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chip-f8.asm
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1000
; CHIP-F8
; -------
; CHIP-8 interpreter for F8
; on the Fairchild Channel F
; made by Tobias V. Langhoff
processor f8
;===================;
; Assembly Switches ;
;===================;
switch_music = 0 ; turns music on and off
;=========;
; Equates ;
;=========;
;------------;
; BIOS Calls ;
;------------;
clrscrn = $00d0 ; uses r31
delay = $008f
pushk = $0107 ; used to allow more subroutine stack space
popk = $011e
drawchar = $0679
;-------------------;
; Color Definitions ;
;-------------------;
red = $40
blue = $80
green = $00
bkg = $C0
clear = $FF
;-----------------;
; RAM Definitions ;
;-----------------;
ram = $2800 ; use Schach RAM to hold CHIP-8 program
screenbuffer = $2f00
chip8offset = $2600
;--------------------;
; Register Reference ;
;--------------------;
; The registers in the f8 are used as both RAM and registers for
; this game. The reference is as follows:
;
; v0-v15 direct access
; delay timer
; sound timer
; r0: scratch
; r1-r2: current opcode
; r3: current palette
; r4-r9: scratch
; r10-r11 (H): PC
; r16-r31: CHIP-8 variables V0–VF
; r40-r41 (o50): I
; r60-63
;===================;
; Main Program Code ;
;===================;
MAC os
; adds missing instruction: OS r
; modifies r0
com
lr 0, a
lr a, {1}
com
ns 0
com
ENDM
;---------------;
; Program Entry ;
;---------------;
org $800
cartridgeStart:
.byte $55, $2B ; cartridge header
cartridgeEntry:
lis 0 ; init the h/w
outs 1
outs 4
outs 5
outs 0
lisu 4 ; r32 = complement flag
lisl 0
lr S, A
li $00 ; set to one color, black background
lr 3, A ; clear screen to black
pi clrscrn
copyGameToRAM subroutine
dci gameROM
xdc
dci ram
.copyByte:
xdc
lm ; copy byte from gameROM to A and advance DC0
xdc ; swap DC
st ; copy byte from A to gameRAM and advance DC0
lr q, dc
lr a, qu ; check counter
xi $2f
bnz .copyByte
initInterpreter:
dci ram ; point at start of ROM
lr h, dc ; store PC in H (r10/r11)
lisu 4 ; set stack pointer
lisl 7
li 40
lr s, a
fetchDecodeLoop subroutine
lr dc, h ; load PC
lm ; fetch [PC] into A and advance
lr 1, a ; store first opcode byte in 1
lm ; fetch PC into A and advance
lr 2, a ; store second opcode byte in 2
lr h, dc ; update PC
dci .jumpTable
lr a, 1
sr 4
adc
adc
lm
lr qu, a
lm
lr ql, a
lr p0, q
.jumpTable:
.word firstDigitZero
.word firstDigitOne
.word firstDigitTwo
.word firstDigitThree
.word firstDigitFour
.word firstDigitFive
.word firstDigitSix
.word firstDigitSeven
.word firstDigitEight
.word firstDigitNine
.word firstDigitA
.word firstDigitB
.word firstDigitC
.word firstDigitD
.word firstDigitE
.word firstDigitF
