Game 1 Unit 14 of 16 1 hr learning time
Life Lost Flash
Make losing a life fair — a border flash marks each hit and doubles as a short window of invulnerability, so a respawn can't die on the spot.
88% of Starfield
Three lives sounds fair until you respawn straight into the enemy that just killed you and lose a second life before you've touched the joystick. The last unit flagged it; this one fixes it. After every hit the ship gets a brief, visible window where it can't be hit — the standard arcade grace that makes a death feel like a setback, not a pile-up. We show that window as a flashing border, and we make the flash mean something.
Where we start
Unit 13 gives the player three lives, but a fresh ship is vulnerable the instant it appears. We add a short timer that both signals the hit and protects the ship while it runs.
Milestone 1 — a flash marks the hit
The tool is the frame counter again: a death_timer set to a frame count when a life is lost, counted down one per frame. While it runs, we flash the border — toggling it between red and black every few frames using a bit of the timer itself — and when it reaches zero we set the border back to black. The flash is loud and central, so a lost life is impossible to miss.
Step 1: flash the border for a moment after each hit
+26
| 16 | 16 | flash_tbl = $0d ; 3 bytes ($0d,$0e,$0f): each enemy's flash timer | |
| 17 | 17 | game_over = $10 ; 0 = playing, 1 = the ship has been hit | |
| 18 | 18 | lives = $11 ; lives remaining (starts at 3) | |
| 19 | + | death_timer = $12 ; frames of post-hit flash (and, in step 2, invulnerability) | |
| 19 | 20 | | |
| 20 | 21 | ; ------------------------------------------------ | |
| 21 | 22 | ; BASIC stub | |
| ... | |||
| 121 | 122 | ; The game starts alive | |
| 122 | 123 | lda #$00 | |
| 123 | 124 | sta game_over | |
| 125 | + | sta death_timer ; not flashing | |
| 124 | 126 | | |
| 125 | 127 | ; ------------------------------------------------ | |
| 126 | 128 | ; Game loop — runs once per frame | |
| ... | |||
| 495 | 497 | lda $d010 | |
| 496 | 498 | and #%11111110 ; clear the ship's 9th bit (back under X=256) | |
| 497 | 499 | sta $d010 | |
| 500 | + | | |
| 501 | + | ; Start the life-lost flash (step 2 makes it an invulnerability window too) | |
| 502 | + | lda #90 | |
| 503 | + | sta death_timer | |
| 498 | 504 | | |
| 499 | 505 | death_sound: | |
| 500 | 506 | ; Death sound — SID voice 3 (plays on every death) | |
| ... | |||
| 512 | 518 | sta $d412 ; sawtooth, gate ON (trigger) | |
| 513 | 519 | | |
| 514 | 520 | no_ship_hit: | |
| 521 | + | | |
| 522 | + | ; --- Life-lost flash: while the timer runs, blink the border --- | |
| 523 | + | lda death_timer | |
| 524 | + | beq flash_done | |
| 525 | + | dec death_timer | |
| 526 | + | lda death_timer | |
| 527 | + | and #%00001000 ; bit 3 toggles every 8 frames | |
| 528 | + | bne flash_bright | |
| 529 | + | lda #$00 ; dark phase | |
| 530 | + | sta $d020 | |
| 531 | + | jmp flash_tick | |
| 532 | + | flash_bright: | |
| 533 | + | lda #$02 ; bright phase (red border) | |
| 534 | + | sta $d020 | |
| 535 | + | flash_tick: | |
| 536 | + | lda death_timer | |
| 537 | + | bne flash_done | |
| 538 | + | lda #$00 ; just expired -> border back to black | |
| 539 | + | sta $d020 | |
| 540 | + | flash_done: | |
| 515 | 541 | | |
| 516 | 542 | jmp game_loop | |
| 517 | 543 | |
The complete step 1 program
; Starfield - Unit 14: Life Lost Flash
; Cumulative steps: step-00 (instant re-death risk) -> step-01 (+ a border flash on each hit) -> step-02 (+ invulnerable while it flashes)
; Assemble: acme -f cbm -o <step>.prg <step>.asm
; ------------------------------------------------
; Zero-page variables
; ------------------------------------------------
bullet_active = $02 ; 0 = no bullet, 1 = active
bullet_y = $03 ; Bullet Y position
laser_timer = $04 ; Frames of laser pitch-sweep remaining (0 = idle)
laser_freq = $05 ; Our copy of the sweep pitch (SID freq regs are write-only)
score = $06 ; Two-digit score, BCD (one decimal digit per nybble)
; Parallel arrays — index 0,1,2 picks enemy 0,1,2 (sprites 2,3,4)
enemy_x_tbl = $07 ; 3 bytes ($07,$08,$09): each enemy's X
enemy_y_tbl = $0a ; 3 bytes ($0a,$0b,$0c): each enemy's Y
flash_tbl = $0d ; 3 bytes ($0d,$0e,$0f): each enemy's flash timer
game_over = $10 ; 0 = playing, 1 = the ship has been hit
lives = $11 ; lives remaining (starts at 3)
death_timer = $12 ; frames of post-hit flash (and, in step 2, invulnerability)
; ------------------------------------------------
; BASIC stub
; ------------------------------------------------
*= $0801
!byte $0c,$08,$0a,$00,$9e,$32,$30,$36,$31,$00,$00,$00
; ------------------------------------------------
; Initialisation
; ------------------------------------------------
*= $080d
start:
; 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)
lda #%00011101
sta $d015 ; Enable sprites 0 (ship), 2-4 (three enemies)
lda #$00
sta $d010 ; sprite high-X bits clear (ship starts under X=256)
; Sprite 1 setup (bullet)
lda #129
sta $07f9 ; Data pointer (block 129 = $2040)
lda #$07
sta $d028 ; Colour (yellow)
; Enemy sprites 2, 3 and 4 share the one enemy shape in block 130.
