Game 1 Unit 1 of 64 1 hr learning time
Hello SID
Your first contact with the SID chip. See the game board, press keys, hear the C64 sing.
2% of SID Symphony
Three tracks. Three keys. Three voices.
This is your first contact with the SID chip — the legendary sound synthesiser that made the Commodore 64 the most musical computer of its era. By the end of this unit, you’ll have a working program that responds to your keypresses with distinct sounds and visual feedback.
Run It
acme -f cbm -o symphony.prg symphony.asmLoad the resulting symphony.prg in VICE (x64sc) or on real hardware. You’ll see three horizontal tracks with a hit zone on the left. Press Z, X, or C to trigger sounds and watch the tracks flash.
What You’re Building
The scaffold provides everything needed for interactive sound:
- Three horizontal tracks displayed on screen
- Hit zones marked in yellow
- SID configured with three distinct voices
- Keyboard reading for Z/X/C keys
- Visual feedback when you press a key
Press Z — voice 1 plays a high sawtooth note, track 1 flashes red. Press X — voice 2 plays a mid-range pulse note, track 2 flashes green. Press C — voice 3 plays a low triangle note, track 3 flashes blue.
Memory Map
The C64 uses memory-mapped I/O — hardware is controlled by writing to specific memory addresses:
; ----------------------------------------------------------------------------
; Memory Addresses
; ----------------------------------------------------------------------------
SCREEN = $0400 ; Screen memory base
COLRAM = $D800 ; Colour RAM base
BORDER = $D020 ; Border colour
BGCOL = $D021 ; Background colour
; SID registers
SID = $D400 ; SID base address
SID_V1_FREQ_LO = $D400 ; Voice 1 frequency low
SID_V1_FREQ_HI = $D401 ; Voice 1 frequency high
SID_V1_CTRL = $D404 ; Voice 1 control register
SID_V1_AD = $D405 ; Voice 1 attack/decay
SID_V1_SR = $D406 ; Voice 1 sustain/release
SID_V2_FREQ_LO = $D407 ; Voice 2 frequency low
SID_V2_FREQ_HI = $D408 ; Voice 2 frequency high
SID_V2_CTRL = $D40B ; Voice 2 control register
SID_V2_AD = $D40C ; Voice 2 attack/decay
SID_V2_SR = $D40D ; Voice 2 sustain/release
SID_V3_FREQ_LO = $D40E ; Voice 3 frequency low
SID_V3_FREQ_HI = $D40F ; Voice 3 frequency high
SID_V3_CTRL = $D412 ; Voice 3 control register
SID_V3_AD = $D413 ; Voice 3 attack/decay
SID_V3_SR = $D414 ; Voice 3 sustain/release
SID_VOLUME = $D418 ; Volume and filter mode
; CIA keyboard
CIA1_PRA = $DC00 ; CIA1 Port A (keyboard column)
CIA1_PRB = $DC01 ; CIA1 Port B (keyboard row)
Key locations:
- $0400 — Screen memory (40×25 characters)
- $D800 — Colour RAM (one byte per character)
- $D400 — SID chip base address
- $DC00 — CIA keyboard interface
Initialising the SID
Before making sound, the SID needs configuration:
; ----------------------------------------------------------------------------
; Initialize SID
; ----------------------------------------------------------------------------
; Sets up SID with three distinct voices ready to play
init_sid:
; Clear all SID registers first
ldx #$18
lda #0
- sta SID,x
dex
bpl -
; Set volume to maximum
lda #$0F
sta SID_VOLUME
; Voice 1 - High pitch, sawtooth wave
lda #$00
sta SID_V1_FREQ_LO
lda #$1C ; High frequency (~523 Hz, C5)
sta SID_V1_FREQ_HI
lda #$09 ; Attack=0, Decay=9
sta SID_V1_AD
lda #$00 ; Sustain=0, Release=0
sta SID_V1_SR
; Voice 2 - Mid pitch, pulse wave
lda #$00