firstDigitZero subroutine
lr a, 2
xi $E0
bz .clearScreen
lr a, 2
xi $EE
bz .returnFromSubroutine
jmp fetchDecodeLoop
.clearScreen:
dci screenbuffer
li 255 ; copy 256 bytes
lr 0, a ; use r0 as counter
.copyByte:
li 0
st ; copy byte from A to screen and advance DC0
ds 0 ; decrement counter
lr a, 0 ; check counter
ns 0 ; AND a with itself
bnz .copyByte
lr 3, A ; clear screen to palette
pi clrscrn ; TODO scratches several registers
jmp fetchDecodeLoop
.returnFromSubroutine:
lisu 4
lisl 7
lr a, s
lr is, a
lr a, i
lr hu, a
lr a, s
lr hl, a
lisu 4
lisl 7
ds s ; decrease stack pointer by two
ds s
jmp fetchDecodeLoop
firstDigitOne:
lr a, 1 ; load first byte of opcode
ni $0F ; remove first nibble
ai $26 ; add RAM offset
lr hu, a ; load into PC
lr a, 2 ; load second byte of opcode
lr hl, a ; load into PC
jmp fetchDecodeLoop
firstDigitTwo:
lisu 4
lisl 7
lr a, s ; load stack pointer
inc ; increase stack pointer by two
inc
lr s, a
lr is, a ; load new stack pointer into ISAR
lr a, hu ; push PC onto stack
lr i, a
lr a, hl
lr s, a
lr a, 1 ; load first byte of opcode
ni $0F ; remove first nibble
ai $26 ; add RAM offset
lr hu, a ; load into PC
lr a, 2 ; load second byte of opcode
lr hl, a ; load into PC
jmp fetchDecodeLoop
firstDigitThree subroutine
pi getX
lr a, 2
xs s
bnz .notEqual
lr dc, h ; load PC
lm
lm
lr h, dc
.notEqual
jmp fetchDecodeLoop
firstDigitFour subroutine
pi getX
lr a, 2
xs s
bz .equal
lr dc, h ; load PC
lm
lm
lr h, dc
.equal:
jmp fetchDecodeLoop
firstDigitFive subroutine
; TODO assert that last digit is 0
pi getX
lr a, s
lr 0, a
pi getY
lr a, s
xs 0
bnz .notEqual
lr dc, h ; load PC
lm
lm
lr h, dc
.notEqual:
jmp fetchDecodeLoop
firstDigitSix:
pi getX
lr a, 2 ; get second byte of opcode
lr s, a ; set it
jmp fetchDecodeLoop
firstDigitSeven:
pi getX
lr a, s ; get current value of VX
as 2 ; add value of second byte of opcode
lr s, a ; set new value of VX
jmp fetchDecodeLoop
firstDigitEight subroutine
dci .jumpTable
lr a, 2 ; get second byte of opcode
ni $0f ; remove first nibble
adc
adc
lm
lr qu, a
lm
lr ql, a
lr p0, q
.jumpTable:
.word .lastDigitZero
.word .lastDigitOne
.word .lastDigitTwo
.word .lastDigitThree
.word .lastDigitFour
.word .lastDigitFive
.word .lastDigitSix
.word .lastDigitSeven
.word .lastDigitEight
.word .lastDigitNine
.word .lastDigitA
.word .lastDigitB
.word .lastDigitC
.word .lastDigitD
.word .lastDigitE
.word .lastDigitF
.lastDigitZero:
pi getY
lr a, s
lr 0, a
pi getX
lr a, 0
lr s, a
jmp fetchDecodeLoop
.lastDigitOne:
pi getY
lr a, s
lr 3, a
pi getX
lr a, s
os 3
lr s, a
jmp fetchDecodeLoop
.lastDigitTwo:
pi getY
lr a, s
lr 0, a
pi getX
lr a, s
ns 0
lr s, a
jmp fetchDecodeLoop
.lastDigitThree:
pi getY
lr a, s
lr 0, a
