Game 1 Unit 12 of 16 1 hr learning time
Game Over
Tell the player it's over and let them try again — print GAME OVER into screen RAM with a text subroutine, then restart the whole game on a press of fire.
75% of Starfield
The game freezes when you die — and just sits there. No word to the player, no way back; the only option is to reset the machine. This unit closes the loop: a GAME OVER message so the player knows what happened, and a press of fire to start again. With it, the game becomes something you can play in rounds.
Where we start
Unit 11 ends in a silent frozen screen the instant the ship dies. We give that screen a voice and an exit.
Milestone 1 — say GAME OVER
In Unit 9 you wrote digits to the top-left corner. A word anywhere on screen is the same idea, twice over: pick the cell, and write each character's screen code. The screen is 40 columns wide and laid out row by row, so the address of any cell is $0400 + row × 40 + col. For the middle of the screen — row 12, column 16 — that's $0400 + 496 = $05f0.
Letters have screen codes too, and they're not the ASCII you might expect: A is 1, B is 2, up to Z at 26, with space at 32. So "GAME OVER" is the codes 7, 1, 13, 5, 32, 15, 22, 5, 18, written into consecutive cells from $05f0, and coloured white through the matching colour-RAM cells at $d9f0. We wrap that in a show_game_over subroutine and call it the moment the ship dies:
Step 1: print GAME OVER on death
+34
| 471 | 471 | sta $d412 ; sawtooth, gate OFF (reset the envelope) | |
| 472 | 472 | lda #$21 | |
| 473 | 473 | sta $d412 ; sawtooth, gate ON (trigger) | |
| 474 | + | | |
| 475 | + | jsr show_game_over ; print the message over the frozen screen | |
| 474 | 476 | | |
| 475 | 477 | no_ship_hit: | |
| 476 | 478 | | |
| ... | |||
| 504 | 506 | !byte $04, $06, $08 ; X offsets: $d004, $d006, $d008 | |
| 505 | 507 | sprite_colour_off: | |
| 506 | 508 | !byte $29, $2a, $2b ; colour offsets: $d029, $d02a, $d02b | |
| 509 | + | | |
| 510 | + | ; ------------------------------------------------ | |
| 511 | + | ; Subroutine: print "GAME OVER" at row 12, column 16 | |
| 512 | + | ; Row 12 x 40 + 16 = 496 = $1f0, so screen RAM $05f0, colour RAM $d9f0 | |
| 513 | + | ; ------------------------------------------------ | |
| 514 | + | show_game_over: | |
| 515 | + | lda #$07 ; G | |
| 516 | + | sta $05f0 | |
| 517 | + | lda #$01 ; A | |
| 518 | + | sta $05f1 | |
| 519 | + | lda #$0d ; M | |
| 520 | + | sta $05f2 | |
| 521 | + | lda #$05 ; E | |
| 522 | + | sta $05f3 | |
| 523 | + | lda #$20 ; (space) | |
| 524 | + | sta $05f4 | |
| 525 | + | lda #$0f ; O | |
| 526 | + | sta $05f5 | |
| 527 | + | lda #$16 ; V | |
| 528 | + | sta $05f6 | |
| 529 | + | lda #$05 ; E | |
| 530 | + | sta $05f7 | |
| 531 | + | lda #$12 ; R | |
| 532 | + | sta $05f8 | |
| 533 | + | ; colour the nine cells white ($d9f0..$d9f8) | |
| 534 | + | lda #$01 | |
| 535 | + | ldx #$00 | |
| 536 | + | - sta $d9f0,x | |
| 537 | + | inx | |
| 538 | + | cpx #$09 | |
| 539 | + | bne - | |
| 540 | + | rts | |
| 507 | 541 | | |
| 508 | 542 | ; ------------------------------------------------ | |
| 509 | 543 | ; Sprite data at $2000 (block 128) — ship |
The complete step 1 program
; Starfield - Unit 12: Game Over
; Cumulative steps: step-00 (silent freeze) -> step-01 (+ a GAME OVER message) -> step-02 (+ press fire to play again)
; 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
; ------------------------------------------------
; 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)
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
; 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 -
; If the game is over, freeze: keep syncing to the raster, do nothing else
lda game_over
bne game_loop
; --- 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:
lda #$01
sta game_over ; the loop freezes from next frame
lda #$02
sta $d027 ; ship turns red
; Death sound — SID voice 3, the third and last voice
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)
jsr show_game_over ; print the message over the frozen screen
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 freeze from Unit 11 still holds the scene, so the message lands on top of it and stays:
Milestone 2 — play again
A dead end isn't a game. We turn the freeze into a prompt: while the game is over, instead of looping forever, we watch the fire button — and when it's pressed, we start the whole game again.