; Colour and position are set per-enemy by spawn_enemy, below.
lda #130
sta $07fa ; sprite 2 data pointer
sta $07fb ; sprite 3 data pointer
sta $07fc ; sprite 4 data pointer
; Bullet starts inactive
lda #$00
sta bullet_active
; 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 #$06
sta $d405 ; Attack=0, Decay=6 (a short, snappy fall)
lda #$00
sta $d406 ; Sustain=0, Release=0
; Score readout: "00" in the top-left, white. (We've written to screen
; RAM since Unit 1's clear loop — now we write characters, not spaces.)
lda #$00
sta score
lda #$30 ; screen code for '0'
sta $0400 ; tens digit (row 0, col 0)
sta $0401 ; ones digit (row 0, col 1)
lda #$01
sta $d800 ; colour both digits white
sta $d801
; Three lives, shown in the top-right corner
lda #$03
sta lives
lda #$33 ; screen code for '3'
sta $0427 ; row 0, col 39
lda #$01
sta $d827 ; colour white
; Spawn the wave at staggered heights (A = start Y, X = enemy index)
lda #$32
ldx #$00
jsr spawn_enemy
lda #$82
ldx #$01
jsr spawn_enemy
lda #$d2
ldx #$02
jsr spawn_enemy
; The game starts alive
lda #$00
sta game_over
sta death_timer ; not flashing
; ------------------------------------------------
; Game loop — runs once per frame
; ------------------------------------------------
game_loop:
; Wait for the raster beam to reach line 255
; This syncs our code to the display (~50Hz PAL)
- lda $d012
cmp #$ff
bne -
; Game over? Wait for fire to restart; otherwise play on.
lda game_over
beq game_active
lda $dc00
and #%00010000 ; fire button (bit 4)
bne game_loop ; not pressed — hold the GAME OVER screen
jmp start ; fire pressed — restart the whole game
game_active:
; --- Read joystick and move ship ---
; UP (bit 0) — clamp to Y >= 50
lda $dc00 ; Read joystick port 2
and #%00000001 ; Isolate bit 0
bne not_up ; Bit is 1 = NOT pressed (active low)
lda $d001
cmp #52 ; 50 + room for a 2-pixel move
bcc not_up ; already at the top — don't move
dec $d001 ; Move ship up (decrease Y)
dec $d001 ; 2 pixels per frame
not_up:
; DOWN (bit 1) — clamp to Y <= 234
lda $dc00
and #%00000010
bne not_down
lda $d001
cmp #233 ; 234 - room for a 2-pixel move
bcs not_down ; already at the bottom — don't move
inc $d001 ; Move ship down (increase Y)
inc $d001
not_down:
; LEFT (bit 2) — 9-bit X, clamp to X >= 24
lda $dc00
and #%00000100
bne not_left
lda $d010
and #$01
bne left_ok ; high bit set: X >= 256, always safe to go left
lda $d000
cmp #26 ; 24 + room for a 2-pixel move
bcc not_left ; already at the left edge — don't move
left_ok:
; before each step, flip the 9th bit when X is about to wrap $00 -> $ff
lda $d000
bne +
lda $d010
eor #$01 ; the eor bit-flip from the Primer, on sprite 0's high X bit
sta $d010
+ dec $d000
lda $d000
bne +
lda $d010
eor #$01
sta $d010
+ dec $d000
not_left:
; RIGHT (bit 3) — 9-bit X, clamp to X <= 320
lda $dc00
and #%00001000
bne not_right
lda $d010
and #$01
beq right_ok ; high bit clear: X < 256, always safe to go right
lda $d000
cmp #63 ; (320 - 256) - room for a 2-pixel move
bcs not_right ; already at the right edge — don't move
right_ok:
; after each step, flip the 9th bit when X wraps $ff -> $00
inc $d000
bne +
lda $d010
eor #$01
sta $d010
+ inc $d000
bne +
lda $d010
eor #$01
sta $d010
+
not_right:
; --- Fire button (bit 4) ---
lda $dc00
and #%00010000
bne no_fire ; Bit is 1 = NOT pressed
; Only spawn if no bullet is already flying
lda bullet_active
bne no_fire
; Spawn the bullet at the ship's position
lda $d000 ; Ship X (low byte) -> bullet X
sta $d002
lda $d001 ; Ship Y -> bullet Y
sta bullet_y
; Copy the ship's 9th X bit (bit 0) to the bullet's (bit 1),
; so a shot fired from the right half spawns under the ship
lda $d010
and #%11111101 ; clear the bullet's 9th bit first
sta $d010
lda $d010
and #$01 ; the ship's 9th bit
asl ; shift it into the bullet's position (bit 1)
ora $d010
sta $d010
; Enable sprite 1 (keep sprite 0 enabled)
lda $d015
ora #%00000010
sta $d015
lda #$01
sta bullet_active
; Trigger laser sound: start the pitch high, gate off then on
lda #$40
sta laser_freq ; start high
sta $d401 ; SID frequency high byte
lda #$20
sta $d404 ; Sawtooth, gate OFF (reset envelope)
lda #$21
sta $d404 ; Sawtooth, gate ON (start sound)
lda #$0a
sta laser_timer ; sweep down over 10 frames
no_fire:
; --- Laser pitch sweep: the 'pew' ---
; Drop the pitch a little each frame while the sweep is running.