sta SID_V2_FREQ_LO
lda #$0E ; Mid frequency (~262 Hz, C4)
sta SID_V2_FREQ_HI
lda #$08 ; 50% pulse width
sta SID_V2_PWHI
lda #$09 ; Attack=0, Decay=9
sta SID_V2_AD
lda #$00 ; Sustain=0, Release=0
sta SID_V2_SR
; Voice 3 - Low pitch, triangle wave
lda #$00
sta SID_V3_FREQ_LO
lda #$07 ; Low frequency (~131 Hz, C3)
sta SID_V3_FREQ_HI
lda #$09 ; Attack=0, Decay=9
sta SID_V3_AD
lda #$00 ; Sustain=0, Release=0
sta SID_V3_SR
rts
Each voice needs:
- Frequency — Pitch of the note (high byte controls most of it)
- ADSR — Attack/Decay/Sustain/Release envelope shaping
- Waveform — Written to the control register to start sound
The three voices use different waveforms:
- Voice 1: Sawtooth ($21) — Bright, buzzy tone
- Voice 2: Pulse ($41) — Hollow, reedy tone
- Voice 3: Triangle ($11) — Pure, flute-like tone
Reading the Keyboard
The C64 keyboard uses a matrix scanned through the CIA chip:
; ----------------------------------------------------------------------------
; Check Keys
; ----------------------------------------------------------------------------
; Reads keyboard and triggers sounds for Z, X, C keys
check_keys:
; Check Z key (row 1, column 2)
; Z is at keyboard matrix position: row=$FD (bit 1 low), col=$10 (bit 4)
lda #$FD ; Select row 1 (bit 1 = 0)
sta CIA1_PRA
lda CIA1_PRB
and #$10 ; Check column 4 (Z key)
bne + ; Branch if not pressed
jsr play_voice1
jsr flash_track1
+
; Check X key (row 2, column 7)
; X is at keyboard matrix position: row=$FB (bit 2 low), col=$80 (bit 7)
lda #$FB ; Select row 2 (bit 2 = 0)
sta CIA1_PRA
lda CIA1_PRB
and #$80 ; Check column 7 (X key)
bne + ; Branch if not pressed
jsr play_voice2
jsr flash_track2
+
; Check C key (row 2, column 4)
; C is at keyboard matrix position: row=$FB (bit 2 low), col=$10 (bit 4)
lda #$FB ; Select row 2 (bit 2 = 0)
sta CIA1_PRA
lda CIA1_PRB
and #$10 ; Check column 4 (C key)
bne + ; Branch if not pressed
jsr play_voice3
jsr flash_track3
+
; Reset keyboard scanning
lda #$FF
sta CIA1_PRA
rts
The keyboard matrix has 8 rows and 8 columns:
- Write to CIA1_PRA to select a row (set one bit low)
- Read CIA1_PRB to see which columns are pressed
- If a bit is low, that key is pressed
Triggering Sounds
Playing a SID note is remarkably simple:
; ----------------------------------------------------------------------------
; Play Voice 1
; ----------------------------------------------------------------------------
; Triggers voice 1 (high, sawtooth)
play_voice1:
lda #$21 ; Gate on + sawtooth waveform
sta SID_V1_CTRL
rts
; ----------------------------------------------------------------------------
; Play Voice 2
; ----------------------------------------------------------------------------
; Triggers voice 2 (mid, pulse)
play_voice2:
lda #$41 ; Gate on + pulse waveform
sta SID_V2_CTRL
rts
; ----------------------------------------------------------------------------
; Play Voice 3
; ----------------------------------------------------------------------------
; Triggers voice 3 (low, triangle)
play_voice3:
lda #$11 ; Gate on + triangle waveform
sta SID_V3_CTRL
rts
The control register combines waveform selection with the gate bit:
- Bit 0 (Gate) — Must be high to start the note
- Bits 4-7 — Select waveform (triangle=$10, sawtooth=$20, pulse=$40, noise=$80)
Setting the gate triggers the Attack phase. The ADSR envelope shapes the volume over time.
