r'
	defb 072h		;014b  'r'
	defb 06fh		;014c  'o'
	defb 072h		;014d  'r'
	defb 020h		;014e  ' '
	defb 023h		;014f  '#'
	defb 032h		;0150  '2'
	defb 020h		;0151  ' '
	defb 03ah		;0152  ':'
	defb 043h		;0153  'C'
	defb 061h		;0154  'a'
	defb 06ch		;0155  'l'
	defb 06ch		;0156  'l'
	defb 020h		;0157  ' '
	defb 073h		;0158  's'
	defb 065h		;0159  'e'
	defb 072h		;015a  'r'
	defb 076h		;015b  'v'
	defb 069h		;015c  'i'
	defb 063h		;015d  'c'
	defb 065h		;015e  'e'
	defb 02eh		;015f  '.'
ERROR2_end:
	halt			;0160  System halt - requires service intervention
; ================================================================================
; FLOPPY DISK BOOT INITIALIZATION  
; ================================================================================
; Memory test passed - prepare for floppy disk boot

floppystart:
	out (0c5h),a		;0161  Clear program map B base (A=0 from memcmp)
	out (0c8h),a		;0163  Clear map and system control (disable banking)
	dec hl			;0165  HL was pointing past end of memory test table
	dec hl			;0166  Move back to get the memory size indicator
	ld a,(hl)		;0167  Load memory configuration value

floppystart2:
	rrca			;0168  Rotate right with carry to get MSB
	and 080h		;0169  Isolate bit 7 (128K vs 256K indicator)
	ld (0ff80h),a		;016b  Store memory size flag: 0x80=256K, 0x00=128K
	ld de,0ffa6h		;016e  Destination: high RAM workspace area
	ld hl,005beh		;0171  Source: ROM data block at 0x05BE
	ld bc,0005ah		;0174  Count: 90 bytes of runtime data
	ldir			;0177  Copy ROM constants to RAM workspace
	call setupclearscreen2	;0179  Clear screen and setup display for boot
; ================================================================================
; DISK BOOT SEQUENCE WITH AUTO-RETRY
; ================================================================================
; ================================================================================
; BOOT DEVICE DETECTION AND PRIORITY SYSTEM
; ================================================================================
; Memory initialization complete - attempt to boot with device auto-detection
; 
; BOOT PRIORITY ORDER:
; 1. Auto-detect controller type (port 0xFF) to determine available devices
; 2. If hard disk detected: Try hard disk FIRST (faster, larger capacity)
; 3. If hard disk fails or not present: Fall back to floppy disk drives
; 4. Try drives in order: Drive 0, 1, 2, 3 (up to 8 total attempts)

readydiskloop:
	call readydisk		;017c  Display "Ready For System Disk" message
	call sub_061ch		;017f  *** CRITICAL *** Initialize disk controller & detect HDD vs floppy
	ld a,0ffh		;0182  Load 0xFF (default retry flag)
	jr c,skipinvert		;0184  Keep 0xFF if carry set (controller error - floppy only mode)
	cpl			;0186  Invert to 0x00 (normal operation - HDD detection successful)
skipinvert:
	ld (0ff93h),a		;0187  Store boot device mode: 0xFF=floppy-only, 0x00=HDD+floppy available
	ld hl,00404h		;018a  H=4 retries, L=4 track number?
	ld (0ff91h),hl		;018d  Store retry count and track parameters
	or a			;0190  Test the boot device mode flag
	jr nz,l01c3h		;0191  If floppy-only mode (0xFF), skip HDD attempts, go to drive loop
	
	; *** HARD DISK BOOT ATTEMPTS (when HDD controller detected) ***
	; Normal boot mode: Try hard disk operations FIRST
	ld c,010h		;0193  Hard disk command 0x10 (seek track 0/recalibrate)
	call sub_065ah		;0195  Execute hard disk command through DMA
	jr c,l01abh		;0198  If HDD error, try alternate HDD command
	ld bc,00000h		;019a  B=drive 0, C=track 0 (hard disk drive 0)
	call sub_040eh		;019d  Attempt to read boot sector from hard disk
	jr c,l01abh		;01a0  If HDD read error, try alternate HDD approach
	call sub_03a2h		;01a2  Process successful HDD sector read
	ld a,(0ff8bh)		;01a5  Get boot validation result from HDD
	ld (0ff91h),a		;01a8  Store HDD boot result

l01abh:
	; *** ALTERNATE HARD DISK ATTEMPT ***
	ld c,011h		;01ab  Hard disk command 0x11 (alternate seek/step command)
	call sub_065ah		;01ad  Execute alternate hard disk command through DMA
	jr c,l01c3h		;01b0  If HDD command error, fall back to floppy drives
	ld bc,0000h		;01b2  B=drive 0, C=track 0 (hard disk drive 0)  
	call sub_040eh		;01b5  Second attempt to read HDD boot sector
	jr c,l01c3h		;01b8  If HDD read still fails, fall back to floppy drives
	call sub_03a2h		;01ba  Process successful HDD sector read
	ld a,(0ff8bh)		;01bd  Get final HDD boot validation result
	ld (0ff92h),a		;01c0  Store HDD boot result flag

; ================================================================================
; FLOPPY DRIVE FALLBACK AND DRIVE SELECTION LOOP
; ================================================================================
; Reached when: 1) Hard disk not detected, 2) Hard disk boot failed, or 3) Force floppy mode
; Systematically tries all floppy drives (0-3) with multiple retry attempts

l01c3h:
	ld a,001h		;01c3  Start with drive bit 0 (will create drive ID 0xA1)
	ld c,008h		;01c5  Retry counter: 8 attempts across all floppy drives

driveloop:
l01c7h:
	; *** FLOPPY DRIVE SELECTION ***
	; Drive encoding: 0xA1=Drive 0, 0xA2=Drive 1, 0xA4=Drive 2, 0xA8=Drive 3
	or 0a0h			;01c7  OR with 0xA0 base: creates floppy controller commands
	out (0c0h),a		;01c9  Select floppy drive via port 0xC0 (different from HDD ports 0x78-0x7B)
	ld b,a			;01cb  Save current drive select value

; Wait for floppy drive ready status
l01cch:
	in a,(0b0h)		;01cc  Read floppy controller status register (port 0xB0, not HDD 0x79)
	rr a			;01ce  Rotate right - check busy bit (bit 0) into carry
	jr c,l01cch		;01d0  Loop while floppy drive is busy (bit 0 = 1)
	in a,(0b0h)		;01d2  Read floppy status again
	add a,a			;01d4  Shift left - check ready bit (bit 7) into carry  
	jr c,l01ddh		;01d5  If floppy drive ready (bit 7 = 1), continue to next drive
	call delayloop		;01d7  Floppy drive not ready - wait a bit
	call loadingdisk	;01da  Display "Loading..." and attempt boot from this floppy drive

; *** DRIVE ADVANCEMENT AND RETRY LOGIC ***
l01ddh:
	inc c			;01dd  Increment attempt counter (8 total attempts across all drives)
	ld a,b			;01de  Restore drive select value from previous iteration
	add a,a			;01df  Shift left to advance to next drive: 0xA1->0xA2->0xA4->0xA8
	and 00fh		;01e0  Mask to keep only drive bits (removes 0xA0 base, leaves 1,2,4,8)
	jr nz,l01c7h		;01e2  Continue loop if more drives to try (bits 1,2,4,8 not all exhausted)
	
; ================================================================================
; ALL FLOPPY DRIVES EXHAUSTED - FINAL FALLBACK ATTEMPTS
; ================================================================================
; Reached when all floppy drives 0-3 have been tried and failed to boot
	
	ld a,(0ff93h)		;01e4  Load boot mode flag from memory
	or a			;01e7  Test if normal boot mode (0x00) or retry mode (0xFF)
	jr nz,l0204h		;01e8  If already in retry mode, skip to "No System Device" error
	in a,(0ffh)		;01ea  Read hardware status port 0xFF (same port used for HDD detection)
	and 020h		;01ec  Check bit 5 - unknown hardware condition/status
	jr nz,l0204h		;01ee  If bit 5 set, skip directly to error message
	
; *** DESPERATE FINAL ATTEMPTS ON DRIVE 0 ONLY ***
; Special recovery commands specifically for Drive 0 as last resort
	ld c,010h		;01f0  Load floppy command 0x10 (likely recalibrate/restore)
	call sub_065ah		;01f2  Execute floppy controller command
	ld b,000h		;01f5  Force selection of Drive 0 specifically  
	call nc,loadingdisk	;01f7  If command succeeded (no carry), try loading from Drive 0
	ld c,011h		;01fa  Load floppy command 0x11 (likely seek home/step in)
	call sub_065ah		;01fc  Execute second recovery command
	ld b,000h		;01ff  Force Drive 0 selection again
	call nc,loadingdisk	;0201  If command succeeded, make final boot attempt from Drive 0

; ================================================================================
; BOOT FAILURE - DISPLAY ERROR AND RESTART BOOT SEQUENCE  
; ================================================================================

l0204h:
	call setupclearscreen2	;0204  Clear screen and setup display for error
	ld bc,002a3h		;0207  B=2 bytes, C=0xA3 port (memory control?)
	ld hl,0061ah		;020a  Source: ROM data at 0x061A
	otir			;020d  Output 2 bytes to port 0xA3 (setup display controller)
	ld de,03000h		;020f  Display memory base address  
	ld hl,NOSYSTEM_start	;0212  Point to "No System Device On Line." message
	jp displaymsg		;0215  Display error message and wait

; ================================================================================
; "NO SYSTEM DEVICE ON LINE" MESSAGE AND RETRY LOOP
; ================================================================================

; BLOCK 'NOSYSTEM' (start 0x0218 end 0x0232)
NOSYSTEM_start:
	defb 019h		;0218  Message length: 25 characters
	defb 04eh		;0219  'N' - "No System Device On Line."
	defb 06fh		;021a  'o'
	defb 020h		;021b  ' '
	defb 053h		;021c  'S'
	defb 079h		;021d  'y'
	defb 073h		;021e  's'
	defb 074h		;021f  't'
	defb 065h		;0220  'e'
	defb 06dh		;0221  'm'
	defb 020h		;0222  ' '
	defb 044h		;0223  'D'
	defb 065h		;0224  'e'
	defb 076h		;0225  'v'
	defb 069h		;0226  'i'
	defb 063h		;0227  'c'
	defb 065h		;0228  'e'
	defb 020h		;0229  ' '
	defb 04fh		;022a  'O'
	defb 06eh		;022b  'n'
	defb 020h		;022c  ' '
	defb 04ch		;022d  'L'
	defb 069h		;022e  'i'
	defb 06eh		;022f  'n'
	defb 065h		;0230  'e'
	defb 02eh		;0231  '.'
NOSYSTEM_end:
	call delayloop		;0232  Wait for user to see message (~1 second)
	call delayloop		;0235  Additional delay (~1 second)  
	call delayloop		;0238  Final delay (~1 second, total ~3 seconds)
	jp readydiskloop	;023b  Restart entire boot sequence - infinite retry loop!