; We keep our own copy because SID frequency registers are write-only.
lda laser_timer
beq no_sweep
lda laser_freq
sec
sbc #$06
sta laser_freq
sta $d401 ; write the new pitch to the SID
dec laser_timer
no_sweep:
; --- Update the bullet ---
lda bullet_active
beq no_bullet
; Move it up 4 pixels a frame
lda bullet_y
sec
sbc #$04
sta bullet_y
sta $d003 ; sprite 1 Y
; Gone off the top? (Y < 30) -> remove it
cmp #$1e
bcs no_bullet
lda #$00
sta bullet_active
lda $d015
and #%11111101 ; disable sprite 1, keep sprite 0
sta $d015
lda $d010
and #%11111101 ; clear the bullet's 9th bit
sta $d010
no_bullet:
; --- Update every enemy: one indexed loop does all of them ---
ldx #$00
enemy_loop:
lda flash_tbl,x
bne do_flash ; this enemy is mid-flash
; not flashing: drift down 1 pixel per frame
lda enemy_y_tbl,x
clc
adc #$01 ; clc before adc, the addition from the Primer
sta enemy_y_tbl,x
cmp #$f8 ; off the bottom? (Y >= 248)
bcc update_sprite ; still on screen
lda #$32 ; respawn this enemy at the top, new column
jsr spawn_enemy
jmp next_enemy
do_flash:
dec flash_tbl,x
bne update_sprite ; still flashing -> stay frozen, white
lda #$32 ; flash done -> respawn (spawn_enemy restores green)
jsr spawn_enemy
jmp next_enemy
update_sprite:
; copy this enemy's position into its VIC-II sprite registers
ldy sprite_pos_off,x
lda enemy_x_tbl,x
sta $d000,y ; sprite X ($d004, $d006, ...)
lda enemy_y_tbl,x
sta $d001,y ; sprite Y ($d005, $d007, ...)
next_enemy:
inx
cpx #$03 ; the full wave of three
bne enemy_loop
; --- Bullet vs the wave: test each enemy until one is hit ---
lda bullet_active
bne check_collision
jmp no_hit
check_collision:
ldx #$00
collision_loop:
lda flash_tbl,x
bne next_collision ; skip an enemy that's already exploding
; Y distance (8-bit subtract wraps, so two ranges count as close)
lda bullet_y
sec
sbc enemy_y_tbl,x
cmp #$10
bcc check_x ; 0..15 apart: close
cmp #$f0
bcc next_collision ; 16..239 apart: too far
check_x:
; A bullet in the right portion (9th bit set) is past X=255, far from
; any enemy, so rule it out before comparing low bytes.
lda $d010
and #%00000010 ; bullet's 9th X bit (sprite 1)
bne next_collision
lda $d002
sec
sbc enemy_x_tbl,x
cmp #$10
bcc hit_enemy ; 0..15 apart: close
cmp #$f0
bcc next_collision ; 16..239 apart: too far
jmp hit_enemy ; 240..255: close from the other side
next_collision:
inx
cpx #$03
bne collision_loop
jmp no_hit
hit_enemy:
; X = the enemy that was hit. Remove the bullet.
lda #$00
sta bullet_active
lda $d015
and #%11111101 ; sprite 1 (bullet) off
sta $d015
; Flash THIS enemy white and start its 8-frame timer. The enemy loop
; freezes it white until the timer runs out, then respawns it.
lda #$08
sta flash_tbl,x
ldy sprite_colour_off,x
lda #$01
sta $d000,y ; this enemy's colour register = white
; Explosion sound — SID voice 2, noise waveform (voice 1 keeps the laser)
lda #$00
sta $d407 ; voice 2 frequency low
lda #$08
sta $d408 ; voice 2 frequency high (a 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 the envelope)
lda #$81
sta $d40b ; noise, gate ON (trigger the burst)
; Score one hit. In decimal mode ADC carries at 10, so the byte stays
; readable as two decimal digits (BCD) — no conversion needed.