The Complete Code
; ============================================================================
; SID SYMPHONY - Unit 1: Hello SID
; ============================================================================
; Your first contact with the SID chip. Three tracks, three keys, three voices.
; Press Z/X/C to trigger sounds and see the tracks flash.
;
; Controls: Z = Track 1 (high), X = Track 2 (mid), C = Track 3 (low)
; ============================================================================
; ----------------------------------------------------------------------------
; Memory Addresses
; ----------------------------------------------------------------------------
SCREEN = $0400 ; Screen memory base
COLRAM = $D800 ; Colour RAM base
BORDER = $D020 ; Border colour
BGCOL = $D021 ; Background colour
; SID registers
SID = $D400 ; SID base address
SID_V1_FREQ_LO = $D400 ; Voice 1 frequency low
SID_V1_FREQ_HI = $D401 ; Voice 1 frequency high
SID_V1_PWLO = $D402 ; Voice 1 pulse width low
SID_V1_PWHI = $D403 ; Voice 1 pulse width high
SID_V1_CTRL = $D404 ; Voice 1 control register
SID_V1_AD = $D405 ; Voice 1 attack/decay
SID_V1_SR = $D406 ; Voice 1 sustain/release
SID_V2_FREQ_LO = $D407 ; Voice 2 frequency low
SID_V2_FREQ_HI = $D408 ; Voice 2 frequency high
SID_V2_PWLO = $D409 ; Voice 2 pulse width low
SID_V2_PWHI = $D40A ; Voice 2 pulse width high
SID_V2_CTRL = $D40B ; Voice 2 control register
SID_V2_AD = $D40C ; Voice 2 attack/decay
SID_V2_SR = $D40D ; Voice 2 sustain/release
SID_V3_FREQ_LO = $D40E ; Voice 3 frequency low
SID_V3_FREQ_HI = $D40F ; Voice 3 frequency high
SID_V3_PWLO = $D410 ; Voice 3 pulse width low
SID_V3_PWHI = $D411 ; Voice 3 pulse width high
SID_V3_CTRL = $D412 ; Voice 3 control register
SID_V3_AD = $D413 ; Voice 3 attack/decay
SID_V3_SR = $D414 ; Voice 3 sustain/release
SID_FLTLO = $D415 ; Filter cutoff low
SID_FLTHI = $D416 ; Filter cutoff high
SID_FLTCTRL = $D417 ; Filter control
SID_VOLUME = $D418 ; Volume and filter mode
; CIA keyboard
CIA1_PRA = $DC00 ; CIA1 Port A (keyboard column)
CIA1_PRB = $DC01 ; CIA1 Port B (keyboard row)
; Colours
BLACK = 0
WHITE = 1
RED = 2
CYAN = 3
PURPLE = 4
GREEN = 5
BLUE = 6
YELLOW = 7
ORANGE = 8
BROWN = 9
LIGHT_RED = 10
DARK_GREY = 11
GREY = 12
LIGHT_GREEN = 13
LIGHT_BLUE = 14
LIGHT_GREY = 15
; Track positions (row on screen)
TRACK1_ROW = 8 ; High voice track
TRACK2_ROW = 12 ; Mid voice track
TRACK3_ROW = 16 ; Low voice track
; Hit zone column
HIT_ZONE_COL = 3 ; Where notes need to be hit
; ----------------------------------------------------------------------------
; BASIC Stub - SYS 2064
; ----------------------------------------------------------------------------
* = $0801
!byte $0C, $08 ; Pointer to next line
!byte $0A, $00 ; Line number 10
!byte $9E ; SYS token
!text "2064" ; Address
!byte $00 ; End of line
!byte $00, $00 ; End of program
; ----------------------------------------------------------------------------
; Main Program
; ----------------------------------------------------------------------------
* = $0810
start:
jsr init_screen ; Set up the display
jsr init_sid ; Configure SID chip
main_loop:
; Wait for raster (smooth timing)
