Game 1 Unit 7 of 128 1 hr learning time

Explosion

The SID chip fires a noise burst on voice 2 while the enemy flashes white — eight frames of audio-visual feedback before the respawn.

5% of Starfield

Mark as complete Completed!

The enemy doesn’t just vanish — it flashes white and the SID fires a burst of noise. Eight frames of feedback, then the enemy respawns. Every hit feels like it matters.

Two Voices

The SID has three independent voices. Voice 1 handles the laser sound from Unit 4. Voice 2 now handles explosions. Each voice has its own frequency registers, ADSR envelope, and waveform selector — they play simultaneously without interfering.

Voice 2’s registers sit seven bytes above voice 1:

Register	Purpose
`$D407`	Frequency low byte
`$D408`	Frequency high byte
`$D40B`	Control register (waveform + gate)
`$D40C`	Attack / Decay
`$D40D`	Sustain / Release

The control register at $D40B works the same way as voice 1’s $D404. Bit 0 is the gate (on/off). Bits 4–7 select the waveform. For explosions, we want noise — bit 7. So $80 means noise with gate off, and $81 means noise with gate on.

The Hit Response

hit_enemy:
        ; Deactivate bullet
        lda #$00
        sta bullet_active
        lda $d015
        and #%11111101          ; Disable sprite 1 (bullet)
        sta $d015

        ; Flash enemy white
        lda #$01
        sta $d029               ; Sprite 2 colour = white

        ; Start flash timer
        lda #$08
        sta flash_timer         ; 8 frames of flash

        ; Explosion sound — SID voice 2 (noise)
        lda #$00
        sta $d407               ; Frequency low
        lda #$08
        sta $d408               ; Frequency high (low rumble)
        lda #$09
        sta $d40c               ; Attack=0, Decay=9
        lda #$00
        sta $d40d               ; Sustain=0, Release=0
        lda #$80
        sta $d40b               ; Noise, gate OFF (reset envelope)
        lda #$81
        sta $d40b               ; Noise, gate ON (trigger)

Three things happen when a bullet hits the enemy.

Deactivate the bullet. Same as Unit 6 — clear bullet_active and disable sprite 1.

Flash the enemy white. Write $01 (white) to $D029, the sprite 2 colour register. Then set flash_timer to 8 — the enemy will stay white for 8 frames.

Trigger the explosion sound. Voice 2 gets a low frequency ($0800) for a bass rumble. The ADSR has zero attack and a moderate decay ($09) — the sound starts loud and fades quickly. The noise waveform is selected by setting bit 7 of $D40B. We write $80 first (gate off) to reset the envelope, then $81 (gate on) to trigger it — the same off-then-on pattern we used for the laser in Unit 4.

No respawn happens here. The enemy freezes in place during the flash.

Timing the Flash

        ; --- Update enemy ---
        lda flash_timer
        beq enemy_move          ; Not flashing, normal movement

        ; Enemy is frozen (flashing white)
        dec flash_timer
        bne no_respawn          ; Still flashing, skip everything

        ; Flash done — restore colour and respawn
        lda #$05
        sta $d029               ; Back to green
        lda #$32
        sta enemy_y
        sta $d005
        lda $d012
        and #$7f
        clc
        adc #$30
        sta enemy_x
        sta $d004
        jmp no_respawn

enemy_move:
        lda enemy_y
        clc
        adc #$01                ; Move down 1 pixel per frame
        sta enemy_y
        sta $d005               ; Update sprite 2 Y position

        ; Check if enemy has left the screen (Y > 248)
        cmp #$f8
        bcc no_respawn          ; Y < 248, still on screen

        ; Respawn at top with new X position
        lda #$32
        sta enemy_y
        sta $d005               ; Reset Y to top

        lda $d012               ; Pseudo-random from raster position
        and #$7f                ; Range 0-127
        clc
        adc #$30                ; Range 48-175
        sta enemy_x
        sta $d004               ; New X position

no_respawn:
        jmp game_loop

The enemy update section now checks flash_timer before doing anything else. If it’s zero, the enemy moves normally — same code as Unit 6. If it’s non-zero, the enemy is frozen mid-explosion.

Each frame, flash_timer decrements by one. While it’s still positive, we skip straight to the end of the loop — no movement, no off-screen check. The enemy just sits there, white.

When the timer reaches zero, we restore the colour to green ($05) and respawn the enemy at the top with a new random X position. Then normal movement resumes.