; ================================================================================
; CRITICAL ERROR HANDLER - SYSTEM HALT
; ================================================================================
; This routine is called for fatal hardware errors that require service

l023eh:
	ld ix,returnsetupcrt1	;023e  Setup return address for CRT initialization
	jp setupcrt		;0242  Initialize CRT controller and return here

returnsetupcrt1:
	call screenclearnetry2	;0245  Clear screen and setup for error display
	rst 10h			;0248  RST 10h: Display message routine

; ================================================================================
; ERROR #0 MESSAGE DATA  
; ================================================================================

; BLOCK 'ERROR0' (start 0x0249 end 0x0261)
ERROR0_start:
	defb 017h		;0249  Message length: 23 characters
	defb 045h		;024a  'E' - "Error #0: Call service."
	defb 072h		;024b  'r'
	defb 072h		;024c  'r'
	defb 06fh		;024d  'o'
	defb 072h		;024e  'r'
	defb 020h		;024f  ' '
	defb 023h		;0250  '#'
	defb 030h		;0251  '0'
	defb 03ah		;0252  ':'
	defb 020h		;0253  ' '
	defb 043h		;0254  'C'
	defb 061h		;0255  'a'
	defb 06ch		;0256  'l'
	defb 06ch		;0257  'l'
	defb 020h		;0258  ' '
	defb 073h		;0259  's'
	defb 065h		;025a  'e'
	defb 072h		;025b  'r'
	defb 076h		;025c  'v'
	defb 069h		;025d  'i'
	defb 063h		;025e  'c'
	defb 065h		;025f  'e'
	defb 02eh		;0260  '.'
ERROR0_end:
	halt			;0261  System halt - requires service call
; ================================================================================
; DISK LOADING ROUTINE - MAIN BOOT SECTOR LOADER
; ================================================================================
; This routine displays "Loading" message and attempts to load the boot sector
; Input: B = drive number (0-3)
; Uses: All registers, modifies disk parameters at 0xFF81-0xFF82

loadingdisk:
	push bc			;0262  Save BC (drive number and parameters)
	xor a			;0263  Clear A register (A = 0)
	ld (0ff81h),a		;0264  Clear disk status parameter 1
	ld (0ff82h),a		;0267  Clear disk status parameter 2  
	rst 8			;026a  RST 8: Display message routine

; ================================================================================
; "LOADING " MESSAGE DATA
; ================================================================================

; BLOCK 'LOADING' (start 0x026b end 0x0274)
LOADING_start:
	defb 008h		;026b  Message length: 8 characters
	defb 04ch		;026c  'L' - "Loading "
	defb 06fh		;026d  'o'
	defb 061h		;026e  'a'
	defb 064h		;026f  'd'
	defb 069h		;0270  'i'
	defb 06eh		;0271  'n'
	defb 067h		;0272  'g'
	defb 020h		;0273  ' '
LOADING_end:
	pop ix			;0274  Restore IX from stack (return address)
	call sub_04a1h		;0276  Initialize disk controller and seek to track 0
	jr c,l023eh		;0279  If seek error, jump to fatal error handler
	ld bc,0000h		;027b  B=track 0, C=sector 0 (boot sector location)
	call sub_040eh		;027e  Read boot sector from disk
	jr c,l02ddh		;0281  If read error, handle disk error
	
	; Check disk status and system configuration
	ld hl,(0ff81h)		;0283  Load disk status parameters (2 bytes)
	ld a,l			;0286  Get low byte of status
	and a			;0287  Test if zero (no special conditions)
	jr z,l029ch		;0288  If zero, continue with normal boot process
	ld a,h			;028a  Get high byte of status  
	and 003h		;028b  Mask bits 0-1 (configuration bits)
	in a,(0ffh)		;028d  Read system status port 0xFF
	jr z,l0298h		;028f  If masked value is zero, check bit 1
	bit 0,a			;0291  Test bit 0 of system status
	jr nz,l029ch		;0293  If bit 0 set, continue normal boot
l0295h:
	jp l034bh		;0295  Jump to alternate boot routine

l0298h:
	bit 1,a			;0298  Test bit 1 of system status
	jr z,l0295h		;029a  If bit 1 clear, jump to alternate boot routine

l029ch:
	call sub_03a2h		;029c  Process and validate boot sector data
	jp c,l0343h		;029f  If validation failed (carry set), handle error
	jr z,l02b6h		;02a2  If zero flag set, skip additional sector read
	call sub_040eh		;02a4  Read additional sector data if needed
	jr c,l02ddh		;02a7  If read error, handle disk error

; ================================================================================
; BOOT SECTOR VALIDATION AND EXECUTION SETUP
; ================================================================================
; Boot sector successfully read - now validate and prepare for execution

	ld hl,0401fh		;02a9  Point to boot sector signature/validity location
	bit 0,(hl)		;02ac  Check boot sector validity bit 0
	jp nz,l034fh		;02ae  If validity bit set, handle invalid boot sector
	call sub_040eh		;02b1  Read final sector data if needed
	jr c,l02ddh		;02b4  If read error, handle disk error

l02b6h:
	exx			;02b6  Switch to alternate register set (BC',DE',HL')
	ld hl,0400eh		;02b7  Point to boot program load address in boot sector
	ld d,(hl)		;02ba  Get high byte of load address
	inc hl			;02bb  Move to next byte
	ld e,(hl)		;02bc  Get low byte of load address  
	push de			;02bd  Save complete load address
	pop iy			;02be  IY = target load address for boot program
	ld hl,04080h		;02c0  Point to boot sector parameter block
	ld c,(hl)		;02c3  Get boot parameter 1
	inc hl			;02c4  Skip reserved byte
	inc hl			;02c5  Point to next parameter
	ld d,(hl)		;02c6  Get parameter high byte
	inc hl			;02c7  Move to next
	ld e,(hl)		;02c8  Get parameter low byte
	inc hl			;02c9  Move to next
	ld a,(hl)		;02ca  Get additional parameter byte
	inc hl			;02cb  Move to final parameter  
	ld l,(hl)		;02cc  Get final parameter byte
	ld h,a			;02cd  Combine A and L into HL
	ex de,hl		;02ce  Exchange DE and HL registers
	push hl			;02cf  Save HL on stack
	ld a,e			;02d0  Get low byte for calculation
	add a,0ffh		;02d1  Add 255 (effectively subtract 1 with carry)
	ld a,d			;02d3  Get high byte
	exx			;02d4  Switch back to main register set
	ld e,000h		;02d5  E = 0 for addition
	adc a,e			;02d7  Add high byte with carry from previous operation
	ld d,a			;02d8  D = calculated high byte for sector loading
	pop hl			;02d9  Restore HL from stack (boot program parameters)
	call sub_0414h		;02da  Load complete boot program from disk into memory

; ================================================================================
; BOOT PROGRAM EXECUTION AND TRANSFER OF CONTROL
; ================================================================================
; Boot program successfully loaded - now transfer control to the loaded code

l02ddh:
	jr c,l0347h		;02dd  If disk load error (carry set), handle error
	ld a,(0ff8bh)		;02df  Get boot completion status flag
	cp 002h			;02e2  Check if status = 2 (special boot mode)
	jr z,l02eeh		;02e4  If special mode, use alternate execution path
	
	; Standard boot execution: Transfer control to loaded program
	xor a			;02e6  Clear A register (A = 0)
	ex af,af'		;02e7  Save AF to alternate AF' register 
	exx			;02e8  Switch to alternate register set
	jp 0ffa6h		;02e9  *** TRANSFER CONTROL TO LOADED BOOT PROGRAM! ***

; ================================================================================
; ALTERNATE BOOT EXECUTION PATH
; ================================================================================
; Special boot mode handling (status = 2)

l02ech:
	ex af,af'		;02ec  Restore AF from alternate register
	adc a,c			;02ed  Add C with carry for parameter processing

l02eeh:
	push ix			;02ee  Save IX register (return address)
	pop bc			;02f0  BC = return address from IX
	ld a,(0ff8bh)		;02f1  Get boot completion status flag
	ld de,00013h		;02f4  DE = 0x0013 (parameter constant)
	ld hl,0ff8dh		;02f7  HL = system parameter storage area
	jr nz,l033fh		;02fa  If status not zero, jump to error 0x33
	cp 002h			;02fc  Compare status with 2 (special boot mode)
	jp nz,0ffb8h		;02fe  If not 2, jump to alternate boot entry point
	