sed ; decimal mode on
lda score
clc
adc #$01
sta score
cld ; decimal mode off (every later ADC/SBC needs it off)
; Refresh the two digits: high nybble -> tens, low nybble -> ones
lda score
lsr
lsr
lsr
lsr ; high nybble down to 0-9
clc
adc #$30 ; to screen code
sta $0400 ; tens digit
lda score
and #$0f ; low nybble, 0-9
clc
adc #$30
sta $0401 ; ones digit
no_hit:
; --- Ship vs the wave: has any enemy reached the ship? ---
ldx #$00
ship_collision_loop:
lda flash_tbl,x
bne next_ship_check ; ignore an exploding enemy
; Y distance: ship Y ($d001) vs this enemy's Y
lda $d001
sec
sbc enemy_y_tbl,x
cmp #$10
bcc check_ship_x
cmp #$f0
bcc next_ship_check
check_ship_x:
; ship past X=255 (9th bit set) is far from any enemy — rule it out
lda $d010
and #%00000001 ; ship's 9th X bit (sprite 0)
bne next_ship_check
lda $d000
sec
sbc enemy_x_tbl,x
cmp #$10
bcc ship_hit
cmp #$f0
bcc next_ship_check
jmp ship_hit ; 240..255: close from the other side
next_ship_check:
inx
cpx #$03
bne ship_collision_loop
jmp no_ship_hit
ship_hit:
; Lose a life and update the readout
dec lives
lda lives
clc
adc #$30
sta $0427 ; lives digit, top-right
lda lives
bne life_lost ; lives remain -> respawn and play on
; Out of lives -> end the game (the freeze + restart from Unit 12)
lda #$01
sta game_over
lda #$02
sta $d027 ; ship turns red
jsr show_game_over
jmp death_sound
life_lost:
; Respawn the ship at its start position
lda #172
sta $d000
lda #220
sta $d001
lda $d010
and #%11111110 ; clear the ship's 9th bit (back under X=256)
sta $d010
; Start the life-lost flash (step 2 makes it an invulnerability window too)
lda #90
sta death_timer
death_sound:
; Death sound — SID voice 3 (plays on every death)
lda #$00
sta $d40e ; voice 3 frequency low
lda #$10
sta $d40f ; voice 3 frequency high
lda #$0a
sta $d413 ; attack 0, decay 10 (a long, slow fade)
lda #$00
sta $d414 ; sustain 0, release 0
lda #$20
sta $d412 ; sawtooth, gate OFF (reset the envelope)
lda #$21
sta $d412 ; sawtooth, gate ON (trigger)
no_ship_hit:
; --- Life-lost flash: while the timer runs, blink the border ---
lda death_timer
beq flash_done
dec death_timer
lda death_timer
and #%00001000 ; bit 3 toggles every 8 frames
bne flash_bright
lda #$00 ; dark phase
sta $d020
jmp flash_tick
flash_bright:
lda #$02 ; bright phase (red border)
sta $d020
flash_tick:
lda death_timer
bne flash_done
lda #$00 ; just expired -> border back to black
sta $d020
flash_done:
jmp game_loop
; ------------------------------------------------
; Subroutine: spawn one enemy
; A = starting Y, X = enemy index (X is preserved)
; ------------------------------------------------
spawn_enemy:
sta enemy_y_tbl,x
lda $d012 ; raster line -> pseudo-random column
and #$7f
clc
adc #$30 ; 48-175, inside the visible width
sta enemy_x_tbl,x
lda #$00
sta flash_tbl,x ; not flashing
ldy sprite_colour_off,x
lda #$05
sta $d000,y ; this enemy's colour = green
ldy sprite_pos_off,x
lda enemy_x_tbl,x
sta $d000,y ; sprite X
lda enemy_y_tbl,x
sta $d001,y ; sprite Y
rts
; Per-enemy VIC-II register offsets (sprites 2, 3, 4)
sprite_pos_off:
!byte $04, $06, $08 ; X offsets: $d004, $d006, $d008
sprite_colour_off:
!byte $29, $2a, $2b ; colour offsets: $d029, $d02a, $d02b
; ------------------------------------------------
; Subroutine: print "GAME OVER" at row 12, column 16
; Row 12 x 40 + 16 = 496 = $1f0, so screen RAM $05f0, colour RAM $d9f0
; ------------------------------------------------
show_game_over:
lda #$07 ; G
sta $05f0
lda #$01 ; A
sta $05f1
lda #$0d ; M
sta $05f2
lda #$05 ; E
sta $05f3
lda #$20 ; (space)
sta $05f4
lda #$0f ; O
sta $05f5
lda #$16 ; V
sta $05f6
lda #$05 ; E
sta $05f7
lda #$12 ; R
sta $05f8
; colour the nine cells white ($d9f0..$d9f8)
lda #$01
ldx #$00
- sta $d9f0,x
inx
cpx #$09
bne -
rts
; ------------------------------------------------
; 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 ;
Lose a life and the screen tells you, unmistakably:
Milestone 2 — the flash is a shield
Right now the flash is only decoration — the ship is still vulnerable during it, so the pile-up the last unit warned about can still happen. We make the flash mean invulnerability with one check: at the top of the ship-enemy collision test, if death_timer is running, skip the whole test. No collision is even looked for while the timer counts down, so for that window the ship is untouchable.