lda #$FF
- cmp $D012
bne -
; Reset track colours to default
jsr reset_track_colours
; Check keyboard and play sounds
jsr check_keys
jmp main_loop
; ----------------------------------------------------------------------------
; Initialize Screen
; ----------------------------------------------------------------------------
; Clears screen and draws the three tracks with hit zones
init_screen:
; Set border and background
lda #BLACK
sta BORDER
sta BGCOL
; Clear screen with spaces
ldx #0
lda #$20 ; Space character
- sta SCREEN,x
sta SCREEN+$100,x
sta SCREEN+$200,x
sta SCREEN+$2E8,x
inx
bne -
; Set all colours to grey
ldx #0
lda #GREY
- sta COLRAM,x
sta COLRAM+$100,x
sta COLRAM+$200,x
sta COLRAM+$2E8,x
inx
bne -
; Draw track lines
jsr draw_tracks
; Draw hit zones
jsr draw_hit_zones
; Draw labels
jsr draw_labels
rts
; ----------------------------------------------------------------------------
; Draw Tracks
; ----------------------------------------------------------------------------
; Draws horizontal lines for each track using minus characters
draw_tracks:
; Track 1 (row 8)
ldx #0
lda #$2D ; Minus character for track line
- sta SCREEN + (TRACK1_ROW * 40),x
inx
cpx #38
bne -
; Track 2 (row 12)
ldx #0
- sta SCREEN + (TRACK2_ROW * 40),x
inx
cpx #38
bne -
; Track 3 (row 16)
ldx #0
- sta SCREEN + (TRACK3_ROW * 40),x
inx
cpx #38
bne -
rts
; ----------------------------------------------------------------------------
; Draw Hit Zones
; ----------------------------------------------------------------------------
; Draws vertical bars at the hit zone position
draw_hit_zones:
; Draw vertical line at hit zone column
; Using pipe character for hit zone marker
lda #$7D ; Pipe character
; Hit zone spans from track 1 to track 3
sta SCREEN + (TRACK1_ROW * 40) + HIT_ZONE_COL
sta SCREEN + ((TRACK1_ROW-1) * 40) + HIT_ZONE_COL
sta SCREEN + ((TRACK1_ROW+1) * 40) + HIT_ZONE_COL
sta SCREEN + (TRACK2_ROW * 40) + HIT_ZONE_COL
sta SCREEN + ((TRACK2_ROW-1) * 40) + HIT_ZONE_COL
sta SCREEN + ((TRACK2_ROW+1) * 40) + HIT_ZONE_COL
sta SCREEN + (TRACK3_ROW * 40) + HIT_ZONE_COL
sta SCREEN + ((TRACK3_ROW-1) * 40) + HIT_ZONE_COL
sta SCREEN + ((TRACK3_ROW+1) * 40) + HIT_ZONE_COL
; Colour the hit zones yellow
lda #YELLOW
sta COLRAM + (TRACK1_ROW * 40) + HIT_ZONE_COL
sta COLRAM + ((TRACK1_ROW-1) * 40) + HIT_ZONE_COL
sta COLRAM + ((TRACK1_ROW+1) * 40) + HIT_ZONE_COL
sta COLRAM + (TRACK2_ROW * 40) + HIT_ZONE_COL
sta COLRAM + ((TRACK2_ROW-1) * 40) + HIT_ZONE_COL
sta COLRAM + ((TRACK2_ROW+1) * 40) + HIT_ZONE_COL
sta COLRAM + (TRACK3_ROW * 40) + HIT_ZONE_COL
sta COLRAM + ((TRACK3_ROW-1) * 40) + HIT_ZONE_COL
sta COLRAM + ((TRACK3_ROW+1) * 40) + HIT_ZONE_COL
rts
; ----------------------------------------------------------------------------
; Draw Labels
; ----------------------------------------------------------------------------
; Draws track labels and instructions
draw_labels:
; Title "SID SYMPHONY" at top
ldx #0
- lda title_text,x
beq +
sta SCREEN + 13,x
lda #WHITE
sta COLRAM + 13,x
inx
bne -
+
; Track labels
; "Z" for track 1
lda #$1A ; Z
sta SCREEN + (TRACK1_ROW * 40)
lda #LIGHT_RED
sta COLRAM + (TRACK1_ROW * 40)
; "X" for track 2
