ProDOS-ROM-Drive/Firmware/Firmware.asm

    writeLatchHigh = $C081
    writeLatchLow = $C080

;temp variables becasue 6502 only has 3 registers
	highLatch = $F8
	lowLatch = $F9
	tempY = $FE
	blockHalfCounter = $FF   

;ProDOS defines
    command = $42   ;ProDOS command
    unit  = $43     ;7=drive 6-4=slot 3-0=not used
    buflo = $44     ;low address of buffer
    bufhi = $45     ;hi address of buffer
    blklo = $46     ;low block
    blkhi = $47     ;hi block
    ioerr = $27     ;I/O error code
    nodev = $28     ;no device connected
    wperr = $2B     ;write protect error

;for relocatable code, address to jump to instead of JSR absolute + RTS
	jumpAddressLo = $FA
	jumpAddressHi = $FB 
	ioAddressLo   = $FC
	ioAddressHi   = $FD
	knownRts      = $FF58

	.org     $C700
			;code is relocatable
			; but set to $c700 for
			; readability

;ID bytes for booting and drive detection
	cpx     #$20    ;ID bytes for ProDOS and the
	cpx     #$00    ; Apple Autostart ROM
	cpx     #$03    ;
	cpx     #$3C    ;this one for older II's

;zero out block numbers and buffer address
	sty		buflo
	sty 	blklo
	sty		blkhi
	iny				;set command = 1 for read block
	sty		command 
	sty		jumpAddressLo ;$01 of $0801 where boot code starts
	jsr 	knownRts ;jump to known RTS to get our address from the stack
	tsx
    lda     $0100,x ;this for example would be $C7 in slot 7
    sta     bufhi   ;keep the slot here
    asl     
    asl     
    asl     
    asl     
    tax				

;display copyright message
    ldy     #<text
drawtxt: lda     (buflo),y
	sta     $07D0,y ;put text on last line
	iny
	bne     drawtxt

;load block 0000 at $0800
	lda     #$08
	sta     bufhi
	sta     jumpAddressHi
	stx     unit
	bne     start
;This is the ProDOS entry point for this card
entry:
	lda     #<knownRts
	sta     jumpAddressLo
	lda     #>knownRts
	sta     jumpAddressHi
start:
	lda     #$C0
	sta     ioAddressHi
	jsr     knownRts
	tsx
	lda     $0100,x
	asl     a
	asl     a
	asl     a
	asl     a
	tax
	cpx     unit	;make sure same as ProDOS
	beq     docmd	;yep, do command
	sec				;nope, set device not connected
	lda     #nodev
	rts             ;go back to ProDOS

docmd:
	lda command
	beq     getstat ;command 0 is GetStatus
	cmp     #$01
	beq     readblk ;command 1 is ReadBlock
	sec             ;Format/Write not permitted
	lda     #wperr  ;write protect error
	rts             ;go back to ProDOS

getstat:  
	clc             ;send back status
	lda     #$00    ;good status
	ldx     #$00    ;1024 blocks
	ldy     #$04    ;
	rts

readblk:
	lda     blkhi	;get hi block
	asl     a       ;shift up to top 3 bits
	asl     a       ;since that's all the high
	asl     a       ;blocks we can handle
	asl     a       ;
	asl     a       ;
	sta     highLatch   ;save it in scratch ram 0
			;so we can stuff it in the
			;high latch later
	lda     blklo   ;get low block
	lsr     a       ;shift so we get the top 5
	lsr     a       ;bits - this also goes in
	lsr     a       ;the high latch
	ora     highLatch ;add it to those top 3 bits
	sta     highLatch ;save it back in scratch ram
	lda     blklo   ;get low block
	asl     a       ;shift it to top 3 bits
	asl     a       ;
	asl     a       ;
	asl     a       ;
	asl     a       ;
	sta     lowLatch
	lda     #$02
	sta     blockHalfCounter

;This gets 256 bytes from the ROM card

read256:
	ldy     #$00
	lda     highLatch	;get high latch value
	sta     writeLatchHigh,x	;set high latch for card
loop256:
	lda     lowLatch
	sta     writeLatchLow,x
	txa
	ora     #$80
	sta     ioAddressLo
loop16:
	sty     tempY
	ldy     #$00
	lda     (ioAddressLo),y
	ldy     tempY
	sta     (buflo),y
	iny
	inc     ioAddressLo
	lda     ioAddressLo
	and     #$0F
	bne     loop16
	inc     lowLatch
	cpy     #$00
	bne     loop256
	dec     blockHalfCounter
	bne     readnext256
	dec     bufhi
	clc             ;clear error code for success
	lda     #$00
	rts             ;return to ProDOS

readnext256:  
	inc     bufhi
    clc
    bcc     read256

;macro for string with high-bit set
.macro aschi str
.repeat .strlen (str), c
.byte .strat (str, c) | $80
.endrep
.endmacro

text:   aschi      "ROM-Drive (c)1998-2021 Terence J. Boldt"
end:
	.byte	 0

; These bytes need to be at the top of the 256 byte firmware as ProDOS
; uses these to find the entry point and drive capabilities

.repeat	251-<end
.byte 0
.endrepeat

	.byte      0,0     ;0000 blocks = check status
	.byte      3       ;bit 0=read 1=status
	.byte     entry&$00FF ;low byte of entry