	; Special boot mode (status = 2) - setup system parameters
	push bc			;0301  Save return address
	bit 3,c			;0302  Test bit 3 of return address low byte
	jr nz,l030bh		;0304  If bit 3 set, skip disk flag setup
	ld a,0ffh		;0306  Set disk parameter flag
	ld (0ff81h),a		;0308  Store at disk parameter location

l030bh:
	in a,(0ffh)		;030b  Read system status port 0xFF
	ld c,a			;030d  Save status in C
	xor a			;030e  Clear A register (A = 0)
	bit 0,c			;030f  Test bit 0 of system status
	jr z,l0315h		;0311  If bit 0 clear, skip
	ld a,00fh		;0313  Set A = 0x0F (enable low nibble)

l0315h:
	bit 1,c			;0315  Test bit 1 of system status
	jr z,l031bh		;0317  If bit 1 clear, skip
	or 0f0h			;0319  Set high nibble (A |= 0xF0)

l031bh:
	ld b,a			;031b  B = combined system configuration mask
	ld a,(0ff81h)		;031c  Get disk parameter flag
	cpl			;031f  Complement (invert all bits)
	and a			;0320  Test if result is zero
	jr z,l0334h		;0321  If zero (was 0xFF), skip complex calculation
	
	; Complex bit manipulation for drive/system configuration
	push bc			;0323  Save BC (config mask and status)
	push ix			;0324  Save IX
	pop bc			;0326  BC = IX (return address)
	ld a,c			;0327  Get low byte of return address
	sub 007h		;0328  Subtract 7 (adjust for calculation)
	ld b,a			;032a  B = adjusted value
	ld a,080h		;032b  Start with bit 7 set (0x80)
	
	; Rotate bit pattern based on adjusted return address
l032dh:
	rlca			;032d  Rotate left with carry (shift bit pattern)
	rlca			;032e  Rotate left again (2 positions per loop)
	djnz l032dh		;032f  Decrement B and loop until B=0
	pop bc			;0331  Restore BC (config mask and status)
	and b			;0332  Mask with configuration bits
	rrca			;0333  Rotate right once (adjust final position)

l0334h:
	xor b			;0334  XOR with configuration mask
	pop bc			;0335  Restore original BC (return address)
	ld (0ff81h),a		;0336  Store final calculated system parameter
	ld a,(0ff8bh)		;0339  Get boot completion status
	jp 0ffb8h		;033c  Jump to final boot entry point in RAM
; ================================================================================
; BOOT ERROR CLASSIFICATION AND DISPLAY  
; ================================================================================
; Various boot errors are classified and appropriate error messages displayed

l033fh:
	ld a,033h		;033f  Error code 0x33 (51 decimal) - set error number
	jr l0351h		;0341  Jump to error display routine

l0343h:
	ld a,030h		;0343  Error code 0x30 (48 decimal) - validation error
	jr l0351h		;0345  Jump to error display routine

l0347h:
	ld a,032h		;0347  Error code 0x32 (50 decimal) - disk read error
	jr l0351h		;0349  Jump to error display routine

l034bh:
	ld a,039h		;034b  Error code 0x39 (57 decimal) - system configuration error
	jr l0351h		;034d  Jump to error display routine

l034fh:
	ld a,031h		;034f  Error code 0x31 (49 decimal) - invalid boot sector

; Common error display routine - converts error code to display
l0351h:
	push ix			;0351  Save IX register (return address)
	push af			;0353  Save error code on stack
	call screenclearnetry2	;0354  Clear screen and setup display
	push de			;0357  Save DE register
	rst 10h			;0358  RST 10h: Display message routine

; ================================================================================
; ERROR #10 MESSAGE DATA - INVALID SYSTEM DISK
; ================================================================================

; BLOCK 'INVALID' (start 0x0359 end 0x0382)  
INVALID_start:
	defb 028h		;0359  Message length: 40 characters
	defb 045h		;035a  'E' - "Error #10: Invalid System Disk in Drive "
	defb 072h		;035b  'r'
	defb 072h		;035c  'r'
	defb 06fh		;035d  'o'
	defb 072h		;035e  'r'
	defb 020h		;035f  ' '
	defb 023h		;0360  '#'
	defb 031h		;0361  '1'
	defb 030h		;0362  '0'
	defb 03ah		;0363  ':'
	defb 020h		;0364  ' '
	defb 049h		;0365  'I'
	defb 06eh		;0366  'n'
	defb 076h		;0367  'v'
	defb 061h		;0368  'a'
	defb 06ch		;0369  'l'
	defb 069h		;036a  'i'
	defb 064h		;036b  'd'
	defb 020h		;036c  ' '
	defb 053h		;036d  'S'
	defb 079h		;036e  'y'
	defb 073h		;036f  's'
	defb 074h		;0370  't'
	defb 065h		;0371  'e'
	defb 06dh		;0372  'm'
	defb 020h		;0373  ' '
	defb 044h		;0374  'D'
	defb 069h		;0375  'i'
	defb 073h		;0376  's'
	defb 06bh		;0377  'k'
	defb 020h		;0378  ' '
	defb 069h		;0379  'i'
	defb 06eh		;037a  'n'
	defb 020h		;037b  ' '
	defb 044h		;037c  'D'
	defb 072h		;037d  'r'
	defb 069h		;037e  'i'
	defb 076h		;037f  'v'
	defb 065h		;0380  'e'
	defb 020h		;0381  ' '
INVALID_end:
	pop hl			;0382  Restore HL register (display parameters)
	pop bc			;0383  Restore BC register (error code and return address)
	ld de,0008h		;0384  DE = 8 (offset for drive number display)
	add hl,de		;0387  HL += 8 (position for drive number)
	add hl,de		;0388  HL += 8 again (total offset = 16)
	inc hl			;0389  HL += 1 (final position for drive character)
	ld a,004h		;038a  Memory bank 4 (display memory access)
	out (0c8h),a		;038c  Select display memory bank
	ld (hl),b		;038e  Store drive number/character in display
	xor a			;038f  Clear A (bank 0)
	out (0c8h),a		;0390  Restore normal memory mapping
	call delayloop		;0392  Wait for user to see error message
	call delayloop		;0395  Additional delay
	call setupclearscreen2	;0398  Clear screen and prepare for retry
	call readydisk		;039b  Display "Ready For System Disk" again
	exx			;039e  Switch to alternate register set
	pop bc			;039f  Restore registers from stack
	scf			;03a0  Set carry flag (error condition)
	ret			;03a1  Return to caller

; ================================================================================
; BOOT SECTOR VALIDATION ROUTINE
; ================================================================================
; Validates boot sector contents and determines next actions
; Input: Boot sector loaded at 0x4000
; Output: Carry set if invalid, Z flag indicates additional reads needed

sub_03a2h:
	ld a,003h		;03a2  Set boot status = 3 (validation in progress)
	ld (0ff8bh),a		;03a4  Store at boot status location
	ld hl,0401dh		;03a7  Point to boot sector validation byte
	ld a,(hl)		;03aa  Load validation byte
	cp 0e5h			;03ab  Check for 0xE5 (invalid marker 1)
	scf			;03ad  Set carry flag (error)
	ret z			;03ae  Return if invalid (0xE5)
	cp 06ch			;03af  Check for 0x6C (invalid marker 2)
	scf			;03b1  Set carry flag (error)
	ret z			;03b2  Return if invalid (0x6C)
	add a,001h		;03b3  Add 1 to validation byte
	jr nc,l03bdh		;03b5  If no carry, continue validation
	ld hl,0ff8bh		;03b7  Point to boot status
	ld (hl),002h		;03ba  Set status = 2 (special boot mode)
	ret			;03bc  Return with status 2

l03bdh:
	inc hl			;03bd  Move to next boot sector parameter
	ld b,(hl)		;03be  Get parameter 1
	ld a,b			;03bf  Copy to A
	inc hl			;03c0  Move to next parameter
	ld c,(hl)		;03c1  Get parameter 2
	add a,001h		;03c2  Add 1 to parameter 1
	adc a,c			;03c4  Add parameter 2 with carry
	ld hl,0ff8bh		;03c5  Point to boot status
	ld (hl),002h		;03c8  Set status = 2 (default)
	jr z,l03d4h		;03ca  If sum is zero, continue
	ld (hl),001h		;03cc  Set status = 1 (normal boot)
	ld a,b			;03ce  Get parameter 1 again
	or c			;03cf  OR with parameter 2
	scf			;03d0  Set carry flag (error)
	ret z			;03d1  Return with error if both parameters are zero
	or a			;03d2  Clear carry flag (success)
	ret			;03d3  Return with success

l03d4h:
	ld bc,0001h		;03d4  BC = 1 (additional sector needed)
	or a			;03d7  Clear carry flag (success, additional read needed)
	ret			;03d8  Return indicating additional sector read required

; ================================================================================
; "READY FOR SYSTEM DISK" MESSAGE DISPLAY
; ================================================================================
; Displays the ready message showing ROM version

readydisk:
	ld de,displaymem	;03d9  DE = display memory base address
	rst 10h			;03dc  RST 10h: Display message routine

; ================================================================================
; "READY FOR SYSTEM DISK" MESSAGE DATA
; ================================================================================

