jblang · September 6, 2017 20:14
diff --git a/a_mind_is_born.asm b/a_mind_is_born.asm
 ; A Mind is Born by Linus Akesson
 ; https://linusakesson.net/scene/a-mind-is-born/index.php
 ; transcribed to 64tass and further commented by J.B. Langston

 ; important locations after program is copied to zero page

 vmptr      = $cb        ; video matrix
 clock      = $13        ; global clock lsb - indicates position within bar
 clock_msb  = $20        ; global clock msb - indicates bar of song
 script     = $21        ; poke table
 const_d3ff = $03        ; location holding constant $d3ff
 const_d01c = $0f        ; location holding constant $d01c
 mel_lfsr   = $14        ; melody LFSR
 sid        = $0a        ; sid register shadow buffer
 mod_op1    = $d5        ; first opcode that gets modified
 mod_op2    = $d7        ; second opcode that gets modified
 basstbl    = $f3        ; bass notes
 freqtbl    = $f7        ; melody notes

        * = $0801

 ; basic bootstrap program

        .byte $0D,$08                   ; pointer to next line
        .byte $FF,$D3                   ; line number 54271
        .byte $9E,$32,$32,$32,$35       ; $9E=SYS + 2225 in PETSCII

 ; end of line/end of program markers overlap with first 3 bytes of sid table

 ; sid register shadow buffer; copied to sid registers at the end of every interrupt cycle
 ; voice 1: kick drum and bass

        .byte $00       ; $D400: voice 1 frequency lsb
        .byte $00       ; $D401: voice 1 frequency msb
        .byte $00       ; $D402: voice 1 pulse width lsb
        .byte $19       ; $D403: voice 1 pulse width msb
        .byte $41       ; $D404: voice 1 control register
        .byte $1C       ; $D405: voice 1 attack/decay
        .byte $D0       ; $D406: voice 1 sustain/release

 ; voice 2: melody

        .byte $00       ; $D407: voice 2 frequency lsb          $D020: border color
        .byte $DC       ; $D408: voice 2 frequency msb          $D021: background color 0
        .byte $00       ; $D409: voice 2 pulse width lsb        $D022: background color 1
        .byte $00       ; $D40A: voice 2 pulse width msb        $D023: background color 2
        .byte $11       ; $D40B: voice 2 control register       $D024: background color 3
        .byte $D0       ; $D40C: voice 2 attack/decay
        .byte $E0       ; $D40D: voice 2 sustain/release

 ; voice 3: drone

        .byte $0B       ; $D40E: voice 3 frequency lsb
        .byte $10       ; $D40F: voice 3 frequency msb
        .byte $33       ; $D410: voice 3 pulse width lsb
        .byte $0E       ; $D411: voice 3 pulse width msb
        .byte $61       ; $D412: voice 3 control register
        .byte $90       ; $D413: voice 3 attack/decay
        .byte $F5       ; $D414: voice 3 sustain/release

 ; filter

        .byte $07       ; $D415: filter cutoff lsb
        .byte $00       ; $D417: filter cutoff msb
        .byte $FF       ; $D418: filter resonance
        .byte $1F       ; $D419: filter control; $1F = low pass filter all channels and max volume

 ; poke table with eight entries: first byte is target address in zero-page, second byte is value to write.
 ; entry for bars $00-$07 overlaps with last two sid registers above; performs dummy write to $ff

        .byte $14,$41   ; bars $08-$0f: repeatedly reset LFSR seed to $41 to stutter melody
        .byte $D5,$24   ; bars $10-$17: overwrite ora opcode with bit to enable color and stop resetting LFSR
        .byte $15,$25   ; bars $18-$1f: change color to green; voice 1 waveform $25 (sounds same as $21)
        .byte $15,$53   ; bars $20-$27: change color to cyan; voice 1 waveform to $53 (hardsync)
        .byte $15,$61   ; bars $28-$2f: change color to white; voice 1 to $61 (disable hardsync)
        .byte $D5,$29   ; bars $30-$37: overwrite bit opcode with and to make visuals brighter
        .byte $1B,$0F   ; bars $38-$3f: change voice 3 pulse width to $f for brighter timbre

 ; interrupt handler

        inc     clock                   ; increment global clock lsb by 2
        inc     clock
        bne     noc1                    ; skip msb unless lsb wrapped to 0
        inc     clock_msb               ; increment global clock msb by 1
 noc1
        lda     #$61                    ; set gate bit for drone
        sta     sid+2*7+4               ; via control reg for voice 3
        lax     clock_msb               ; load current bar # into X and A
        cpx     #$3f                    ; switch to high pass filter for final bar
        beq     highpass
        bcc     noend                   ; skip finale until after final bar
        lsr     $d011                   ; fade to black
        jmp     ($fffc)                 ; kernal reset vector
 highpass
        ldy     #$6d
        sty     sid+$18                 ; switch for high pass filter 
        sty     mod_op2                 ; change ora to adc (stop blinking)
 noend
        lsr                             ; calculate address into poke table:
        asr     #$1c                    ;       ((bar >> 1) and $1c) >> 1
        tay                             ; save result fo later