Step 2: skip ship collision while the flash runs
+5
| 437 | 437 | no_hit: | |
| 438 | 438 | | |
| 439 | 439 | ; --- Ship vs the wave: has any enemy reached the ship? --- | |
| 440 | + | ; ...but not while the life-lost flash runs — the ship is invulnerable | |
| 441 | + | lda death_timer | |
| 442 | + | beq do_ship_collision ; not flashing -> run the check | |
| 443 | + | jmp no_ship_hit ; flashing -> skip it (jmp, the target is far) | |
| 444 | + | do_ship_collision: | |
| 440 | 445 | ldx #$00 | |
| 441 | 446 | ship_collision_loop: | |
| 442 | 447 | lda flash_tbl,x |
The complete program
; Starfield - Unit 14: Life Lost Flash
; Cumulative steps: step-00 (instant re-death risk) -> step-01 (+ a border flash on each hit) -> step-02 (+ invulnerable while it flashes)
; Assemble: acme -f cbm -o <step>.prg <step>.asm
; ------------------------------------------------
; Zero-page variables
; ------------------------------------------------
bullet_active = $02 ; 0 = no bullet, 1 = active
bullet_y = $03 ; Bullet Y position
laser_timer = $04 ; Frames of laser pitch-sweep remaining (0 = idle)
laser_freq = $05 ; Our copy of the sweep pitch (SID freq regs are write-only)
score = $06 ; Two-digit score, BCD (one decimal digit per nybble)
; Parallel arrays — index 0,1,2 picks enemy 0,1,2 (sprites 2,3,4)
enemy_x_tbl = $07 ; 3 bytes ($07,$08,$09): each enemy's X
enemy_y_tbl = $0a ; 3 bytes ($0a,$0b,$0c): each enemy's Y
flash_tbl = $0d ; 3 bytes ($0d,$0e,$0f): each enemy's flash timer
game_over = $10 ; 0 = playing, 1 = the ship has been hit
lives = $11 ; lives remaining (starts at 3)
death_timer = $12 ; frames of post-hit flash (and, in step 2, invulnerability)
; ------------------------------------------------
; BASIC stub
; ------------------------------------------------
*= $0801
!byte $0c,$08,$0a,$00,$9e,$32,$30,$36,$31,$00,$00,$00
; ------------------------------------------------
; Initialisation
; ------------------------------------------------
*= $080d
start:
; 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)
lda #%00011101
sta $d015 ; Enable sprites 0 (ship), 2-4 (three enemies)
lda #$00
sta $d010 ; sprite high-X bits clear (ship starts under X=256)
; Sprite 1 setup (bullet)
lda #129
sta $07f9 ; Data pointer (block 129 = $2040)
lda #$07
sta $d028 ; Colour (yellow)
; Enemy sprites 2, 3 and 4 share the one enemy shape in block 130.
; Colour and position are set per-enemy by spawn_enemy, below.
lda #130
sta $07fa ; sprite 2 data pointer
sta $07fb ; sprite 3 data pointer
sta $07fc ; sprite 4 data pointer
; Bullet starts inactive
lda #$00
sta bullet_active
; 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 #$06
sta $d405 ; Attack=0, Decay=6 (a short, snappy fall)
lda #$00
sta $d406 ; Sustain=0, Release=0
; Score readout: "00" in the top-left, white. (We've written to screen
; RAM since Unit 1's clear loop — now we write characters, not spaces.)
lda #$00
sta score
lda #$30 ; screen code for '0'
sta $0400 ; tens digit (row 0, col 0)
sta $0401 ; ones digit (row 0, col 1)
lda #$01
sta $d800 ; colour both digits white
sta $d801
; Three lives, shown in the top-right corner
lda #$03
sta lives
lda #$33 ; screen code for '3'
sta $0427 ; row 0, col 39
lda #$01
sta $d827 ; colour white
; Spawn the wave at staggered heights (A = start Y, X = enemy index)
lda #$32
ldx #$00
jsr spawn_enemy
lda #$82
ldx #$01
jsr spawn_enemy
lda #$d2
ldx #$02
jsr spawn_enemy
; The game starts alive
lda #$00
sta game_over
sta death_timer ; not flashing
; ------------------------------------------------
; Game loop — runs once per frame
; ------------------------------------------------
game_loop:
; Wait for the raster beam to reach line 255
; This syncs our code to the display (~50Hz PAL)
- lda $d012
cmp #$ff
bne -
; Game over? Wait for fire to restart; otherwise play on.