; BLOCK 'READY' (start 0x03dd end 0x03fb)
READY_start:
	defb 01dh		;03dd  Message length: 29 characters
	defb 052h		;03de  'R' - "R3-00C Ready For System Disk "
	defb 033h		;03df  '3'
	defb 02dh		;03e0  '-'
	defb 030h		;03e1  '0'
	defb 030h		;03e2  '0'
	defb 043h		;03e3  'C'
	defb 020h		;03e4  ' '
	defb 052h		;03e5  'R'
	defb 065h		;03e6  'e'
	defb 061h		;03e7  'a'
	defb 064h		;03e8  'd'
	defb 079h		;03e9  'y'
	defb 020h		;03ea  ' '
	defb 046h		;03eb  'F'
	defb 06fh		;03ec  'o'
	defb 072h		;03ed  'r'
	defb 020h		;03ee  ' '
	defb 053h		;03ef  'S'
	defb 079h		;03f0  'y'
	defb 073h		;03f1  's'
	defb 074h		;03f2  't'
	defb 065h		;03f3  'e'
	defb 06dh		;03f4  'm'
	defb 020h		;03f5  ' '
	defb 044h		;03f6  'D'
	defb 069h		;03f7  'i'
	defb 073h		;03f8  's'
	defb 06bh		;03f9  'k'
	defb 020h		;03fa  ' '
READY_end:
	call delayloop		;03fb  Wait briefly after displaying message
	ret			;03fe  Return to caller

; ================================================================================
; INTERNAL DELAY ROUTINE (PART OF READY FUNCTION)
; ================================================================================
; This is part of the ready function flow, not a standalone delay routine

	ld b,00ah		;03ff  B = 10 (outer loop counter)
	ld hl,0000h		;0401  HL = 0 (inner loop counter)
l0404h:
	inc hl			;0404  Increment HL
	ld a,l			;0405  Test if HL reached 0x0000 again
	or h			;0406  (after wrapping around 65536 times)
	jr nz,l0404h		;0407  Continue inner loop until HL wraps to 0
	djnz l0404h		;0409  Decrement B and repeat outer loop
	jp memcmp		;040b  Jump to memory compare routine

; ================================================================================
; DISK SECTOR READ ROUTINES
; ================================================================================
; Main disk I/O routines for reading boot sectors and program data

sub_040eh:
	ld de,00100h		;040e  DE = 0x0100 (256 bytes = 1 sector)
	ld hl,04000h		;0411  HL = 0x4000 (sector buffer address)

sub_0414h:
	push bc			;0414  Save BC (track/sector parameters)
	push ix			;0415  Save IX
	pop bc			;0417  BC = IX (return address)
	ld a,c			;0418  Get low byte of return address
	pop bc			;0419  Restore original BC
	cp 010h			;041a  Check if return address low byte < 0x10
	jp nc,l06c1h		;041c  If >= 0x10, jump to alternate routine

l041fh:
	ld a,005h		;041f  A = 5 (retry counter for disk operations)

l0421h:
	ex af,af'		;0421  Save retry counter in AF'
	push bc			;0422  Save BC (track/sector)
	ex (sp),hl		;0423  Exchange HL with top of stack
	ld a,l			;0424  Get low byte 
	and 00fh		;0425  Mask to lower 4 bits
	push bc			;0427  Save BC again
	push af			;0428  Save masked value
	ld a,(0ff81h)		;0429  Get disk parameter flag
	and a			;042c  Test if zero
	ld b,000h		;042d  B = 0 (default sector offset)
	jr z,l0433h		;042f  If flag is zero, use offset 0
	ld b,010h		;0431  B = 0x10 (alternate sector offset)

l0433h:
	pop af			;0433  Restore masked value from stack
	add a,b			;0434  Add sector offset (0 or 0x10)
	inc a			;0435  Add 1 (sectors are 1-based)
	out (0b2h),a		;0436  Write to floppy sector register
	add hl,hl		;0438  HL *= 2 (shift left)
	add hl,hl		;0439  HL *= 2 (shift left again)
	add hl,hl		;043a  HL *= 2 (shift left again)  
	add hl,hl		;043b  HL *= 2 (total: HL *= 16 for track calculation)
	cp 011h			;043c  Compare sector with 17
	ld a,h			;043e  Get high byte (track number)
	out (0b3h),a		;043f  Write to floppy track register
	jr c,l0453h		;0441  If sector < 17, skip extended handling
	rra			;0443  Rotate right (divide by 2)
	out (0b3h),a		;0444  Write modified track to floppy register
	push ix			;0446  Save IX
	pop bc			;0448  BC = IX 
	ld a,b			;0449  Get high byte of IX
	jr nc,l044eh		;044a  If no carry from rotate, skip
	or 010h			;044c  Set bit 4 (extended track flag)

l044eh:
	ld (0ff82h),a		;044e  Store drive control byte
	out (0c0h),a		;0451  Write to floppy drive control register

l0453h:
	pop bc			;0453  Restore BC (track/sector parameters)
	pop hl			;0454  Restore HL (buffer address)
	ld a,01dh		;0455  Floppy command 0x1D (read sector with retry)
	call setfloppystatus	;0457  Execute floppy command and wait for completion
	and 098h		;045a  Check error bits: bit 7=not ready, 4=CRC error, 3=lost data
	jr nz,l0486h		;045c  If any error bits set, handle error
	call sub_04c1h		;045e  Setup DMA transfer parameters

; Multi-sector read loop
l0461h:
	ld a,(0ff82h)		;0461  Get drive control byte
	bit 4,a			;0464  Test bit 4 (extended track flag)
	ld a,082h		;0466  Floppy command 0x82 (read data)
	jr z,l046ch		;0468  If bit 4 clear, use standard command
	ld a,08ah		;046a  Floppy command 0x8A (read data - extended)

l046ch:
	call setfloppystatus	;046c  Execute read command and wait
	and 09ch		;046f  Check error bits: bit 7=not ready, 4=CRC, 3=lost data, 2=record not found
	jr nz,l0486h		;0471  If any error, handle it
	inc h			;0473  Increment buffer high byte (next page)
	inc bc			;0474  Increment sector count
	dec d			;0475  Decrement remaining sector count
	ret z			;0476  Return if all sectors read (D=0)
	ld a,c			;0477  Get current sector number
	and 00fh		;0478  Mask to lower 4 bits (sectors 0-15)
	jr z,l041fh		;047a  If wrapped to 0, start new track
	in a,(0b2h)		;047c  Read current sector register
	inc a			;047e  Increment to next sector
	out (0b2h),a		;047f  Write back to sector register
	ld a,005h		;0481  Reset retry counter to 5

l0483h:
	ex af,af'		;0483  Save retry counter
	jr l0461h		;0484  Continue reading next sector

; Error handling for disk read operations
l0486h:
	scf			;0486  Set carry flag (error)
	bit 7,a			;0487  Test bit 7 (drive not ready)
	ret nz			;0489  Return immediately if drive not ready
	ex af,af'		;048a  Get retry counter
	dec a			;048b  Decrement retry count
	jr nz,l0421h		;048c  If retries left, try again
	ex af,af'		;048e  Restore error status
	bit 4,a			;048f  Test bit 4 (CRC error)
	jr z,l049fh		;0491  If not CRC error, return failure
	ld a,(0ff81h)		;0493  Get disk parameter flag
	and a			;0496  Test if zero
	jr nz,l049fh		;0497  If non-zero, return failure
	cpl			;0499  Complement (0x00 -> 0xFF)
	ld (0ff81h),a		;049a  Store inverted flag (try alternate sectors)
	jr l041fh		;049d  Retry with alternate sector numbering

l049fh:
	scf			;049f  Set carry flag (permanent error)
	ret			;04a0  Return with error

; ================================================================================
; DISK INITIALIZATION AND SEEK ROUTINE
; ================================================================================
; Initialize floppy controller and seek to track 0

sub_04a1h:
	push bc			;04a1  Save BC register
	push ix			;04a2  Save IX register
	pop bc			;04a4  BC = IX (return address)
	ld a,c			;04a5  Get low byte of return address
	pop bc			;04a6  Restore original BC
	cp 010h			;04a7  Check if return address low byte < 0x10
	jp nc,l0788h		;04a9  If >= 0x10, jump to alternate routine
	ld a,001h		;04ac  Floppy command 0x01 (restore/recalibrate to track 0)
	call setfloppystatus	;04ae  Execute restore command
	and 004h		;04b1  Check bit 2 (seek error/track 0 not found)
	ret nz			;04b3  Return with error if seek failed
	scf			;04b4  Set carry flag (error - unexpected!)
	ret			;04b5  Return with error

; ================================================================================
; FLOPPY CONTROLLER COMMAND EXECUTION
; ================================================================================
; Sends command to floppy controller and waits for completion
; Input: A = floppy command
; Output: A = final status register

setfloppystatus:
	out (0b0h),a		;04b6  Write command to floppy controller
	
	; Wait for controller ready (INTRQ signal)
l04b8h:
	in a,(0b4h)		;04b8  Read PIO A data register  
	bit 4,a			;04ba  Test bit 4 (floppy controller INTRQ signal)
	jr z,l04b8h		;04bc  Wait until INTRQ goes high (command complete)
	in a,(0b0h)		;04be  Read final status from floppy controller
	ret			;04c0  Return with status in A

; ================================================================================
; DMA SETUP ROUTINES
; ================================================================================
; Configure Z80 DMA controller for floppy disk data transfers

sub_04c1h:
	push bc			;04c1  Save BC register
	ld bc,0b301h		;04c2  B=0xB3 (DMA port), C=0x01 (DMA command)
	jr l04cbh		;04c5  Jump to common DMA setup

sub_04c7h:
	push bc			;04c7  Save BC register  
	ld bc,07803h		;04c8  B=0x78 (different setting), C=0x03 (DMA command)

l04cbh:
	ld a,b			;04cb  Get DMA parameter
	ld (0ff8ch),a		;04cc  Store DMA configuration
	ld a,c			;04cf  Get DMA command
	out (0b4h),a		;04d0  Write to PIO A data register (DMA control)
	ld bc,006ach		;04d2  B=6 (loop counter), C=0xAC (DMA port)
	ld a,0c3h		;04d5  DMA reset command

l04d7h:
	out (c),a		;04d7  Send reset command to DMA controller
	djnz l04d7h		;04d9  Repeat 6 times (ensure DMA is reset)
	ld a,07dh		;04db  DMA disable command
	out (c),a		;04dd  Send disable to DMA controller
	out (c),l		;04df  Send L register to DMA (low address byte)
	out (c),h		;04e1  Send H register to DMA (high address byte)
	out (c),e		;04e3  Send E register to DMA (low count byte)
	out (c),d		;04e5  Send D register to DMA (high count byte)
	push hl			;04e7  Save HL register
	ld hl,004fbh		;04e8  Point to DMA parameter table
	ld b,003h		;04eb  Send 3 bytes
	otir			;04ed  Output 3 bytes from (HL) to port C
	ld a,(0ff8ch)		;04ef  Get stored DMA configuration
	out (c),a		;04f2  Send to DMA controller
	ld b,005h		;04f4  Send 5 more bytes
	otir			;04f6  Output 5 bytes from (HL) to port C
	pop hl			;04f8  Restore HL register
	pop bc			;04f9  Restore BC register
	ret			;04fa  Return