 ; beat generation

        lda     clock                   ; check what part of current bar we're in
        and     #$30                    ; only duck during third quarter of beat
        bne     noduck
        dec     sid+2*7+4               ; turn off gate bit to start release of note`
 noduck
        cpx     #$2f                    ; turn bass off during bar 2f
        beq     bassoff
        bcs     nointro                 ; During bars $00-$2e we keep playing the same bass
        ldx     #2                      ; note, from offset 2 in the bass table.
 nointro
        cmp     #$10                    ; turn bass off after first quarter of beat
        beq     bassoff
        txa                             ; x contains either 2 or current bar count
        and     #3                      ; mask off last two bits of bar
        tax                             
        lda     basstbl,x               ; use as index into bass note table
        sta     sid+0*7+0               ; poke into sid voice 1 frequency msb

        .byte    $2d                    ; and absolute will eat the next instr and its operand
 bassoff
        lax     #0                      ; Safe when the operand is zero.
        bcs     bassdone                ; Carry will be set if we got here via bassoff.

        ; Carry was not set by cmp #$10, so we are in the
        ; first 25% of a beat. Throw away the computed bass note
        ; and play a drum instead.

        ; But handle the script first.

        lax     script+1,y              ; load poke byte into A+X
        ldx     script,y                ; load poke address into X
        sta     0,x                     ; poke accumulator into address from script

        lda     clock                   ; get clock
        asr     #$0e                    ; mask 00001110, then shift right
        tax                             ; A goes from 0 to 7 during the first 25% of the beat
        sbx     #256-8                  ; update the video matrix pointer
        stx     vmptr+1
        eor     #$07                    ; invert the value to obtain the pitch of the drum sound
 bassdone
        sta     sid+0*7+1               ; store in voice 1 freq msb

 ; melody generation

        lda     clock                   ; 16th notes last for for 8 interrupts
        and     #$0f
        bne     nomel

        lda     #$b8                    ; load lfsr seed
        sre     mel_lfsr                ; shift right and exclusive or
        bcc     noc2                    ; only write it back if a bit shifted out

        sta     mel_lfsr
 noc2
        and     #7                      ; use bottom 3 bits of lfsr as index into note table
        tax
        lda     freqtbl,x               ; look up note from note table
        sta     sid+1*7+1               ; store in voice 2 freq msb
 nomel
        ldy     #8
 vicloop
        lax     sid+3,y                 ; copy stuff from shadow buffer to vic registers $d01c-$d024
        sta     (const_d01c),y
        dey
        bpl     vicloop
        tay                             ; loop leaves $19 in accumulator; transfer to Y
 loop
        lax     sid-1,y                 ; copy stuff from shadow buffer to sid registers $d400-$d418
        sta     (const_d3ff),y
        dey
        bne     loop
        jmp     $ea7e                   ; jump into to last part of standard interrupt handler

 ; initialization

        sei
        stx     $286                    ; set foreground text color to black
        stx     $d021                   ; set background 0 color to black
        jsr     $e544                   ; kernal clear screen routine

        ldx     #$fd                    ; copy program from $0802-$08FF to zero page ($02-$FF)
 initloop
        lda     $802,x
        sta     $02,x
        dex
        bne     initloop

        stx     $315                    ; clear high byte of interrupt vector; int handler now = $31
        jmp     $cc                     ; jump to main routine in zero page

 ; main routine

        lda     #$50                    ; enable extended color text mode; set YSCROLL to 0
        sta     $d011
        cli                             ; enable interrupts

 mainloop
        lda     $dc04                   ; get timer A low byte
 ;mod_op1
        ldy     #$c3                    ; forces background color 2 or 3 
 ;mod_op2
        ora     $d41c                   ; manipulate font using opcode set by poke table
        pha                             ; push it on the stack, where the vic is looking for font data
        asr     #$04
        ldy     #$30                    ; tell vic to find video matrix at $0c00, font at $0000.
        sty     $d018
        adc     (vmptr),y               ; read and combine two consecutive values (horizontal neighbors)
        inc     vmptr
        adc     (vmptr),y
        ror
        ora     clock_msb
        ldy     #$30+40                 ; write back 40 bytes later
        ora     mod_op1                 ; only use characters from stack page
        sta     (vmptr),y
        bne     mainloop                ; Always branches.