lda #$18 ; X
sta SCREEN + (TRACK2_ROW * 40)
lda #LIGHT_GREEN
sta COLRAM + (TRACK2_ROW * 40)
; "C" for track 3
lda #$03 ; C
sta SCREEN + (TRACK3_ROW * 40)
lda #LIGHT_BLUE
sta COLRAM + (TRACK3_ROW * 40)
; Instructions at bottom
ldx #0
- lda instr_text,x
beq +
sta SCREEN + (23 * 40) + 5,x
lda #GREY
sta COLRAM + (23 * 40) + 5,x
inx
bne -
+
rts
title_text:
!scr "sid symphony"
!byte 0
instr_text:
!scr "press z, x, c to play"
!byte 0
; ----------------------------------------------------------------------------
; Initialize SID
; ----------------------------------------------------------------------------
; Sets up SID with three distinct voices ready to play
init_sid:
; Clear all SID registers first
ldx #$18
lda #0
- sta SID,x
dex
bpl -
; Set volume to maximum
lda #$0F
sta SID_VOLUME
; Voice 1 - High pitch, sawtooth wave
lda #$00
sta SID_V1_FREQ_LO
lda #$1C ; High frequency (~523 Hz, C5)
sta SID_V1_FREQ_HI
lda #$09 ; Attack=0, Decay=9
sta SID_V1_AD
lda #$00 ; Sustain=0, Release=0
sta SID_V1_SR
; Voice 2 - Mid pitch, pulse wave
lda #$00
sta SID_V2_FREQ_LO
lda #$0E ; Mid frequency (~262 Hz, C4)
sta SID_V2_FREQ_HI
lda #$08 ; 50% pulse width
sta SID_V2_PWHI
lda #$09 ; Attack=0, Decay=9
sta SID_V2_AD
lda #$00 ; Sustain=0, Release=0
sta SID_V2_SR
; Voice 3 - Low pitch, triangle wave
lda #$00
sta SID_V3_FREQ_LO
lda #$07 ; Low frequency (~131 Hz, C3)
sta SID_V3_FREQ_HI
lda #$09 ; Attack=0, Decay=9
sta SID_V3_AD
lda #$00 ; Sustain=0, Release=0
sta SID_V3_SR
rts
; ----------------------------------------------------------------------------
; Reset Track Colours
; ----------------------------------------------------------------------------
; Returns tracks to their default colours
reset_track_colours:
; Track 1 - default grey
ldx #0
lda #GREY
- sta COLRAM + (TRACK1_ROW * 40),x
inx
cpx #38
bne -
; Track 2 - default grey
ldx #0
- sta COLRAM + (TRACK2_ROW * 40),x
inx
cpx #38
bne -
; Track 3 - default grey
ldx #0
- sta COLRAM + (TRACK3_ROW * 40),x
inx
cpx #38
bne -
; Restore key labels
lda #LIGHT_RED
sta COLRAM + (TRACK1_ROW * 40)
lda #LIGHT_GREEN
sta COLRAM + (TRACK2_ROW * 40)
lda #LIGHT_BLUE
sta COLRAM + (TRACK3_ROW * 40)
; Restore hit zone colours
lda #YELLOW
sta COLRAM + (TRACK1_ROW * 40) + HIT_ZONE_COL
sta COLRAM + (TRACK2_ROW * 40) + HIT_ZONE_COL
sta COLRAM + (TRACK3_ROW * 40) + HIT_ZONE_COL
rts
; ----------------------------------------------------------------------------
; Check Keys
; ----------------------------------------------------------------------------
; Reads keyboard and triggers sounds for Z, X, C keys
check_keys:
; Check Z key (row 1, column 2)
; Z is at keyboard matrix position: row=$FD (bit 1 low), col=$10 (bit 4)
lda #$FD ; Select row 1 (bit 1 = 0)
sta CIA1_PRA
lda CIA1_PRB
and #$10 ; Check column 4 (Z key)
bne + ; Branch if not pressed
jsr play_voice1
jsr flash_track1
+
; Check X key (row 2, column 7)
; X is at keyboard matrix position: row=$FB (bit 2 low), col=$80 (bit 7)
lda #$FB ; Select row 2 (bit 2 = 0)
sta CIA1_PRA
lda CIA1_PRB
and #$80 ; Check column 7 (X key)
bne + ; Branch if not pressed
jsr play_voice2
jsr flash_track2
+
; Check C key (row 2, column 4)