pi getX
lr a, s
xs 0
lr s, a
jmp fetchDecodeLoop
.lastDigitFour:
pi getY
lr a, s
lr 0, a
pi getX
lr a, s
as 0
lr s, a
; Set VF
lisu 3
lisl 7
lr j, w
lr a, j
ni 2
sr 1
lr s, a
jmp fetchDecodeLoop
.lastDigitFive:
pi getY
lr a, s
com
inc
lr 0, a
pi getX
lr a, s
as 0
lr s, a
; Set VF
lisu 3
lisl 7
lr j, w
lr a, j
ni 2
sr 1
lr s, a
jmp fetchDecodeLoop
.lastDigitSix:
; TODO quirk
pi getY
lr a, s
lr 0, a
pi getX
lr a, 0
sr 1
lr s, a
; Set VF
lisu 3
lisl 7
lr a, 0
ni 1
lr s, a
jmp fetchDecodeLoop
.lastDigitSeven:
pi getX
lr a, s
com
inc
lr 0, a
pi getY
lr a, s
as 0
lr 0, a
; Set VF
lisu 3
lisl 7
lr j, w
lr a, j
ni 2
sr 1
lr s, a
pi getX
lr a, 0
lr s, a
jmp fetchDecodeLoop
.lastDigitEight:
jmp fetchDecodeLoop
.lastDigitNine:
jmp fetchDecodeLoop
.lastDigitA:
jmp fetchDecodeLoop
.lastDigitB:
jmp fetchDecodeLoop
.lastDigitC:
jmp fetchDecodeLoop
.lastDigitD:
jmp fetchDecodeLoop
.lastDigitE:
; TODO quirk
pi getY
lr a, s
lr 0, a
pi getX
lr a, 0
sl 1
lr s, a
; Set VF
lisu 3
lisl 7
lr a, 0
ni 8
sr 4
sr 1
sr 1
sr 1
lr s, a
jmp fetchDecodeLoop
.lastDigitF:
jmp fetchDecodeLoop
firstDigitNine subroutine
; TODO assert that last digit is 0
pi getX
lr a, s
lr 0, a
pi getY
lr a, s
xs 0
bz .equal
lr dc, h ; load PC
lm
lm
lr h, dc
.equal:
jmp fetchDecodeLoop
firstDigitA:
lisu 4
lisl 1
lr a, 1 ; load first byte of opcode
ni $0F ; remove first nibble
ai $26 ; add RAM offset
lr i, a ; load into I
lr a, 2 ; load second byte of opcode
lr s, a ; load into I
jmp fetchDecodeLoop
firstDigitB:
lr a, 1 ; load first byte of opcode
ni $0F ; remove first nibble
ai $26 ; add RAM offset
lr qu, a ; load into PC
lr a, 2 ; load second byte of opcode
lr ql, a ; load into PC
lr dc, q
lisu 2
lisu 0
lr a, s
adc ; TODO this is used as a two's complement number!!
lr h, dc ; load into PC
jmp fetchDecodeLoop
firstDigitC:
jmp firstDigitSix
;jmp fetchDecodeLoop
firstDigitD:
pi getX
lr a, s ; get X value
lr 5, a ; store X in scratch 5
ni $07 ; bit offset of first bit of sprite data
lr 7, a ; store bit offset in scratch 7
lr a, 5 ; get X again
ni $3F ; modulo 64
sr 1 ; position in pixel row of first byte that will contain sprite data
sr 1
sr 1
lr 8, a ; store position in scratch 8
pi getY
lr a, s ; get Y value
ni $1F ; modulo 32
sl 1 ; position in display memory of the first row that will contain sprite data
sl 1
sl 1
lr 6, a ; store Y in scratch 6
as 8 ; r6 + r8
lr ql, a
li $2f ; screenbuffer
lr qu, a
lr dc, q
lr a, 2 ; get second byte of opcode
ni $0f ; remove first nibble
lr 9, a ; save in scratch 9 as display row counter
; clear VF before drawing
lisu 3
lisl 7
clr
lr s, a
xdc
; load I into DC
lisu 4
lisl 1
lr a, i
lr qu, a
lr a, s
lr ql, a
lr dc, Q ; load I into DC
; just to recap...