Restarting cleanly is easier than it sounds. The init code at the top already sets everything to a known state: black screen cleared, ship white at the bottom, score 00, the wave spawned, game_over cleared. So "restart" is just "run the init again" — we label its entry start and jmp there. The screen clear wipes the GAME OVER text for free.
Step 2: poll fire and restart from the top
+8-2
| 26 | 26 | ; Initialisation | |
| 27 | 27 | ; ------------------------------------------------ | |
| 28 | 28 | *= $080d | |
| 29 | + | start: | |
| 29 | 30 | ; Black screen | |
| 30 | 31 | lda #$00 | |
| 31 | 32 | sta $d020 ; Border colour | |
| ... | |||
| 122 | 123 | cmp #$ff | |
| 123 | 124 | bne - | |
| 124 | 125 | | |
| 125 | - | ; If the game is over, freeze: keep syncing to the raster, do nothing else | |
| 126 | + | ; Game over? Wait for fire to restart; otherwise play on. | |
| 126 | 127 | lda game_over | |
| 127 | - | bne game_loop | |
| 128 | + | beq game_active | |
| 129 | + | lda $dc00 | |
| 130 | + | and #%00010000 ; fire button (bit 4) | |
| 131 | + | bne game_loop ; not pressed — hold the GAME OVER screen | |
| 132 | + | jmp start ; fire pressed — restart the whole game | |
| 133 | + | game_active: | |
| 128 | 134 | | |
| 129 | 135 | ; --- Read joystick and move ship --- | |
| 130 | 136 | |
The complete program
; Starfield - Unit 12: Game Over
; Cumulative steps: step-00 (silent freeze) -> step-01 (+ a GAME OVER message) -> step-02 (+ press fire to play again)
; 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
; ------------------------------------------------
; 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
; 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:
lda #$01
sta game_over ; the loop freezes from next frame
lda #$02
sta $d027 ; ship turns red
; Death sound — SID voice 3, the third and last voice
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)
jsr show_game_over ; print the message over the frozen screen
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,$00 ;
The game-over branch now reads fire ($dc00 bit 4) and jumps to start when it's pressed; otherwise it holds the screen. Press fire and the round resets — message gone, ship white, score back to 00, a fresh wave on its way down:
When it's wrong, see why
Text and restart fail in their own ways — the cell and the flag tell you which:
- No message, or it's in the wrong place. The address maths.
$0400 + row × 40 + col— row 12, col 16 is$05f0; an off-by-40 lands it on the wrong row. And the colour cells ($d9f0+) must be set, or white-on-black text is invisible until something else colours them. - Gibberish instead of letters. Screen codes, not ASCII.
Ais1, not$41; writing$41shows a graphics character. Check the codes againstA=1 …Z=26. - Fire doesn't restart. The poll or the jump. In the game-over branch,
$dc00bit 4 should send you tostart; if nothing happens, the branch is reading the wrong bit or jumping to the wrong label. - Restart leaves stale state. Something the init doesn't reset. The score, the ship colour,
game_over, and the wave must all be set atstart— if the score keeps its old value or the ship stays red, that init step is missing.
Before and after
We started with a dead-end freeze and finished with a real round: the game announces its end and offers a way back, all from one screen-writing subroutine and a jump to the top. The clean-slate restart is almost free — because the init already does the work of setting a fresh game, "play again" is just "do that again."
Try this
- Centre it properly. "GAME OVER" is nine characters; true-centre on a 40-column row starts at column 15 or 16. Nudge the start address and see the word shift. While you're there, add a second line — PRESS FIRE — a row below, so the prompt is explicit.
- A beat before restart. Right now any fire press restarts instantly, even one held from the shot that killed you. Add a short countdown — a frame counter, like the flash — before the game starts accepting fire, so the round doesn't restart by accident.
What you've learnt
- Screen position maths — any cell is
$0400 + row × 40 + col; the screen is row-major, 40 wide. - Letter screen codes —
A=1 …Z=26, space 32 — distinct from ASCII; they're not offset like the digits were. - A text subroutine — wrapping a fixed message in
jsr/rtsso it's one call from anywhere. - Restart by re-init — labelling the init
startand jumping to it gives a clean slate without duplicating the setup.
What's next
Right now one hit ends everything. Next you'll give the player three lives — a death costs a life and respawns the ship, and only running out of lives brings up GAME OVER.