lda game_over
beq game_active
lda $dc00
and #%00010000 ; fire button (bit 4)
bne game_loop ; not pressed — hold the GAME OVER screen
jmp start ; fire pressed — restart the whole game
game_active:
; --- Read joystick and move ship ---
; UP (bit 0) — clamp to Y >= 50
lda $dc00 ; Read joystick port 2
and #%00000001 ; Isolate bit 0
bne not_up ; Bit is 1 = NOT pressed (active low)
lda $d001
cmp #52 ; 50 + room for a 2-pixel move
bcc not_up ; already at the top — don't move
dec $d001 ; Move ship up (decrease Y)
dec $d001 ; 2 pixels per frame
not_up:
; DOWN (bit 1) — clamp to Y <= 234
lda $dc00
and #%00000010
bne not_down
lda $d001
cmp #233 ; 234 - room for a 2-pixel move
bcs not_down ; already at the bottom — don't move
inc $d001 ; Move ship down (increase Y)
inc $d001
not_down:
; LEFT (bit 2) — 9-bit X, clamp to X >= 24
lda $dc00
and #%00000100
bne not_left
lda $d010
and #$01
bne left_ok ; high bit set: X >= 256, always safe to go left
lda $d000
cmp #26 ; 24 + room for a 2-pixel move
bcc not_left ; already at the left edge — don't move
left_ok:
; before each step, flip the 9th bit when X is about to wrap $00 -> $ff
lda $d000
bne +
lda $d010
eor #$01 ; the eor bit-flip from the Primer, on sprite 0's high X bit
sta $d010
+ dec $d000
lda $d000
bne +
lda $d010
eor #$01
sta $d010
+ dec $d000
not_left:
; RIGHT (bit 3) — 9-bit X, clamp to X <= 320
lda $dc00
and #%00001000
bne not_right
lda $d010
and #$01
beq right_ok ; high bit clear: X < 256, always safe to go right
lda $d000
cmp #63 ; (320 - 256) - room for a 2-pixel move
bcs not_right ; already at the right edge — don't move
right_ok:
; after each step, flip the 9th bit when X wraps $ff -> $00
inc $d000
bne +
lda $d010
eor #$01
sta $d010
+ inc $d000
bne +
lda $d010
eor #$01
sta $d010
+
not_right:
; --- Fire button (bit 4) ---
lda $dc00
and #%00010000
bne no_fire ; Bit is 1 = NOT pressed
; Only spawn if no bullet is already flying
lda bullet_active
bne no_fire
; Spawn the bullet at the ship's position
lda $d000 ; Ship X (low byte) -> bullet X
sta $d002
lda $d001 ; Ship Y -> bullet Y
sta bullet_y
; Copy the ship's 9th X bit (bit 0) to the bullet's (bit 1),
; so a shot fired from the right half spawns under the ship
lda $d010
and #%11111101 ; clear the bullet's 9th bit first
sta $d010
lda $d010
and #$01 ; the ship's 9th bit
asl ; shift it into the bullet's position (bit 1)
ora $d010
sta $d010
; Enable sprite 1 (keep sprite 0 enabled)
lda $d015
ora #%00000010
sta $d015
lda #$01
sta bullet_active
; Trigger laser sound: start the pitch high, gate off then on
lda #$40
sta laser_freq ; start high
sta $d401 ; SID frequency high byte
lda #$20
sta $d404 ; Sawtooth, gate OFF (reset envelope)
lda #$21
sta $d404 ; Sawtooth, gate ON (start sound)
lda #$0a
sta laser_timer ; sweep down over 10 frames
no_fire:
; --- Laser pitch sweep: the 'pew' ---
; Drop the pitch a little each frame while the sweep is running.
; We keep our own copy because SID frequency registers are write-only.
lda laser_timer
beq no_sweep
lda laser_freq
sec
sbc #$06
sta laser_freq
sta $d401 ; write the new pitch to the SID
dec laser_timer
no_sweep:
; --- Update the bullet ---
lda bullet_active
beq no_bullet
; Move it up 4 pixels a frame
lda bullet_y
sec
sbc #$04
sta bullet_y
sta $d003 ; sprite 1 Y
; Gone off the top? (Y < 30) -> remove it
cmp #$1e
bcs no_bullet
lda #$00
sta bullet_active
lda $d015
and #%11111101 ; disable sprite 1, keep sprite 0
sta $d015
lda $d010
and #%11111101 ; clear the bullet's 9th bit
sta $d010
no_bullet:
; --- Update every enemy: one indexed loop does all of them ---
ldx #$00
enemy_loop:
lda flash_tbl,x
bne do_flash ; this enemy is mid-flash
; not flashing: drift down 1 pixel per frame
lda enemy_y_tbl,x
clc
adc #$01 ; clc before adc, the addition from the Primer
sta enemy_y_tbl,x
cmp #$f8 ; off the bottom? (Y >= 248)
bcc update_sprite ; still on screen
lda #$32 ; respawn this enemy at the top, new column
jsr spawn_enemy
jmp next_enemy
do_flash:
dec flash_tbl,x
bne update_sprite ; still flashing -> stay frozen, white
lda #$32 ; flash done -> respawn (spawn_enemy restores green)
jsr spawn_enemy
jmp next_enemy
update_sprite:
; copy this enemy's position into its VIC-II sprite registers
ldy sprite_pos_off,x
lda enemy_x_tbl,x
sta $d000,y ; sprite X ($d004, $d006, ...)