 ;basstbl
        .byte $2B,$AA,$02,$62           ; bass notes
 ;freqtbl
        .byte $00,$18,$26,$20           ; melody notes
        .byte $12,$24,$13,$10
	; A Mind is Born by Linus Akesson
	; https://linusakesson.net/scene/a-mind-is-born/index.php
	; transcribed to 64tass and further commented by J.B. Langston

	; important locations after program is copied to zero page

	vmptr = $cb ; video matrix
	clock = $13 ; global clock lsb - indicates position within bar
	clock_msb = $20 ; global clock msb - indicates bar of song
	script = $21 ; poke table
	const_d3ff = $03 ; location holding constant $d3ff
	const_d01c = $0f ; location holding constant $d01c
	mel_lfsr = $14 ; melody LFSR
	sid = $0a ; sid register shadow buffer
	mod_op1 = $d5 ; first opcode that gets modified
	mod_op2 = $d7 ; second opcode that gets modified
	basstbl = $f3 ; bass notes
	freqtbl = $f7 ; melody notes

	* = $0801

	; basic bootstrap program

	.byte $0D,$08 ; pointer to next line
	.byte $FF,$D3 ; line number 54271
	.byte $9E,$32,$32,$32,$35 ; $9E=SYS + 2225 in PETSCII

	; end of line/end of program markers overlap with first 3 bytes of sid table

	; sid register shadow buffer; copied to sid registers at the end of every interrupt cycle
	; voice 1: kick drum and bass

	.byte $00 ; $D400: voice 1 frequency lsb
	.byte $00 ; $D401: voice 1 frequency msb
	.byte $00 ; $D402: voice 1 pulse width lsb
	.byte $19 ; $D403: voice 1 pulse width msb
	.byte $41 ; $D404: voice 1 control register
	.byte $1C ; $D405: voice 1 attack/decay
	.byte $D0 ; $D406: voice 1 sustain/release

	; voice 2: melody

	.byte $00 ; $D407: voice 2 frequency lsb $D020: border color
	.byte $DC ; $D408: voice 2 frequency msb $D021: background color 0
	.byte $00 ; $D409: voice 2 pulse width lsb $D022: background color 1
	.byte $00 ; $D40A: voice 2 pulse width msb $D023: background color 2
	.byte $11 ; $D40B: voice 2 control register $D024: background color 3
	.byte $D0 ; $D40C: voice 2 attack/decay
	.byte $E0 ; $D40D: voice 2 sustain/release

	; voice 3: drone

	.byte $0B ; $D40E: voice 3 frequency lsb
	.byte $10 ; $D40F: voice 3 frequency msb
	.byte $33 ; $D410: voice 3 pulse width lsb
	.byte $0E ; $D411: voice 3 pulse width msb
	.byte $61 ; $D412: voice 3 control register
	.byte $90 ; $D413: voice 3 attack/decay
	.byte $F5 ; $D414: voice 3 sustain/release

	; filter

	.byte $07 ; $D415: filter cutoff lsb
	.byte $00 ; $D417: filter cutoff msb
	.byte $FF ; $D418: filter resonance
	.byte $1F ; $D419: filter control; $1F = low pass filter all channels and max volume

	; poke table with eight entries: first byte is target address in zero-page, second byte is value to write.
	; entry for bars $00-$07 overlaps with last two sid registers above; performs dummy write to $ff

	.byte $14,$41 ; bars $08-$0f: repeatedly reset LFSR seed to $41 to stutter melody
	.byte $D5,$24 ; bars $10-$17: overwrite ora opcode with bit to enable color and stop resetting LFSR
	.byte $15,$25 ; bars $18-$1f: change color to green; voice 1 waveform $25 (sounds same as $21)
	.byte $15,$53 ; bars $20-$27: change color to cyan; voice 1 waveform to $53 (hardsync)
	.byte $15,$61 ; bars $28-$2f: change color to white; voice 1 to $61 (disable hardsync)
	.byte $D5,$29 ; bars $30-$37: overwrite bit opcode with and to make visuals brighter
	.byte $1B,$0F ; bars $38-$3f: change voice 3 pulse width to $f for brighter timbre

