Game 1 Unit 13 of 16 1 hr learning time
Three Lives
Soften the one-hit ending — give the player three lives, a counter that ticks down on each hit and respawns the ship, with game over held back until the last life is gone.
81% of Starfield
One touch and it's over — which is honest, but harsh. A single misjudged move ends the whole run, and the game punishes the smallest mistake as hard as the biggest. Almost every arcade game softened this the same way: a stock of lives. This unit gives the player three. A hit now costs a life and puts the ship back; only when the last one is spent does the game end.
Where we start
Unit 12 ends the game on the first hit. We turn that single, fatal collision into one of three.
Milestone 1 — three lives, one spent per hit
A life count is a number on screen you already know how to make: store it in a byte, show it as a digit. We keep lives in zero page, start it at 3, and display it in the top-right corner (the score sits top-left). On a hit, instead of ending the game, we decrement the count, redraw the digit, and put the ship back at its start position to fly again.
Step 1: a lives counter; a hit costs one and respawns the ship
+25-7
| 15 | 15 | enemy_y_tbl = $0a ; 3 bytes ($0a,$0b,$0c): each enemy's Y | |
| 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 | + | lives = $11 ; lives remaining (starts at 3) | |
| 18 | 19 | | |
| 19 | 20 | ; ------------------------------------------------ | |
| 20 | 21 | ; BASIC stub | |
| ... | |||
| 97 | 98 | lda #$01 | |
| 98 | 99 | sta $d800 ; colour both digits white | |
| 99 | 100 | sta $d801 | |
| 101 | + | | |
| 102 | + | ; Three lives, shown in the top-right corner | |
| 103 | + | lda #$03 | |
| 104 | + | sta lives | |
| 105 | + | lda #$33 ; screen code for '3' | |
| 106 | + | sta $0427 ; row 0, col 39 | |
| 107 | + | lda #$01 | |
| 108 | + | sta $d827 ; colour white | |
| 100 | 109 | | |
| 101 | 110 | ; Spawn the wave at staggered heights (A = start Y, X = enemy index) | |
| 102 | 111 | lda #$32 | |
| ... | |||
| 459 | 468 | jmp no_ship_hit | |
| 460 | 469 | | |
| 461 | 470 | ship_hit: | |
| 462 | - | lda #$01 | |
| 463 | - | sta game_over ; the loop freezes from next frame | |
| 464 | - | lda #$02 | |
| 465 | - | sta $d027 ; ship turns red | |
| 471 | + | ; Lose a life and update the readout | |
| 472 | + | dec lives | |
| 473 | + | lda lives | |
| 474 | + | clc | |
| 475 | + | adc #$30 | |
| 476 | + | sta $0427 ; lives digit, top-right | |
| 466 | 477 | | |
| 467 | - | ; Death sound — SID voice 3, the third and last voice | |
| 478 | + | ; Respawn the ship at its start position | |
| 479 | + | lda #172 | |
| 480 | + | sta $d000 | |
| 481 | + | lda #220 | |
| 482 | + | sta $d001 | |
| 483 | + | lda $d010 | |
| 484 | + | and #%11111110 ; clear the ship's 9th bit (back under X=256) | |
| 485 | + | sta $d010 | |
| 486 | + | | |
| 487 | + | ; Death sound — SID voice 3 (plays on every life lost) | |
| 468 | 488 | lda #$00 | |
| 469 | 489 | sta $d40e ; voice 3 frequency low | |
| 470 | 490 | lda #$10 | |
| ... | |||
| 477 | 497 | sta $d412 ; sawtooth, gate OFF (reset the envelope) | |
| 478 | 498 | lda #$21 | |
| 479 | 499 | sta $d412 ; sawtooth, gate ON (trigger) | |
| 480 | - | | |
| 481 | - | jsr show_game_over ; print the message over the frozen screen | |
| 482 | 500 | | |
| 483 | 501 | no_ship_hit: | |
| 484 | 502 | |
The complete step 1 program
; Starfield - Unit 13: Three Lives
; Cumulative steps: step-00 (one hit ends it) -> step-01 (+ three lives: a hit costs one) -> step-02 (+ last life triggers game over)
; 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)
; ------------------------------------------------
; 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
; ------------------------------------------------
; 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
; 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
; Death sound — SID voice 3 (plays on every life lost)
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:
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 ;
The ship-hit handler is now a survivable event: dec lives, redraw, and reset $d000/$d001 (and clear the ship's ninth bit, in case it died over on the right). The death tone still plays — a hit should still sting — but the wave rolls on:
Milestone 2 — the last life ends it
As written, the count would tick straight past zero into nonsense and the ship would never stop respawning. The fix is one comparison: after decrementing, check whether any lives remain. If they do, respawn as before; if not, fall into the Unit 11–12 ending — game over, red ship, the GAME OVER message, the freeze.