	; DMA parameter table
	inc d			;04fb  DMA parameter byte 1
	jr z,l0483h		;04fc  DMA parameter bytes 2-3 
	adc a,d			;04fe  DMA parameter byte 4
	rst 8			;04ff  DMA parameter byte 5 (restart vector)
	ld bc,087cfh		;0500  DMA parameter bytes 6-7-8

; ================================================================================
; SCREEN SETUP AND CLEAR ROUTINES
; ================================================================================
; Initialize CRT controller and clear display memory

setupclearscreen2:
	pop ix			;0503  Get return address from stack

setupclearscreen:
	; Entry point during initialization with IX=0x0092
	xor a			;0505  A = 0 (starting register number)
	ld bc,00eb9h		;0506  C=0xB9 (CRT data port), B=14 (register count)
	ld hl,0054fh		;0509  Point to CRT register table

setupclearloop:
	out (0b8h),a		;050c  Write register number to CRT register select
	inc a			;050e  Increment to next register number
	outi			;050f  Output (HL) to port C, increment HL, decrement B
	jr nz,setupclearloop	;0511  Loop until B=0 (all 14 registers written)
	ld de,03002h		;0513  DE = display memory + 2 (destination)
	ld hl,displaymem	;0516  HL = display memory base (source)
	ld bc,00ffeh		;0519  BC = 4094 bytes (2K display memory - 2)

; RST 08 jumps here with different parameters: HL=300C, DE=300E, BC=004E, IX=032F
l051ch:
	ld a,004h		;051c  Memory bank 4 (display memory)
	out (0c8h),a		;051e  Select display memory bank
	xor a			;0520  A = 0 (clear character)
	ld (hl),a		;0521  Store 0x00 at display memory (character)
	inc hl			;0522  Move to attribute byte
	ld (hl),020h		;0523  Store 0x20 (space character attribute)
	dec hl			;0525  Go back to character position
	ldir			;0526  Copy pattern throughout display memory
	xor a			;0528  A = 0 (normal memory bank)
	out (0c8h),a		;0529  Restore normal memory mapping
	jp (ix)			;052b  Continue execution at IX

; ================================================================================
; CLEAR TOP LINE ROUTINE
; ================================================================================
; Clears a single line (80 characters) at the specified address

cleartopline:
	push de			;052d  Save original DE
	ld l,e			;052e  HL = DE (copy destination address)
	ld h,d			;052f
	inc de			;0530  DE += 2 (destination for copy)
	inc de			;0531
	ld bc,0004eh		;0532  BC = 78 words (80 chars - 2 = 78 more chars)
	push ix			;0535  Save original IX
	ld ix,clearlineend	;0537  Set return address to 0x053D
	jr l051ch		;053b  Jump to screen clear routine

	; Screen clear routine returns here
clearlineend:
	pop ix			;053d  Restore original IX
	pop de			;053f  Restore original DE
	ret			;0540  Return to caller

; ================================================================================
; TEXT DISPLAY ROUTINE  
; ================================================================================
; Displays text messages on screen using character/attribute pairs
; Input: BC = character count, HL = message source, DE = screen target

printroutine:
	ld a,004h		;0541  Memory bank 4 (display memory)
	out (0c8h),a		;0543  Select display memory bank

printcharloop:
	inc de			;0545  Skip to attribute byte (every other address)
	ldi			;0546  Copy (HL) to (DE), increment both, decrement BC
	jp pe,printcharloop	;0548  Continue while BC ≠ 0 (parity even = BC not zero)
	xor a			;054b  A = 0 (normal memory bank)
	out (0c8h),a		;054c  Restore normal memory mapping
	jp (hl)			;054e  Jump to address in HL

; ================================================================================
; CRT CONTROLLER REGISTER TABLE (6845 CRTC)
; ================================================================================
; 14-byte table of values for CRT controller registers R0-R13

CRT_REGISTER_TABLE:
	defb 069h		;054f  R0: Horizontal Total = 105 characters
	defb 050h		;0550  R1: Horizontal Displayed = 80 characters  
	defb 056h		;0551  R2: Horizontal Sync Position = 86
	defb 00bh		;0552  R3: Horizontal Sync Width = 11
	defb 019h		;0553  R4: Vertical Total = 25 rows
	defb 003h		;0554  R5: Vertical Total Adjust = 3
	defb 018h		;0555  R6: Vertical Displayed = 24 rows
	defb 018h		;0556  R7: Vertical Sync Position = 24
	defb 000h		;0557  R8: Interlace Mode = 0 (non-interlaced)
	defb 00bh		;0558  R9: Maximum Scan Line = 11 (12 lines per char)
	defb 020h		;0559  R10: Cursor Start = 32 (cursor off)
	defb 000h		;055a  R11: Cursor End = 0
	defb 000h		;055b  R12: Start Address High = 0
	defb 000h		;055c  R13: Start Address Low = 0
CRT_TABLE_END:

; ================================================================================
; ADVANCED SCREEN CLEAR ROUTINES
; ================================================================================

	pop hl			;055d  Restore HL register
	jr l0562h		;055e  Jump to screen clear loop

screenclearloop:
	otir			;0560  Output B bytes from (HL) to port C
	
l0562h:
	ld c,(hl)		;0562  Get I/O port number from table
	inc hl			;0563  Move to count byte
	ld b,(hl)		;0564  Get byte count
	inc hl			;0565  Move to data
	ld a,b			;0566  Test count
	and a			;0567  Check if zero
	jr nz,screenclearloop	;0568  If not zero, output data block
	jp (hl)			;056a  Jump to next routine

screenclearnetry2:
	ld de,03100h		;056b  DE = 0x3100 (display line 1 start address)
	ld a,00fh		;056e  CRT register 15 (cursor position low byte)
	out (0b8h),a		;0570  Select CRT register 15
	xor a			;0572  A = 0 (cursor position = 0)
	out (0b9h),a		;0573  Write cursor position to CRT
	in a,(0bbh)		;0575  Read from port 0xBB (keyboard status?)
	or a			;0577  Test status
	ret nz			;0578  Return if non-zero (key pressed?)
	ld de,030a0h		;0579  DE = 0x30A0 (alternate display position)
	ret			;057c  Return

; ================================================================================
; DELAY LOOP ROUTINE
; ================================================================================
; Provides more precise timing delays using stack manipulation

delayloop:
	ld hl,04600h		;057d  HL = 17920 (delay counter)
	
l0580h:
	dec hl			;0580  Decrement counter
	ex (sp),hl		;0581  Exchange HL with top of stack (add delay)
	ex (sp),hl		;0582  Exchange back (more delay)
	ex (sp),hl		;0583  Exchange again (even more delay)
	ex (sp),hl		;0584  Exchange back (maximum delay per loop)
	ld a,l			;0585  Test if HL reached zero
	or h			;0586
	jr nz,l0580h		;0587  Continue loop until HL = 0
	ret			;0589  Return after ~17920 * 4 clock cycles

	pop ix			;058a  Clean up stack (dead code?)

; ================================================================================
; CRT CONTROLLER INITIALIZATION
; ================================================================================
; Complete CRT setup with buzzer control and I/O initialization

setupcrt:
	ld a,00fh		;058c  CRT register 15 (cursor position low)
	out (0b8h),a		;058e  Select CRT register 15
	xor a			;0590  A = 0 (cursor off)
	out (0b9h),a		;0591  Write to CRT data register
	in a,(0bbh)		;0593  Read keyboard/system status
	and a			;0595  Test status
	jr nz,l059ch		;0596  If non-zero, skip buzzer
	ld a,081h		;0598  Buzzer control byte
	out (0d4h),a		;059a  Turn on buzzer/sound

l059ch:
	ld bc,006cch		;059c  B=6 bytes, C=0xCC (I/O port for system control)
	ld hl,005b8h		;059f  Point to I/O initialization table
	otir			;05a2  Output 6 bytes to port 0xCC (system setup)
	ld bc,0000h		;05a4  BC = 0 (delay counter)

	; Delay loop for system settling
l05a7h:
	ex (sp),hl		;05a7  Exchange HL with stack top (timing delay)
	ex (sp),hl		;05a8  Exchange back (more timing delay)
	dec bc			;05a9  Decrement delay counter
	ld a,b			;05aa  Test if BC reached zero
	or c			;05ab
	jr nz,l05a7h		;05ac  Continue delay loop
	ld a,080h		;05ae  Buzzer off command
	out (0d4h),a		;05b0  Turn off buzzer
	ld a,09fh		;05b2  System control value
	out (0cch),a		;05b4  Final system control setup
	jp (ix)			;05b6  Continue execution at IX

; ================================================================================
; I/O INITIALIZATION TABLE
; ================================================================================
; 6-byte table for system controller setup (port 0xCC)