This frame counter pattern — set a value, decrement each frame, act when it reaches zero — is one you’ll use constantly. Invulnerability windows, animation timers, delayed events. It’s the simplest form of game timing.

The Complete Code

; Starfield - Unit 7: Explosion
; Assemble with: acme -f cbm -o starfield.prg starfield.asm

; ------------------------------------------------
; Zero-page variables
; ------------------------------------------------
bullet_active = $02     ; 0 = no bullet, 1 = active
bullet_y      = $03     ; Bullet Y position
enemy_x       = $04     ; Enemy X position
enemy_y       = $05     ; Enemy Y position
flash_timer   = $06     ; Explosion flash countdown (0 = no flash)

; ------------------------------------------------
; BASIC stub
; ------------------------------------------------
*= $0801
!byte $0c,$08,$0a,$00,$9e,$32,$30,$36,$31,$00,$00,$00

; ------------------------------------------------
; Initialisation
; ------------------------------------------------
*= $080d
        ; Black screen
        lda #$00
        sta $d020           ; Border colour
        sta $d021           ; Background colour

        ; Clear the screen
        ldx #$00
-       lda #$20
        sta $0400,x
        sta $0500,x
        sta $0600,x
        sta $0700,x
        inx
        bne -

        ; Sprite 0 setup (ship)
        lda #128
        sta $07f8           ; Data pointer (block 128 = $2000)
        lda #172
        sta $d000           ; X position
        lda #220
        sta $d001           ; Y position
        lda #$01
        sta $d027           ; Colour (white)

        ; Sprite 1 setup (bullet)
        lda #129
        sta $07f9           ; Data pointer (block 129 = $2040)
        lda #$07
        sta $d028           ; Colour (yellow)

        ; Sprite 2 setup (enemy)
        lda #130
        sta $07fa           ; Data pointer (block 130 = $2080)
        lda #$05
        sta $d029           ; Colour (green)

        ; Enemy starting position
        lda #100
        sta enemy_x
        sta $d004           ; X position
        lda #$32
        sta enemy_y
        sta $d005           ; Y position

        ; Enable sprites 0 and 2 (bullet starts disabled)
        lda #%00000101
        sta $d015

        ; Bullet starts inactive
        lda #$00
        sta bullet_active

        ; Flash timer starts at zero (no flash)
        lda #$00
        sta flash_timer

        ; SID setup — voice 1 laser sound
        lda #$0f
        sta $d418           ; Volume to maximum

        lda #$00
        sta $d400           ; Frequency low byte
        lda #$10
        sta $d401           ; Frequency high byte ($1000 = mid-high pitch)

        lda #$09
        sta $d405           ; Attack=0, Decay=9
        lda #$00
        sta $d406           ; Sustain=0, Release=0

!ifdef SCREENSHOT_MODE {
        lda $d000
        sta $d002           ; Bullet X = ship X
        lda #140
        sta $d003           ; Bullet Y mid-screen

        ; Show enemy flashing white (explosion in progress)
        lda #$01
        sta $d029           ; White

        lda #%00000111
        sta $d015           ; Enable all three sprites
}

; ------------------------------------------------
; Game loop — runs once per frame
; ------------------------------------------------
game_loop:
        ; Wait for the raster beam to reach line 255
-       lda $d012
        cmp #$ff
        bne -

        ; --- Read joystick and move ship ---

        ; UP (bit 0)
        lda $dc00
        and #%00000001
        bne not_up
        dec $d001
        dec $d001
not_up:

        ; DOWN (bit 1)
        lda $dc00
        and #%00000010
        bne not_down
        inc $d001
        inc $d001
not_down:

        ; LEFT (bit 2)
        lda $dc00
        and #%00000100
        bne not_left
        dec $d000
        dec $d000
not_left:

        ; RIGHT (bit 3)
        lda $dc00
        and #%00001000
        bne not_right
        inc $d000
        inc $d000
not_right:

        ; --- Fire button (bit 4) ---
        lda $dc00
        and #%00010000
        bne no_fire         ; Bit is 1 = NOT pressed

        ; Fire is pressed — only spawn if no bullet active
        lda bullet_active
        bne no_fire         ; Already active, skip

        ; Spawn bullet at ship position
        lda $d000           ; Ship X → bullet X
        sta $d002
        lda $d001           ; Ship Y → bullet Y
        sta bullet_y