	; interrupt handler

	inc clock ; increment global clock lsb by 2
	inc clock
	bne noc1 ; skip msb unless lsb wrapped to 0
	inc clock_msb ; increment global clock msb by 1
	noc1
	lda #$61 ; set gate bit for drone
	sta sid+2*7+4 ; via control reg for voice 3
	lax clock_msb ; load current bar # into X and A
	cpx #$3f ; switch to high pass filter for final bar
	beq highpass
	bcc noend ; skip finale until after final bar
	lsr $d011 ; fade to black
	jmp ($fffc) ; kernal reset vector
	highpass
	ldy #$6d
	sty sid+$18 ; switch for high pass filter
	sty mod_op2 ; change ora to adc (stop blinking)
	noend
	lsr ; calculate address into poke table:
	asr #$1c ; ((bar >> 1) and $1c) >> 1
	tay ; save result fo later

	; beat generation

	lda clock ; check what part of current bar we're in
	and #$30 ; only duck during third quarter of beat
	bne noduck
	dec sid+2*7+4 ; turn off gate bit to start release of note`
	noduck
	cpx #$2f ; turn bass off during bar 2f
	beq bassoff
	bcs nointro ; During bars $00-$2e we keep playing the same bass
	ldx #2 ; note, from offset 2 in the bass table.
	nointro
	cmp #$10 ; turn bass off after first quarter of beat
	beq bassoff
	txa ; x contains either 2 or current bar count
	and #3 ; mask off last two bits of bar
	tax
	lda basstbl,x ; use as index into bass note table
	sta sid+0*7+0 ; poke into sid voice 1 frequency msb

	.byte $2d ; and absolute will eat the next instr and its operand
	bassoff
	lax #0 ; Safe when the operand is zero.
	bcs bassdone ; Carry will be set if we got here via bassoff.

	; Carry was not set by cmp #$10, so we are in the
	; first 25% of a beat. Throw away the computed bass note
	; and play a drum instead.

	; But handle the script first.

	lax script+1,y ; load poke byte into A+X
	ldx script,y ; load poke address into X
	sta 0,x ; poke accumulator into address from script

	lda clock ; get clock
	asr #$0e ; mask 00001110, then shift right
	tax ; A goes from 0 to 7 during the first 25% of the beat
	sbx #256-8 ; update the video matrix pointer
	stx vmptr+1
	eor #$07 ; invert the value to obtain the pitch of the drum sound
	bassdone
	sta sid+0*7+1 ; store in voice 1 freq msb

	; melody generation

	lda clock ; 16th notes last for for 8 interrupts
	and #$0f
	bne nomel

	lda #$b8 ; load lfsr seed
	sre mel_lfsr ; shift right and exclusive or
	bcc noc2 ; only write it back if a bit shifted out

	sta mel_lfsr
	noc2
	and #7 ; use bottom 3 bits of lfsr as index into note table
	tax
	lda freqtbl,x ; look up note from note table
	sta sid+1*7+1 ; store in voice 2 freq msb
	nomel
	ldy #8
	vicloop
	lax sid+3,y ; copy stuff from shadow buffer to vic registers $d01c-$d024
	sta (const_d01c),y
	dey
	bpl vicloop
	tay ; loop leaves $19 in accumulator; transfer to Y
	loop
	lax sid-1,y ; copy stuff from shadow buffer to sid registers $d400-$d418
	sta (const_d3ff),y
	dey
	bne loop
	jmp $ea7e ; jump into to last part of standard interrupt handler

	; initialization

	sei
	stx $286 ; set foreground text color to black
	stx $d021 ; set background 0 color to black
	jsr $e544 ; kernal clear screen routine

	ldx #$fd ; copy program from $0802-$08FF to zero page ($02-$FF)
	initloop
	lda $802,x
	sta $02,x
	dex
	bne initloop

	stx $315 ; clear high byte of interrupt vector; int handler now = $31
	jmp $cc ; jump to main routine in zero page

	; main routine

	lda #$50 ; enable extended color text mode; set YSCROLL to 0
	sta $d011
	cli ; enable interrupts

	mainloop
	lda $dc04 ; get timer A low byte
	;mod_op1
	ldy #$c3 ; forces background color 2 or 3
	;mod_op2
	ora $d41c ; manipulate font using opcode set by poke table
	pha ; push it on the stack, where the vic is looking for font data
	asr #$04
	ldy #$30 ; tell vic to find video matrix at $0c00, font at $0000.
	sty $d018
	adc (vmptr),y ; read and combine two consecutive values (horizontal neighbors)
	inc vmptr
	adc (vmptr),y
	ror
	ora clock_msb
	ldy #$30+40 ; write back 40 bytes later
	ora mod_op1 ; only use characters from stack page
	sta (vmptr),y
	bne mainloop ; Always branches.

	;basstbl
	.byte $2B,$AA,$02,$62 ; bass notes
	;freqtbl
	.byte $00,$18,$26,$20 ; melody notes
	.byte $12,$24,$13,$10