IO_SETUP_TABLE:
	defb 08eh		;05b8  I/O setup byte 1 (adc a,(hl) opcode)
	defb 004h		;05b9  I/O setup byte 2 (inc b opcode)
	defb 092h		;05ba  I/O setup byte 3 (sub d opcode)
	defb 0bfh		;05bb  I/O setup byte 4 (cp a opcode)
	defb 0dfh		;05bc  I/O setup byte 5 (rst 18h opcode)
	defb 0ffh		;05bd  I/O setup byte 6 (rst 38h opcode)

; ================================================================================
; MEMORY COPY ROUTINE WITH BANKING
; ================================================================================
; Copies data using banked memory (used during boot program loading)

l05beh:
	ld a,001h		;05be  Memory bank 1
	out (0c8h),a		;05c0  Select bank 1 for memory operations

l05c2h:
	ex af,af'		;05c2  Save AF in alternate register
	adc a,(hl)		;05c3  Load byte from source (HL)
	ex af,af'		;05c4  Restore AF
	inc hl			;05c5  Increment source pointer
	dec de			;05c6  Decrement byte counter
	ld a,e			;05c7  Test if DE reached zero
	or d			;05c8
	jr nz,l05c2h		;05c9  Continue copy loop if more bytes
	out (0c8h),a		;05cb  A=0, restore normal memory bank
	jp l02ech		;05cd  Jump back to boot execution routine

;
; ANALYSIS OF ALTERNATIVE BOOT PATH (0x05D0-0x0616):
; ==================================================
; 
; This represents a remarkable discovery of the Monroe OC8820's dual-OS boot capability.
; The system implements support for two operating systems through calculated 
; indirect jumps, creating a sophisticated OS-selection boot system.
;
; OPERATING SYSTEM BOOT MODES:
; ----------------------------
; Mode 0/1: Standard boot path - Monroe OS8 (Monroe's proprietary operating system)
; Mode 2:   CP/M boot path - Industry standard OS for business applications
; Mode 2+:  Advanced Monroe OS8 features and configurations
;
; DUAL-OS ENTRY MECHANISM:
; ------------------------
; 1. Boot disk contains OS identification in banked memory (0xFF80-0xFF92 range)
; 2. Main boot code reads OS type and calculates appropriate boot vector
; 3. Address calculation: Base(4) + OS_Parameter + AdvancedFeatures = OS_BootVector
; 4. Target address stored in IX via push BC/pop IX mechanism
; 5. Later jp (ix) instruction jumps to OS-specific initialization code
; 
; OS CONFIGURATION DATA STRUCTURE (in banked memory):
; --------------------------------------------------
; 0xFF80: Base OS configuration flag (combined with runtime parameters)
; 0xFF81: OS-specific configuration byte (CP/M BIOS params or Monroe OS8 settings)
; 0xFF8D: 4-byte OS parameter block #1 (disk geometry, memory layout)
; 0xFF8F: OS validation signature (0x95 = validated Monroe OS8 disk with advanced features)
; 0xFF91: OS boot data byte (entry points, system calls)
; 0xFF92: OS boot data byte (additional OS parameters)
;
; CP/M vs MONROE OS8 BOOT DIFFERENTIATION:
; ----------------------------------------
; - If mode ≠ 2: Monroe OS8 boot (standard proprietary OS)
; - If mode = 2 and parameter < 16: CP/M boot (industry standard)
; - If mode = 2 and parameter ≥ 16: Check for Monroe OS8 advanced features
; - If signature = 0x95: Monroe OS8 with advanced features (+1 offset)
; - Final jump target = 4 + (OS_parameter-16) + advanced_features_bonus
;
; The final jp (iy) transfers control to the selected OS kernel!
;
;; ================================================================================
; Entry point: 0x05D0 (reached via jp (ix) with calculated address from 0x05F2-0x05F3)
; This represents a sophisticated secondary boot path for handling different system configurations
; 
; Entry conditions:
; - A register contains boot mode indicator
; - C register contains configuration parameter  
; - System expects specific memory layout in banked memory

l05d0h:
	cp 002h			;05d0  Compare A with 2 (OS boot mode check: 2 = CP/M selection)
	ld a,001h		;05d2  Load memory bank 1 selector
	out (0c8h),a		;05d4  Switch to memory bank 1 for OS-specific operations
	jr nz,l05f5h		;05d6  If A≠2, use Monroe OS8 (native OS), skip CP/M setup
	
	; CP/M specific initialization (A was equal to 2)
	ld a,(0ff81h)		;05d8  Load CP/M configuration byte from boot disk
	ld (0022h),a		;05db  Store CP/M config in system variable (BIOS parameters)
	ld a,c			;05de  Get CP/M parameter from C register
	sub 010h		;05df  Subtract 16 (check if parameter >= 16 for advanced CP/M)
	jr c,l05f5h		;05e1  If C<16, use basic CP/M boot, jump to common path
	ld c,a			;05e3  Update C with adjusted value (C = C-16)
	jr z,l05eeh		;05e4  If result is zero (C was exactly 16), basic advanced CP/M
	
	; Advanced Monroe OS8 features when booting from CP/M mode
	ld a,(0ff8fh)		;05e6  Load validation signature from boot disk
	cp 095h			;05e9  Compare with Monroe OS8 advanced feature signature 0x95
	jr nz,l05eeh		;05eb  If not Monroe OS8 signature, use standard CP/M boot
	inc c			;05ed  Valid Monroe OS8 features: increment C (Monroe-specific CP/M extensions)

l05eeh:
	; Calculate OS-specific jump target address
	ld a,004h		;05ee  Base offset = 4
	add a,c			;05f0  Add OS-derived offset to base
	ld c,a			;05f1  C now contains calculated OS boot vector offset
	push bc			;05f2  Push BC to stack
	pop ix			;05f3  Pop into IX - now IX = OS boot vector (B×256 + OS_offset)
	; This creates the target address for OS-specific initialization

l05f5h:
	; Common path for both operating systems (Monroe OS8 and CP/M)
	; OS-specific data preparation and parameter setup
	ld a,(0ff80h)		;05f5  Load base OS configuration from boot disk
	or c			;05f8  Combine with OS-specific offset parameter
	ld c,a			;05f9  Store combined OS configuration
	ld a,(0ff91h)		;05fa  Load OS boot parameter 1 from disk (entry points, BIOS vectors)
	ld (de),a		;05fd  Store to OS parameter area (DE points to system area)
	inc de			;05fe  Advance destination pointer
	
	; Copy 4-byte OS parameter block from boot disk
	push bc			;05ff  Save BC (OS configuration)
	ld bc,00004h		;0600  Set count = 4 bytes
	ldir			;0603  Copy 4 bytes: Monroe OS8 or CP/M system parameters
	
	; Set up for second OS parameter block copy
	ld de,00023h		;0605  DE = destination address 0x0023 (OS parameter area)
	ld hl,0ff8dh		;0608  HL = source in banked memory 0xFF8D (OS data block 2)
	ld a,(0ff92h)		;060b  Load OS boot parameter 2 (system vectors, disk geometry)
	ld (de),a		;060e  Store at 0x0023
	inc de			;060f  DE = 0x0024
	
	; Copy second 4-byte OS parameter block
	ld bc,00004h		;0610  Set count = 4 bytes  
	ldir			;0613  Copy 4 bytes: Monroe OS8 system tables or CP/M BIOS tables
	
	pop bc			;0615  Restore BC (OS configuration)
	jp (iy)			;0616  Jump to selected OS kernel entry point! (Monroe OS8 or CP/M)

; ================================================================================
; DATA TABLE AND DISK CONTROLLER INITIALIZATION
; ================================================================================
	defb 005h		;0618  Controller parameter 1
	defb 080h		;0619  Controller parameter 2 (0x80 = enable flag?)
	defb 005h		;061a  Controller parameter 3 
	defb 000h		;061b  Controller parameter 4 (terminator?)

; ================================================================================
; DISK CONTROLLER SETUP AND STATUS CHECK ROUTINE
; ================================================================================
; Initializes the disk controller and waits for ready status
; Uses the 4-byte parameter table above (0x0618-0x061B)
; Returns: Carry set if timeout/error, clear if success

sub_061ch:
	ld bc,002a3h		;061c  B=2 bytes, C=0xA3 (port number for disk controller)
	ld hl,00618h		;061f  HL points to controller parameter table above
	otir			;0622  Output 2 bytes from (HL) to port 0xA3, auto-increment HL
	ld b,0ffh		;0624  B = 255 (timeout counter for controller ready)

l0626h:
	; Wait for disk controller ready status
	in a,(079h)		;0626  Read disk controller status port
	cp 0c7h			;0628  Compare with ready status value 0xC7
	jr z,l0630h		;062a  If ready (0xC7), continue with operation
	djnz l0626h		;062c  Decrement timeout counter, loop if not expired
	scf			;062e  Set carry flag (timeout error)
	ret			;062f  Return with error

l0630h:
	; Controller is ready - execute read operation
	call reading		;0630  Execute disk read command
	ret c			;0633  Return if read error occurred
	
	; Analyze controller response for configuration
	in a,(0ffh)		;0634  Read controller configuration register
	rra			;0636  Rotate right (check bit 0)
	rra			;0637  Rotate right again (check bit 1) 
	and 007h		;0638  Mask to get lower 3 bits (configuration value 0-7)
	