        ; Enable sprite 1 (keep other sprites enabled)
        lda $d015
        ora #%00000010
        sta $d015

        ; Mark bullet active
        lda #$01
        sta bullet_active

        ; Trigger laser sound (gate off then on to retrigger)
        lda #$20
        sta $d404           ; Sawtooth, gate OFF (reset envelope)
        lda #$21
        sta $d404           ; Sawtooth, gate ON (start sound)

no_fire:

        ; --- Update bullet ---
        lda bullet_active
        beq no_bullet       ; Not active, skip

        ; Move bullet up (4 pixels per frame)
        lda bullet_y
        sec
        sbc #$04
        sta bullet_y
        sta $d003           ; Update sprite 1 Y position

        ; Check if bullet has left the screen (Y < 30)
        cmp #$1e
        bcs no_bullet       ; Y >= 30, still on screen

        ; Deactivate bullet
        lda #$00
        sta bullet_active

        ; Disable sprite 1 (keep other sprites enabled)
        lda $d015
        and #%11111101
        sta $d015

no_bullet:

        ; --- Check bullet-enemy collision ---
        lda bullet_active
        beq no_hit              ; No bullet, skip

        ; Check Y distance
        lda bullet_y
        sec
        sbc enemy_y
        cmp #$10                ; Within 16 pixels? (positive direction)
        bcc y_close
        cmp #$f0                ; Within 16 pixels? (negative/wrapped)
        bcc no_hit              ; Too far apart
y_close:
        ; Check X distance
        lda $d002               ; Bullet X
        sec
        sbc enemy_x
        cmp #$10                ; Within 16 pixels? (positive direction)
        bcc hit_enemy
        cmp #$f0                ; Within 16 pixels? (negative/wrapped)
        bcc no_hit              ; Too far apart

hit_enemy:
        ; Deactivate bullet
        lda #$00
        sta bullet_active
        lda $d015
        and #%11111101          ; Disable sprite 1 (bullet)
        sta $d015

        ; Flash enemy white
        lda #$01
        sta $d029               ; Sprite 2 colour = white

        ; Start flash timer
        lda #$08
        sta flash_timer         ; 8 frames of flash

        ; Explosion sound — SID voice 2 (noise)
        lda #$00
        sta $d407               ; Frequency low
        lda #$08
        sta $d408               ; Frequency high (low rumble)
        lda #$09
        sta $d40c               ; Attack=0, Decay=9
        lda #$00
        sta $d40d               ; Sustain=0, Release=0
        lda #$80
        sta $d40b               ; Noise, gate OFF (reset envelope)
        lda #$81
        sta $d40b               ; Noise, gate ON (trigger)

no_hit:

        ; --- Update enemy ---
        lda flash_timer
        beq enemy_move          ; Not flashing, normal movement

        ; Enemy is frozen (flashing white)
        dec flash_timer
        bne no_respawn          ; Still flashing, skip everything

        ; Flash done — restore colour and respawn
        lda #$05
        sta $d029               ; Back to green
        lda #$32
        sta enemy_y
        sta $d005
        lda $d012
        and #$7f
        clc
        adc #$30
        sta enemy_x
        sta $d004
        jmp no_respawn

enemy_move:
        lda enemy_y
        clc
        adc #$01                ; Move down 1 pixel per frame
        sta enemy_y
        sta $d005               ; Update sprite 2 Y position

        ; Check if enemy has left the screen (Y > 248)
        cmp #$f8
        bcc no_respawn          ; Y < 248, still on screen

        ; Respawn at top with new X position
        lda #$32
        sta enemy_y
        sta $d005               ; Reset Y to top

        lda $d012               ; Pseudo-random from raster position
        and #$7f                ; Range 0-127
        clc
        adc #$30                ; Range 48-175
        sta enemy_x
        sta $d004               ; New X position

no_respawn:
        jmp game_loop

; ------------------------------------------------
; Sprite data at $2000 (block 128) — ship
; ------------------------------------------------
*= $2000
        !byte $00,$18,$00   ;        ##
        !byte $00,$3c,$00   ;       ####
        !byte $00,$3c,$00   ;       ####
        !byte $00,$7e,$00   ;      ######
        !byte $00,$7e,$00   ;      ######
        !byte $00,$ff,$00   ;     ########
        !byte $00,$ff,$00   ;     ########
        !byte $01,$ff,$80   ;    ##########
        !byte $03,$ff,$c0   ;   ############
        !byte $07,$ff,$e0   ;  ##############
        !byte $07,$ff,$e0   ;  ##############
        !byte $07,$e7,$e0   ;  ###..####..###
        !byte $03,$c3,$c0   ;   ##....##....##
        !byte $01,$ff,$80   ;    ##########
        !byte $00,$ff,$00   ;     ########
        !byte $00,$ff,$00   ;     ########
        !byte $00,$db,$00   ;     ##.##.##
        !byte $00,$db,$00   ;     ##.##.##
        !byte $00,$66,$00   ;      ##..##
        !byte $00,$24,$00   ;       #..#
        !byte $00,$00,$00   ;