; C is at keyboard matrix position: row=$FB (bit 2 low), col=$10 (bit 4)
lda #$FB ; Select row 2 (bit 2 = 0)
sta CIA1_PRA
lda CIA1_PRB
and #$10 ; Check column 4 (C key)
bne + ; Branch if not pressed
jsr play_voice3
jsr flash_track3
+
; Reset keyboard scanning
lda #$FF
sta CIA1_PRA
rts
; ----------------------------------------------------------------------------
; Play Voice 1
; ----------------------------------------------------------------------------
; Triggers voice 1 (high, sawtooth)
play_voice1:
lda #$21 ; Gate on + sawtooth waveform
sta SID_V1_CTRL
rts
; ----------------------------------------------------------------------------
; Play Voice 2
; ----------------------------------------------------------------------------
; Triggers voice 2 (mid, pulse)
play_voice2:
lda #$41 ; Gate on + pulse waveform
sta SID_V2_CTRL
rts
; ----------------------------------------------------------------------------
; Play Voice 3
; ----------------------------------------------------------------------------
; Triggers voice 3 (low, triangle)
play_voice3:
lda #$11 ; Gate on + triangle waveform
sta SID_V3_CTRL
rts
; ----------------------------------------------------------------------------
; Flash Track 1
; ----------------------------------------------------------------------------
; Highlights track 1 in red
flash_track1:
ldx #0
lda #RED
- sta COLRAM + (TRACK1_ROW * 40),x
inx
cpx #38
bne -
; Keep label visible
lda #WHITE
sta COLRAM + (TRACK1_ROW * 40)
rts
; ----------------------------------------------------------------------------
; Flash Track 2
; ----------------------------------------------------------------------------
; Highlights track 2 in green
flash_track2:
ldx #0
lda #GREEN
- sta COLRAM + (TRACK2_ROW * 40),x
inx
cpx #38
bne -
; Keep label visible
lda #WHITE
sta COLRAM + (TRACK2_ROW * 40)
rts
; ----------------------------------------------------------------------------
; Flash Track 3
; ----------------------------------------------------------------------------
; Highlights track 3 in blue
flash_track3:
ldx #0
lda #BLUE
- sta COLRAM + (TRACK3_ROW * 40),x
inx
cpx #38
bne -
; Keep label visible
lda #WHITE
sta COLRAM + (TRACK3_ROW * 40)
rts
Try This: Change the Pitch
Find the frequency values in init_sid:
lda #$1C ; Voice 1 - high frequency
sta SID_V1_FREQ_HITry changing $1C to:
$38— One octave higher$0E— One octave lower$24— A different note
Experiment with all three voices. Notice how the relationship between frequencies creates harmony or dissonance.
Try This: Change the Waveform
In play_voice1, change the waveform:
lda #$21 ; Try $11 (triangle), $41 (pulse), $81 (noise)
sta SID_V1_CTRLEach waveform has a distinct character:
- Triangle — Pure, soft
- Sawtooth — Bright, brassy
- Pulse — Hollow, variable
- Noise — Harsh, percussive
What You’ve Learnt
- Memory-mapped I/O — Hardware control through memory addresses
- SID basics — Voices, frequencies, waveforms, ADSR
- Keyboard scanning — Matrix reading through CIA
- Visual feedback — Colour RAM for instant response
What’s Next
In Unit 2, notes will scroll across the tracks. You’ll see the rhythm game take shape as notes approach the hit zone, waiting to be played.