	; *** BOOT DEVICE SELECTION LOGIC ***
	; Configuration value determines boot device priority:
	; 0 = Default (floppy-only system)
	; 1 = Hard disk + floppy system (hard disk priority)  
	; 2+ = Advanced hard disk system (hard disk with extended features)
	
	ld hl,007e2h		;063a  Default: point to floppy-only configuration table
	jr z,l064fh		;063d  If value=0, use floppy-only mode, skip device selection
	ld hl,007d6h		;063f  Point to hard disk + floppy configuration table
	dec a			;0642  Decrement configuration value
	jr z,l0648h		;0643  If value was 1, use hard disk + floppy mode
	ld hl,007cah		;0645  Point to advanced hard disk configuration (value ≥ 2)

l0648h:
	call sub_076bh		;0648  Execute configuration-specific routine
	call reading		;064b  Execute another disk read
	ret c			;064e  Return if error

l064fh:
	; Copy configuration data to system memory
	ld de,0ff8dh		;064f  DE = destination in banked memory (system parameters)
	ld bc,00004h		;0652  BC = 4 bytes to copy
	ldir			;0655  Copy 4 bytes from (HL) to system memory
	jp l077fh		;0657  Jump to next stage of disk initialization
; ================================================================================
; DISK COMMAND EXECUTION ROUTINE
; ================================================================================
; Executes specific floppy disk commands (0x10, 0x11) used during boot sequence
; This routine handles low-level disk operations like recalibrate and seek
; Input: C = disk command (0x10 = recalibrate, 0x11 = seek/position)
; Output: Carry flag set if error, clear if success

sub_065ah:
	push bc			;065a  Save BC register
	pop ix			;065b  IX = BC (save original command for later use)
	ld a,010h		;065d  A = 0x10 (base command value)
	sub c			;065f  A = 0x10 - C (calculate command offset)
	jr z,l0664h		;0660  If C=0x10 (recalibrate), use A=0 parameter
	ld a,020h		;0662  A = 0x20 (seek command parameter)

l0664h:
	; Build disk command parameter block
	ld hl,0ff83h		;0664  Point to disk parameter buffer in banked memory
	ld b,004h		;0667  B = 4 (number of parameters to set)
	ld (hl),008h		;0669  Store 0x08 (command type/drive select)
	inc hl			;066b  Move to next parameter slot
	ld (hl),a		;066c  Store command-specific parameter (0x00 or 0x20)

l066dh:
	; Clear remaining parameters
	inc hl			;066d  Move to next parameter slot
	ld (hl),000h		;066e  Clear parameter (set to 0x00)
	djnz l066dh		;0670  Clear remaining 3 parameters (total 6 bytes)
	
	; Execute the disk command
	call reading		;0672  Execute the prepared disk command
	ret c			;0675  Return immediately if command failed
	call moveoutreseta	;0676  Move/output result data  
	call reading2		;0679  Read final status
	and 002h		;067c  Check bit 1 (error flag)
	ret z			;067e  Return success if bit 1 clear
	scf			;067f  Set carry flag (error)
	ret			;0680  Return with error
; ================================================================================
; CRITICAL DISK ERROR HANDLER  
; ================================================================================
; Handles severe disk system errors that prevent successful boot
; This is called when disk controller hardware fails or becomes unresponsive
; Displays Error #1 message and enters service mode

print_error1:
	push bc			;0681  Save BC register state
	call screenclearnetry2		;0682  Clear screen and setup error display
	rst 10h			;0685  RST 10h: Display error message (points to message below)

; ================================================================================
; ERROR #1 MESSAGE DATA - CRITICAL DISK SYSTEM ERROR
; ================================================================================
; "Error 1: Disk system error, Call Service" - 40 characters
; This indicates hardware-level disk controller failure requiring service

ERROR1_start:
	defb 028h		;0686  Message length: 40 characters
	defb 045h		;0687  'E'
	defb 072h		;0688  'r'
	defb 072h		;0689  'r'
	defb 06fh		;068a  'o'
	defb 072h		;068b  'r'
	defb 020h		;068c  ' '
	defb 031h		;068d  '1'
	defb 03ah		;068e  ':'
	defb 020h		;068f  ' '
	defb 044h		;0690  'D'
	defb 069h		;0691  'i'
	defb 073h		;0692  's'
	defb 06bh		;0693  'k'
	defb 020h		;0694  ' '
	defb 073h		;0695  's'
	defb 079h		;0696  'y'
	defb 073h		;0697  's'
	defb 074h		;0698  't'
	defb 065h		;0699  'e'
	defb 06dh		;069a  'm'
	defb 020h		;069b  ' '
	defb 065h		;069c  'e'
	defb 072h		;069d  'r'
	defb 072h		;069e  'r'
	defb 06fh		;069f  'o'
	defb 072h		;06a0  'r'
	defb 02ch		;06a1  ','
	defb 020h		;06a2  ' '
	defb 043h		;06a3  'C'
	defb 061h		;06a4  'a'
	defb 06ch		;06a5  'l'
	defb 06ch		;06a6  'l'
	defb 020h		;06a7  ' '
	defb 053h		;06a8  'S'
	defb 065h		;06a9  'e'
	defb 072h		;06aa  'r'
	defb 076h		;06ab  'v'
	defb 069h		;06ac  'i'
	defb 063h		;06ad  'c'
	defb 065h		;06ae  'e'
ERROR1_end:

; ================================================================================
; ERROR RECOVERY AND SYSTEM RESTART
; ================================================================================
; After displaying the error, system performs cleanup and restart sequence

error1_recovery:
	call delayloop		;06af  Delay to allow user to read error message
	call delayloop		;06b2  Additional delay for visibility
	call delayloop		;06b5  Final delay before restart attempt
	call setupclearscreen2	;06b8  Clear screen and reinitialize display system
	call readydisk		;06bb  Attempt to ready disk system for retry
	pop bc			;06be  Restore BC register from stack
	scf			;06bf  Set carry flag (indicate error condition)
	ret			;06c0  Return with error status

; ================================================================================
; DISK SECTOR READ WITH RETRY LOGIC
; ================================================================================
; Advanced disk read routine with automatic retry and error recovery
; Input: D = sector count, BC = parameters, HL = buffer pointer
; Output: Carry set if unrecoverable error, clear if success

l06c1h:
	call sub_04c7h		;06c1  Setup disk operation parameters
	ld a,b			;06c4  Get B register (track/cylinder)
	ld (0ff85h),a		;06c5  Store track parameter in system memory
	ld a,c			;06c8  Get C register (sector/head)
	ld (0ff86h),a		;06c9  Store sector parameter in system memory

l06cch:
	ld a,d			;06cc  Get D register (sector count)
	ld (0ff87h),a		;06cd  Store sector count parameter
	push bc			;06d0  Save BC registers (track/sector info)
	push hl			;06d1  Save HL registers (buffer pointer)
	call reading		;06d2  Execute disk read command
	jp c,l06fbh		;06d5  If read error, jump to cleanup and return
	
	; Successful read - transfer parameter data
	ld hl,0ff83h		;06d8  Point to disk parameter buffer
	ld bc,00678h		;06db  B=6 bytes to transfer, C=0x78 (I/O port for data)

l06deh:
	; Wait for controller ready and transfer data
	in a,(079h)		;06de  Read disk controller status port
	and 041h		;06e0  Check bits 6 and 0 (ready and data request flags)
	jr nz,l06deh		;06e2  Wait until controller indicates ready for transfer
	outi			;06e4  Transfer byte: output (HL) to port C, increment HL, decrement B
	jr nz,l06deh		;06e6  Continue until all 6 parameter bytes transferred
	out (07ah),a		;06e8  Write final control byte to data port 0x7A
	nop			;06ea  Short timing delay for controller

l06ebh:
	; Wait for final status and complete operation
	in a,(079h)		;06eb  Read disk controller status
	and 044h		;06ed  Check bits 6 and 2 (completion status flags)
	jr nz,l06ebh		;06ef  Wait for completion status change
	out (07bh),a		;06f1  Write completion status to control port 0x7B
	call reading2		;06f3  Read final operation status
	and 002h		;06f6  Check bit 1 (error indication)
	call nz,sub_06ffh	;06f8  If error bit set, call error recovery routine

l06fbh:
	; Cleanup and return
	pop hl			;06fb  Restore HL registers (buffer pointer)
	pop bc			;06fc  Restore BC registers (track/sector)
	inc bc			;06fd  Increment BC to next sector position
	ret			;06fe  Return to caller

; ================================================================================
; ADVANCED DISK ERROR RECOVERY ROUTINE
; ================================================================================
; Sophisticated error recovery with retry logic and validation
; Called when initial read fails - attempts recovery and validation
; Returns: Carry set if unrecoverable, clear if recovery successful

sub_06ffh:
	call reading		;06ff  Attempt another disk read operation
	ret c			;0702  Return immediately if read still failing
	ld a,003h		;0703  A = 3 (retry/recovery command code)
	call moveout		;0705  Execute move/output operation with retry command
	ld bc,00478h		;0708  B=4 bytes to read, C=0x78 (I/O data port)
	ld hl,0ff89h		;070b  Point to error recovery buffer in system memory

l070eh:
	; Read recovery data from controller
	in a,(079h)		;070e  Read disk controller status port
	cp 00fh			;0710  Compare with 0x0F (ready for data transfer)
	jr nz,l070eh		;0712  Wait until controller indicates ready (status = 0x0F)
	ini			;0714  Input byte: read from port C to (HL), increment HL, decrement B
	jr nz,l070eh		;0716  Continue until all 4 recovery bytes read
	call reading2		;0718  Read final status after recovery attempt
	ld a,(0ff89h)		;071b  Get first byte from recovery buffer
	sub 098h		;071e  Compare with expected recovery value 0x98
	ret z			;0720  Return success (Z flag) if recovery data matches
	scf			;0721  Set carry flag (recovery failed)
	ret			;0722  Return with unrecoverable error status