Step 2: out of lives means game over
+14-1
| 474 | 474 | clc | |
| 475 | 475 | adc #$30 | |
| 476 | 476 | sta $0427 ; lives digit, top-right | |
| 477 | + | | |
| 478 | + | lda lives | |
| 479 | + | bne life_lost ; lives remain -> respawn and play on | |
| 480 | + | | |
| 481 | + | ; Out of lives -> end the game (the freeze + restart from Unit 12) | |
| 482 | + | lda #$01 | |
| 483 | + | sta game_over | |
| 484 | + | lda #$02 | |
| 485 | + | sta $d027 ; ship turns red | |
| 486 | + | jsr show_game_over | |
| 487 | + | jmp death_sound | |
| 477 | 488 | | |
| 489 | + | life_lost: | |
| 478 | 490 | ; Respawn the ship at its start position | |
| 479 | 491 | lda #172 | |
| 480 | 492 | sta $d000 | |
| ... | |||
| 484 | 496 | and #%11111110 ; clear the ship's 9th bit (back under X=256) | |
| 485 | 497 | sta $d010 | |
| 486 | 498 | | |
| 487 | - | ; Death sound — SID voice 3 (plays on every life lost) | |
| 499 | + | death_sound: | |
| 500 | + | ; Death sound — SID voice 3 (plays on every death) | |
| 488 | 501 | lda #$00 | |
| 489 | 502 | sta $d40e ; voice 3 frequency low | |
| 490 | 503 | lda #$10 |
The complete program
; Starfield - Unit 13: Three Lives
; Cumulative steps: step-00 (one hit ends it) -> step-01 (+ three lives: a hit costs one) -> step-02 (+ last life triggers game over)
; 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)
; ------------------------------------------------
; 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
; ------------------------------------------------
; 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
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:
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 ;
Now the stock matters. Each hit spends a life — 3, 2, 1 — and only the loss of the last one brings the game to a halt:
When it's wrong, see why
The counter and the branch are the two moving parts:
- No number in the top-right. The display write. The digit goes to
$0427(row 0, column 39), coloured via$d827, and the value is a screen code —3is$33, a0–9value plus$30. Miss the colour and white-on-black is invisible. - The count runs past zero into letters. The zero check is missing or after the respawn.
dec livesthenlda lives/bne— the branch to game over must happen when the result is zero, before anything respawns. - The ship never comes back. The respawn writes. On a non-final hit,
$d000/$d001must be set to the start position; if they aren't, the ship stays where it died (often off-screen or on top of the enemy that hit it). - A single hit drains several lives at once. The ship respawns on top of, or right next to, the enemy that just hit it, and is hit again immediately. There's no grace period yet — that's the next unit. For now, the fixed respawn point keeps it mostly clear.
Before and after
We started with a one-hit kill and finished with a stock of three lives: a hit costs one and returns the ship, and the GAME OVER ending waits for the last life to go. The whole change rides on a counter you display the same way as the score and a single branch that chooses between "respawn" and "end the game."
Try this
- Change the stock. Start
livesat 5 for a gentler game, or 1 for a brutal one. The starting number is the difficulty dial — set it where the game feels fair. - An extra life for a good run. When the score passes a milestone — say
$50— add one tolivesand redraw the digit. Rewarding survival with more survival is one of the oldest hooks in the medium.
What you've learnt
- A lives counter — a byte shown as a digit, decremented on each hit, the same display technique as the score.
- Respawn versus game over — one branch on the remaining count chooses between putting the ship back and ending the game.
- Reusing the ending — the game-over path from the last two units is called only when the stock is empty, no new code for it.
What's next
Respawning straight back into the wave is rough — the ship can reappear on top of an enemy and lose another life instantly. Next you'll add a brief flash of invulnerability after each death, so a lost life always gives the player a fair moment to recover.