; ------------------------------------------------
; Sprite data at $2040 (block 129) — bullet
; ------------------------------------------------
*= $2040
        !byte $00,$00,$00   ;
        !byte $00,$00,$00   ;
        !byte $00,$00,$00   ;
        !byte $00,$00,$00   ;
        !byte $00,$00,$00   ;
        !byte $00,$00,$00   ;
        !byte $00,$00,$00   ;
        !byte $00,$18,$00   ;        ##
        !byte $00,$18,$00   ;        ##
        !byte $00,$18,$00   ;        ##
        !byte $00,$18,$00   ;        ##
        !byte $00,$18,$00   ;        ##
        !byte $00,$18,$00   ;        ##
        !byte $00,$00,$00   ;
        !byte $00,$00,$00   ;
        !byte $00,$00,$00   ;
        !byte $00,$00,$00   ;
        !byte $00,$00,$00   ;
        !byte $00,$00,$00   ;
        !byte $00,$00,$00   ;
        !byte $00,$00,$00   ;

; ------------------------------------------------
; Sprite data at $2080 (block 130) — enemy
; ------------------------------------------------
*= $2080
        !byte $00,$66,$00   ;      ##..##
        !byte $00,$3c,$00   ;       ####
        !byte $00,$7e,$00   ;      ######
        !byte $00,$db,$00   ;     ##.##.##
        !byte $00,$ff,$00   ;     ########
        !byte $01,$ff,$80   ;    ##########
        !byte $01,$7e,$80   ;    #.######.#
        !byte $01,$3c,$80   ;    #..####..#
        !byte $00,$a5,$00   ;     #.#..#.#
        !byte $01,$81,$80   ;    ##......##
        !byte $00,$00,$00   ;
        !byte $00,$00,$00   ;
        !byte $00,$00,$00   ;
        !byte $00,$00,$00   ;
        !byte $00,$00,$00   ;
        !byte $00,$00,$00   ;
        !byte $00,$00,$00   ;
        !byte $00,$00,$00   ;
        !byte $00,$00,$00   ;
        !byte $00,$00,$00   ;
        !byte $00,$00,$00   ;

If It Doesn’t Work

No explosion sound? Check $D418 volume is $0F (set during init) and that voice 2’s gate is toggled — $80 then $81 written to $D40B. If you only write $81, the envelope may not retrigger.
Flash doesn’t show? Make sure $D029 is set to $01 in the hit block and restored to $05 when the timer expires.
Enemy keeps moving during flash? The flash_timer check must come before the normal movement code. If you put it after, the enemy moves for one frame before freezing.
Sound plays but wrong? Make sure you’re writing to $D407–$D40D (voice 2), not $D400–$D406 (voice 1). Easy mistake — the registers look identical, just offset by 7.

Try This: Different Explosion Sounds

Change voice 2’s frequency and envelope to hear different effects:

$D408 = $02 — Very low. Subterranean rumble.
$D408 = $18 — Higher. More of a crackle.
$D40C = $0F — Longer decay. The explosion lingers.
$D40C = $03 — Short decay. A sharp pop.

Try This: Longer Flash

Change flash_timer from 8 to 16. Or try 4 for a quick blink. At 50 fps (PAL), 8 frames is about 160ms — just long enough to notice.

What You’ve Learnt

SID voice 2 — an independent sound channel with its own registers, offset 7 bytes from voice 1.
Noise waveform — bit 7 of the control register. Good for explosions, static, percussion.
Frame counter — set a value, decrement each frame, act when it reaches zero. The simplest game timer.
Timed visual effects — temporary colour change with a countdown, frozen movement during the effect.

What’s Next

The enemy explodes but there’s no record of the carnage. In Unit 8, screen RAM gives you a score display — proof of every hit.