; ================================================================================
; LOW-LEVEL DISK CONTROLLER COMMUNICATION ROUTINES
; ================================================================================
; Core routines for sending commands and reading status from disk controller

reading:
	; Initiate disk read command with timeout protection
	in a,(079h)		;0723  Read disk controller status port
	and 080h		;0725  Check bit 7 (controller ready flag)
	scf			;0727  Set carry flag (assume error initially)
	ret z			;0728  Return with error if controller not ready
	ld a,001h		;0729  A = 1 (start read command)
	out (079h),a		;072b  Send start command to disk controller
	ld b,0ffh		;072d  B = 255 (timeout counter - ~255 loop iterations)

l072fh:
	; Wait for command completion with timeout
	in a,(079h)		;072f  Read controller status port
	and 080h		;0731  Check ready bit (bit 7)
	ret z			;0733  Return success (carry clear) if ready bit cleared
	djnz l072fh		;0734  Decrement timeout counter and continue waiting
	scf			;0736  Set carry flag (timeout error)
	ret			;0737  Return with timeout error

reading2:
	in a,(079h)		;0738  Read disk controller status
	and 044h		;073a  Check bits 6 and 2 (busy/status flags)
	jr nz,reading2		;073c  Wait until both flags clear
	in a,(078h)		;073e  Read result from data port
	ld b,a			;0740  Save result in B

l0741h:
	in a,(079h)		;0741  Read controller status again
	and 042h		;0743  Check bits 6 and 1 (different status bits)
	jr nz,l0741h		;0745  Wait until these flags clear
	in a,(078h)		;0747  Read final data byte
	ld a,b			;0749  Restore original result to A
	ret			;074a  Return with status in A

; ================================================================================
; DISK COMMAND OUTPUT ROUTINES
; ================================================================================
; Send command sequences to disk controller

moveoutreseta:
	ld a,000h		;074b  A = 0 (reset command)

moveout:
	call sub_0760h		;074d  Send A to disk controller
	ld a,(0ff84h)		;0750  Get stored command parameter
	call sub_0760h		;0753  Send parameter to controller
	ld a,000h		;0756  A = 0 (padding/terminator)
	ld b,004h		;0758  B = 4 (send 4 zero bytes)

l075ah:
	call sub_0760h		;075a  Send zero byte to controller
	djnz l075ah		;075d  Repeat 4 times
	ret			;075f  Return

; ================================================================================
; LOW-LEVEL DISK CONTROLLER I/O
; ================================================================================
; Fundamental routine for sending bytes to disk controller

sub_0760h:
	push af			;0760  Save byte to send
	
l0761h:
	in a,(079h)		;0761  Read controller status
	and 041h		;0763  Check bits 6 and 0 (ready/busy flags)
	jr nz,l0761h		;0765  Wait until controller ready to receive
	pop af			;0767  Restore byte to send
	out (078h),a		;0768  Send byte to controller data port
	ret			;076a  Return

; ================================================================================
; DISK CONTROLLER DATA TRANSFER
; ================================================================================
; Transfers data block from controller

sub_076bh:
	ld a,00ch		;076b  A = 0x0C (data transfer command)
	call moveout		;076d  Send command to controller
	ld bc,00878h		;0770  B=8 bytes, C=0x78 (data port)

l0773h:
	in a,(079h)		;0773  Read controller status
	cp 007h			;0775  Compare with 0x07 (data ready)
	jr nz,l0773h		;0777  Wait until data ready
	outi			;0779  Input byte: (HL) ← (C), HL++, B--
	jr nz,l0773h		;077b  Continue until 8 bytes transferred
	jr reading2		;077d  Jump to read final status

; ================================================================================
; DISK ERROR RECOVERY AND ALTERNATE ROUTINES
; ================================================================================

l077fh:
	ld a,0e4h		;077f  A = 0xE4 (error recovery command)
	call steproutine	;0781  Execute step/recovery routine
	ret z			;0784  Return if successful
	jp print_error1		;0785  Jump to disk error handler

l0788h:
	call reading		;0788  Attempt disk read
	ret c			;078b  Return if error
	ld a,001h		;078c  A = 0x01 (step command)
	call steproutine	;078e  Execute step routine
	ret z			;0791  Return if successful
	jr print_error3		;0792  Jump to alternate error handling

; ================================================================================
; DISK STEP/SEEK ROUTINE
; ================================================================================
; Executes disk step commands (seek operations)

steproutine:
	call moveout		;0794  Send step command to controller
	call reading2		;0797  Read controller status
	and 002h		;079a  Check bit 1 (error flag)
	ret			;079c  Return with Z flag set if no error

; ================================================================================
; DISK MALFUNCTION ERROR HANDLER
; ================================================================================

print_error3:
	call screenclearnetry2	;079d  Clear screen and setup display
	rst 10h			;07a0  RST 10h: Display error message

; ================================================================================
; ERROR #3 MESSAGE DATA - DISK MALFUNCTION
; ================================================================================

; BLOCK 'ERROR3' (start 0x07a1 end 0x07c9)
ERROR3_start:
	defb 027h		;07a1  Message length: 39 characters
	defb 045h		;07a2  'E' - "Error #3: Disk Malfunction, Call Service"
	defb 072h		;07a3  'r'
	defb 072h		;07a4  'r'
	defb 06fh		;07a5  'o'
	defb 072h		;07a6  'r'
	defb 020h		;07a7  ' '
	defb 033h		;07a8  '3'
	defb 03ah		;07a9  ':'
	defb 020h		;07aa  ' '
	defb 044h		;07ab  'D'
	defb 069h		;07ac  'i'
	defb 073h		;07ad  's'
	defb 06bh		;07ae  'k'
	defb 020h		;07af  ' '
	defb 04dh		;07b0  'M'
	defb 061h		;07b1  'a'
	defb 06ch		;07b2  'l'
	defb 066h		;07b3  'f'
	defb 075h		;07b4  'u'
	defb 06eh		;07b5  'n'
	defb 063h		;07b6  'c'
	defb 074h		;07b7  't'
	defb 069h		;07b8  'i'
	defb 06fh		;07b9  'o'
	defb 06eh		;07ba  'n'
	defb 02ch		;07bb  ','
	defb 020h		;07bc  ' '
	defb 043h		;07bd  'C'
	defb 061h		;07be  'a'
	defb 06ch		;07bf  'l'
	defb 06ch		;07c0  'l'
	defb 020h		;07c1  ' '
	defb 053h		;07c2  'S'
	defb 065h		;07c3  'e'
	defb 072h		;07c4  'r'
	defb 076h		;07c5  'v'
	defb 069h		;07c6  'i'
	defb 063h		;07c7  'c'
	defb 065h		;07c8  'e'
ERROR3_end:
	halt			;07c9  System halt - requires service intervention

; ================================================================================
; DATA TABLES AND CONSTANTS
; ================================================================================
; Various data tables used by system routines

l07cah:
	defb 001h		;07ca  Data table entry 1
	defb 032h		;07cb  Data table entry 2
	defb 004h		;07cc  Data table entry 3
	defb 001h		;07cd  Data table entry 4
	defb 031h		;07ce  Data table entry 5
	defb 000h		;07cf  Data table entry 6
	defb 040h		;07d0  Data byte
	defb 00bh		;07d1  Data byte
	defb 006h		;07d2  Data byte
	defb 000h		;07d3  Data byte
	defb 095h		;07d4  Data byte
	defb 0ffh		;07d5  Data byte
	defb 001h		;07d6  Data byte
l07d7h:
	defb 032h		;07d7  Data byte
	defb 002h		;07d8  Data byte  
	defb 001h		;07d9  Data byte
	defb 031h		;07da  Data table entry 1
	defb 000h		;07db  Data table entry 1
	defb 040h		;07dc  Data table entry 2
	defb 00bh		;07dd  Data table entry 2
	defb 006h		;07de  Data byte
	defb 000h		;07df  Data byte
	defb 04ah		;07e0  Data byte
	defb 0ffh		;07e1  Data byte

l07e2h:
	defb 000h		;07e2  Data byte 'Q'
	defb 000h		;07e3  Data byte (NOP padding)
	defb 04ah		;07e4  Data byte (NOP padding)
l07e5h:
	defb 0ffh		;07e5  Data byte 'J'
	
; ROM unused space filled with RST 38h (0FFh) - typical EPROM pattern

	defb 0ffh		;07e6  Unused ROM space
	defb 0ffh		;07e7  Unused ROM space
	defb 0ffh		;07e8  Unused ROM space
	defb 0ffh		;07e9  Unused ROM space
	defb 0ffh		;07ea  Unused ROM space
	defb 0ffh		;07eb  Unused ROM space
	defb 0ffh		;07ec  Unused ROM space
	defb 0ffh		;07ed  Unused ROM space
	defb 0ffh		;07ee  Unused ROM space
	defb 0ffh		;07ef  Unused ROM space
	defb 0ffh		;07f0  Unused ROM space
	defb 0ffh		;07f1  Unused ROM space
	defb 0ffh		;07f2  Unused ROM space
	defb 0ffh		;07f3  Unused ROM space
	defb 0ffh		;07f4  Unused ROM space
	defb 0ffh		;07f5  Unused ROM space
	defb 0ffh		;07f6  Unused ROM space
	defb 0ffh		;07f7  Unused ROM space
	defb 0ffh		;07f8  Unused ROM space
	defb 0ffh		;07f9  Unused ROM space
	defb 0ffh		;07fa  Unused ROM space
	defb 0ffh		;07fb  Unused ROM space
	defb 0ffh		;07fc  Unused ROM space
	defb 0ffh		;07fd  Unused ROM space
	defb 0ffh		;07fe  Unused ROM space
	defb 0ffh		;07ff  End of 2K ROM - TMS2516 EPROM

; ================================================================================
; END OF ROM - MONROE OC8820 BOOT ROM R3.00C
; ================================================================================
; Total ROM size: 2048 bytes (0x0000-0x07FF)
; Boot process complete - system transfers control to OS at 0xFFA6
; ================================================================================