; r0 scratch
; r1 first opcode
; r2 second opcode
; r3 left byte of assembled sprite
; r4 right byte of assembled sprite
; r5 X
; r6 Y position in display memory of the first row that will contain sprite data
; r7 bit offset
; r8 position in pixel row of first byte that will contain sprite data
; r9 row counter
; DC0 I
; DC1 screen buffer with offset
.nextSpriteRow:
clr
lisu 4
lisl 3
lr i, a
lr d, a
lr a, 9 ; get row counter
ns 9 ; AND a with itself
bz .displaySprite ; we're done, reset I and display the sprite
.dontResetI:
ds 9 ; decrease row counter
lm ; get one byte of sprite data from I and advance I
lr s, a ; put byte in scratch 32 ; at 0095 now...
lr a, 7 ; get bit offset for first bit of sprite data
lr 3, a ; put byte in scratch 33 to use as bit counter
.splitSpriteRow:
lr a, 3 ; get current bit count
ns 3 ; AND a with itself
bz .storeSpriteRow ; sprite data is now split across two rows
lr a, s ; load byte into a
ni 1
bz .rightShiftWithNoCarry
.rightShiftWithCarry:
lr a, s ; load byte into a
sr 1 ; shift it right once
lr i, a ; load the shifted byte back
lr a, s ; load new byte into a
sr 1
oi $80
lr d, a ; load shifted byte into a
ds 3
jmp .splitSpriteRow
.rightShiftWithNoCarry:
lr a, s ; load byte into a
sr 1 ; shift it right once
lr i, a ; load the shifted byte back
lr a, s ; load new byte into a
sr 1
lr d, a ; load shifted byte into a
ds 3
jmp .splitSpriteRow
.storeSpriteRow:
xdc ; switch to screen buffer pointer
lr q, dc ; store DC in Q so we can revert here
lisu 4 ; take the assembled bytes
lisl 3
lr a, s ; first assembled byte
nm ; and with screen data to detect collision
bz .noCollision1
li 1
lr 0, a
.noCollision1:
lr dc, q ; restore dc
lr a, i ; first assembled byte
xm ; xor with screen data (this increments dc)
lr dc, q ; restore dc
st ; store xor-ed result
; check for screen boundary, we don't wrap sprites
lr a, ql
lr 3, a
lr q, dc ; store DC in Q so we can revert here
lr a, ql
xs 3 ; xor old and new ql
ni $08 ; see if we crossed from $2F?7 to $2F?8; if so, we've wrapped around X
li 7 ; go to next row in screen data
bnz .outOfBounds
lr a, s
nm ; and with screen data
bz .noCollision2
li 1
lr 0, a
.noCollision2:
lr dc, q ; restore dc
lr a, d ; second assembled byte
xm ; xor with screen data (this increments dc)
lr dc, q ; restore dc
st ; store xor-ed result
li 6 ; go to next row in screen data
.outOfBounds:
adc
xdc ; swap back to I pointer for next sprite row
; load collision flag into VF
lisu 3
lisl 7
lr a, 0
lr s, a
jmp .nextSpriteRow
.displaySprite:
.blitSprite:
; Right now we blit the entire screen buffer
; but we probably want to only blit the area
; the sprite was drawn to. To do that we need
; to change a lot of register use.
dci screenparams
pi blitGraphic
jmp fetchDecodeLoop
firstDigitE:
lr a, 2
xi $9e
bnz .ex9e
lr a, 2
xi $a1
bnz .exa1
jmp fetchDecodeLoop
.ex9e:
jmp fetchDecodeLoop
.exa1:
lr dc, h
lm
lm
lr h, dc
jmp fetchDecodeLoop
firstDigitF:
; TODO convert below to jump table (or branch/offset table if space permits)
lr a, 2 ; get second byte of opcode
xi $07
bnz .not07
jmp .lastNibble07
.not07:
lr a, 2
xi $0A
bnz .not0A
jmp .lastNibble0A
.not0A:
lr a, 2
xi $15
bnz .not15
jmp .lastNibble15
.not15:
lr a, 2
xi $18
bnz .not18
jmp .lastNibble18
.not18:
lr a, 2
xi $1E
bnz .not1E
jmp .lastNibble1E
.not1E:
lr a, 2
xi $29
bnz .not29
jmp .lastNibble29
.not29:
lr a, 2
xi $33
bnz .not33
jmp .lastNibble33
.not33:
lr a, 2
xi $55
bnz .not55
jmp .lastNibble55
.not55:
lr a, 2
xi $65
bnz .not65
jmp .lastNibble65
.not65:
jmp fetchDecodeLoop
.lastNibble07:
jmp fetchDecodeLoop
.lastNibble0A:
; decrement PC by 2
lr dc, h
li $fe
adc
lr h, dc
jmp fetchDecodeLoop
.lastNibble15:
jmp fetchDecodeLoop
.lastNibble18:
jmp fetchDecodeLoop
.lastNibble1E:
lisu 4
lisl 1
lr a, i
lr qu, a
lr a, s
lr ql, a
lr dc, q
pi getX
; Now we want to add the value in the register selected by the ISAR to the address in DC.