lda enemy_y_tbl,x
sta $d001,y ; sprite Y ($d005, $d007, ...)
next_enemy:
inx
cpx #$03 ; the full wave of three
bne enemy_loop
; --- Bullet vs the wave: test each enemy until one is hit ---
lda bullet_active
bne check_collision
jmp no_hit
check_collision:
ldx #$00
collision_loop:
lda flash_tbl,x
bne next_collision ; skip an enemy that's already exploding
; Y distance (8-bit subtract wraps, so two ranges count as close)
lda bullet_y
sec
sbc enemy_y_tbl,x
cmp #$10
bcc check_x ; 0..15 apart: close
cmp #$f0
bcc next_collision ; 16..239 apart: too far
check_x:
; A bullet in the right portion (9th bit set) is past X=255, far from
; any enemy, so rule it out before comparing low bytes.
lda $d010
and #%00000010 ; bullet's 9th X bit (sprite 1)
bne next_collision
lda $d002
sec
sbc enemy_x_tbl,x
cmp #$10
bcc hit_enemy ; 0..15 apart: close
cmp #$f0
bcc next_collision ; 16..239 apart: too far
jmp hit_enemy ; 240..255: close from the other side
next_collision:
inx
cpx #$03
bne collision_loop
jmp no_hit
hit_enemy:
; X = the enemy that was hit. Remove the bullet.
lda #$00
sta bullet_active
lda $d015
and #%11111101 ; sprite 1 (bullet) off
sta $d015
; Flash THIS enemy white and start its 8-frame timer. The enemy loop
; freezes it white until the timer runs out, then respawns it.
lda #$08
sta flash_tbl,x
ldy sprite_colour_off,x
lda #$01
sta $d000,y ; this enemy's colour register = white
; Explosion sound — SID voice 2, noise waveform (voice 1 keeps the laser)
lda #$00
sta $d407 ; voice 2 frequency low
lda #$08
sta $d408 ; voice 2 frequency high (a 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 the envelope)
lda #$81
sta $d40b ; noise, gate ON (trigger the burst)
; Score one hit. In decimal mode ADC carries at 10, so the byte stays
; readable as two decimal digits (BCD) — no conversion needed.
sed ; decimal mode on
lda score
clc
adc #$01
sta score
cld ; decimal mode off (every later ADC/SBC needs it off)
; Refresh the two digits: high nybble -> tens, low nybble -> ones
lda score
lsr
lsr
lsr
lsr ; high nybble down to 0-9
clc
adc #$30 ; to screen code
sta $0400 ; tens digit
lda score
and #$0f ; low nybble, 0-9
clc
adc #$30
sta $0401 ; ones digit
no_hit:
; --- Ship vs the wave: has any enemy reached the ship? ---
; ...but not while the life-lost flash runs — the ship is invulnerable
lda death_timer
beq do_ship_collision ; not flashing -> run the check
jmp no_ship_hit ; flashing -> skip it (jmp, the target is far)
do_ship_collision:
ldx #$00
ship_collision_loop:
lda flash_tbl,x
bne next_ship_check ; ignore an exploding enemy
; Y distance: ship Y ($d001) vs this enemy's Y
lda $d001
sec
sbc enemy_y_tbl,x
cmp #$10
bcc check_ship_x
cmp #$f0
bcc next_ship_check
check_ship_x:
; ship past X=255 (9th bit set) is far from any enemy — rule it out
lda $d010
and #%00000001 ; ship's 9th X bit (sprite 0)
bne next_ship_check
lda $d000
sec
sbc enemy_x_tbl,x
cmp #$10
bcc ship_hit
cmp #$f0
bcc next_ship_check
jmp ship_hit ; 240..255: close from the other side
next_ship_check:
inx
cpx #$03
bne ship_collision_loop
jmp no_ship_hit
ship_hit:
; Lose a life and update the readout
dec lives
lda lives
clc
adc #$30
sta $0427 ; lives digit, top-right
lda lives
bne life_lost ; lives remain -> respawn and play on
; Out of lives -> end the game (the freeze + restart from Unit 12)
lda #$01
sta game_over
lda #$02
sta $d027 ; ship turns red
jsr show_game_over
jmp death_sound
life_lost:
; Respawn the ship at its start position
lda #172
sta $d000
lda #220
sta $d001
lda $d010
and #%11111110 ; clear the ship's 9th bit (back under X=256)
sta $d010
; Start the life-lost flash (step 2 makes it an invulnerability window too)
lda #90
sta death_timer
death_sound:
; Death sound — SID voice 3 (plays on every death)
lda #$00
sta $d40e ; voice 3 frequency low
lda #$10
sta $d40f ; voice 3 frequency high
lda #$0a
sta $d413 ; attack 0, decay 10 (a long, slow fade)
lda #$00
sta $d414 ; sustain 0, release 0
lda #$20
sta $d412 ; sawtooth, gate OFF (reset the envelope)
lda #$21
sta $d412 ; sawtooth, gate ON (trigger)
no_ship_hit:
; --- Life-lost flash: while the timer runs, blink the border ---
lda death_timer
beq flash_done
dec death_timer
lda death_timer
and #%00001000 ; bit 3 toggles every 8 frames
bne flash_bright
lda #$00 ; dark phase
sta $d020
jmp flash_tick
flash_bright:
lda #$02 ; bright phase (red border)
sta $d020
flash_tick:
lda death_timer
bne flash_done
lda #$00 ; just expired -> border back to black
sta $d020
flash_done:
jmp game_loop
; ------------------------------------------------
