ProDOS-ROM-Drive/Firmware/Firmware.asm
2021-12-12 10:31:01 -05:00

194 lines
4.3 KiB
NASM

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