; But ADC treats the value in A as a two's complement number, so we need to do this in a roundabout way:
lr a, s ; load VX offset into A
ns s ; AND the offset with itself to check the upper bit
bp .adcHack ; if bit 7 is not set, we just proceed as normal
sr 1 ; if bit 7 is set, shift right
adc ; add this offset to DC
adc ; twice
lr a, s ; load the original offset again
ni 1 ; get bit 0 so we can finally add that
.adcHack:
adc ; add offset
lisu 4 ; set ISAR to I
lisl 1
lr q, dc ; load DC back into Q
lr a, qu
lr i, a
lr a, ql
lr s, a
jmp fetchDecodeLoop
.lastNibble29:
pi getX
lr a, s ; get number in VX
sl 1 ; multiply by 4
sl 1
as s ; add the number, so we multiply by 5
dci font ; set DC to the fontset's address
adc ; add the offset for the current character
lr q, dc ; load it into Q
lisu 4 ; set ISAR to I
lisl 1
lr a, qu ; load Q into I via A
lr i, a
lr a, ql
lr s, a
jmp fetchDecodeLoop
.lastNibble33:
pi getX
; BCD routine courtesy of https://my.eng.utah.edu/~nmcdonal/Tutorials/BCDTutorial/BCDConversion.html
li 0
lr 0, a ; bit counter
lr 1, a ; hundreds (treat as nibble)
lr 2, a ; tens (treat as nibble)
lr 3, a ; ones (treat as nibble)
lr a, s
lr 4, a ; r4: the binary number to convert to BCD
li 8
lr 0, a
.loop:
lr a, 1 ; if hundreds > 4 then add 3
ci 4
bp .testTens
ai 3
lr 1, a
.testTens: ; if tens > 4 then add 3
lr a, 2
ci 4
bp .testOnes
ai 3
lr 2, a
.testOnes: ; if ones > 4 then add 3
lr a, 3
ci 4
bp .shift
ai 3
lr 3, a
.shift:
lr a, 1 ; shift hundreds nibble left
sl 1
ni $0f
lr 1, a
lr a, 2
ni $08 ; if tens most significant bit is 1, carry it over to hundreds
bz .shiftTens
lr a, 1
inc
lr 1, a
.shiftTens: ; shift tens nibble left
lr a, 2
sl 1
ni $0f
lr 2, a
lr a, 3
ni $08 ; if ones most significant bit is 1, carry it over to tens
bz .shiftOnes
lr a, 2
inc
lr 2, a
.shiftOnes: ; shift ones nibble left
lr a, 3
sl 1
ni $0f
lr 3, a
lr a, 4
ni $80 ; if binary number's most significant bit is 1, carry it over to ones
bz .shiftByte
lr a, 3
inc
lr 3, a
.shiftByte: ; shift binary number left
lr a, 4
sl 1
lr 4, a
ds 0 ; decrement bit counter
bnz .loop ; if we haven't processed the entire binary number, continue
lisu 4
lisl 1