; Subroutine: spawn one enemy
; A = starting Y, X = enemy index (X is preserved)
; ------------------------------------------------
spawn_enemy:
sta enemy_y_tbl,x
lda $d012 ; raster line -> pseudo-random column
and #$7f
clc
adc #$30 ; 48-175, inside the visible width
sta enemy_x_tbl,x
lda #$00
sta flash_tbl,x ; not flashing
ldy sprite_colour_off,x
lda #$05
sta $d000,y ; this enemy's colour = green
ldy sprite_pos_off,x
lda enemy_x_tbl,x
sta $d000,y ; sprite X
lda enemy_y_tbl,x
sta $d001,y ; sprite Y
rts
; Per-enemy VIC-II register offsets (sprites 2, 3, 4)
sprite_pos_off:
!byte $04, $06, $08 ; X offsets: $d004, $d006, $d008
sprite_colour_off:
!byte $29, $2a, $2b ; colour offsets: $d029, $d02a, $d02b
; ------------------------------------------------
; Subroutine: print "GAME OVER" at row 12, column 16
; Row 12 x 40 + 16 = 496 = $1f0, so screen RAM $05f0, colour RAM $d9f0
; ------------------------------------------------
show_game_over:
lda #$07 ; G
sta $05f0
lda #$01 ; A
sta $05f1
lda #$0d ; M
sta $05f2
lda #$05 ; E
sta $05f3
lda #$20 ; (space)
sta $05f4
lda #$0f ; O
sta $05f5
lda #$16 ; V
sta $05f6
lda #$05 ; E
sta $05f7
lda #$12 ; R
sta $05f8
; colour the nine cells white ($d9f0..$d9f8)
lda #$01
ldx #$00
- sta $d9f0,x
inx
cpx #$09
bne -
rts
; ------------------------------------------------
; 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,$00 ;
One detail in that diff is worth a look: the skip is written as beq do_ship_collision / jmp no_ship_hit instead of a single bne no_ship_hit. A branch only reaches about 127 bytes, and the collision code it has to jump over is longer than that — so we branch a short way over an unconditional jmp, which has no range limit. It's a pattern you hit whenever a conditional skip spans a big block.
Now the window after a hit is genuinely safe — the ship flies back into the wave and keeps the lives it has left, for exactly as long as the border flashes:
When it's wrong, see why
The timer drives both the look and the safety — watch it:
- No flash. The timer isn't being set, or the border isn't being written. On a life lost,
death_timershould become non-zero; each frame it runs,$d020should change. If the value sets but the border stays black, the flash branch isn't writing the bright colour. - The border flashes forever. The countdown or the restore.
dec death_timermust run every frame, and when it reaches zero the border must be forced back to$00— miss that and it sticks on whatever the last flash phase wrote. - Still dying on respawn. The invulnerability check is missing or in the wrong place. The
death_timertest must sit at the top of the ship-collision section and skip the whole thing; if it's after the loop, the collision still runs. - The skip won't assemble. "Target out of range" — a
bne/beqcan't reach across the long collision block. Branch over ajmpinstead, as the diff does.
Before and after
We started with a respawn that could die on the spot and finished with a fair one: every lost life flashes the border and grants a short, visible stretch of invulnerability. It's the same frame-counter pattern as the hit flash, used twice over — once to show the moment, once to protect it — and it's the difference between a death that feels cheap and one that feels earned.
Try this
- Tune the grace. The window is about a second and a half. Shorten it for a tense game where you must reposition fast, lengthen it for a forgiving one. Too long and the ship feels like a ghost; too short and the pile-up returns.
- Flash the ship, not the border. Instead of (or as well as) the border, blink sprite 0 on and off with the timer — toggle its bit in
$d015. The blinking-ship invulnerability is the look most arcade games used; see which reads better to you.
What you've learnt
- A timer that drives a visual — a frame counter whose value both counts down a window and, through one of its bits, toggles a colour.
- Temporary invulnerability — gating a collision check on a timer, so an object can't be hit for a stretch.
- Branching over a jump — the fix when a conditional skip has to clear a block bigger than a branch can reach.
- Game feel as fairness — a respawn that the player can survive is the difference between difficulty and cheapness.
What's next
The game plays fair now, but the ship and its wave hang in flat black space. Next you'll fill that space with a scrolling starfield — layers of stars drifting at different speeds — and the game finally looks like it's moving through somewhere.