Game 1 Unit 11 of 128 1 hr learning time
Game Over
Screen codes write GAME OVER to the display, fire button restarts via an init_game subroutine that resets all state cleanly.
9% of Starfield
The ship turns red and the game freezes. But there’s no message and no way to try again. This unit adds “GAME OVER” text to the screen and lets the player restart by pressing fire.
Screen Codes
The VIC-II reads character data from screen RAM at $0400. Each byte is a screen code — a number that maps to a character in the ROM character set. Screen codes are not the same as PETSCII (the C64’s text encoding). The mapping for letters: A=1, B=2, C=3, … Z=26. Digits stay at $30–$39. Space is $20.
To display “GAME OVER”:
- G =
$07, A =$01, M =$0D, E =$05 - Space =
$20 - O =
$0F, V =$16, E =$05, R =$12
Positioning Text on Screen
Screen RAM is a 40×25 grid. Row 0 starts at $0400, row 1 at $0400 + 40 = $0428, and so on. The formula: $0400 + (row × 40) + column.
“GAME OVER” is 9 characters. Centred on a 40-column screen: column 16 (since (40 − 9) ÷ 2 ≈ 15.5). Row 12 puts it vertically centred. That gives $0400 + (12 × 40) + 16 = $05F0.
Colour RAM mirrors the layout at $D800. The colour for position $05F0 is at $D9F0. Write $01 (white) to make the text visible against the black background.
The Game Over Display
; ------------------------------------------------
; Subroutine: show_game_over
; Writes "GAME OVER" to screen RAM, row 12, col 16
; ------------------------------------------------
show_game_over:
; Screen codes: G=7, A=1, M=13, E=5, space=32, O=15, V=22, E=5, R=18
; Position: row 12 × 40 + col 16 = 496 → $0400 + $01F0 = $05F0
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 to white
lda #$01
sta $d9f0
sta $d9f1
sta $d9f2
sta $d9f3
sta $d9f4
sta $d9f5
sta $d9f6
sta $d9f7
sta $d9f8
rts
Nine LDA/STA pairs write the characters, then nine more set the colours. It’s direct and clear. A loop with a data table would be shorter, but for nine characters the explicit writes are easier to follow — and every character’s screen code is visible in the source.
Press Fire to Restart
The game loop already checks game_state at the top. In Unit 10, when game_state was 1, the loop just jumped back — frozen. Now it does something: poll the fire button. When pressed, clear the screen and reinitialise.
; --- Check game state ---
lda game_state
beq game_active
; --- Game over: poll fire button ---
lda $dc00
and #%00010000
bne game_loop ; Not pressed, keep waiting
; Fire pressed — restart the game
jsr clear_screen
jsr init_game
jmp game_loop
game_active:
; ... input, movement, collision code continues here
clear_screen fills screen RAM with spaces, wiping the “GAME OVER” text and the old score display. init_game resets everything — ship position, enemy positions, score, game state. Then the loop continues from the top with game_state back at 0.
Extracting Initialisation
The init code from Unit 10 was inline — it ran once at startup and couldn’t be reused. Now it’s a subroutine: init_game. The boot sequence calls JSR clear_screen then JSR init_game. The restart path calls the same two subroutines. Same code, two contexts.
Hardware setup that never changes (sprite data pointers, SID voice configuration, score colour) stays outside init_game in the one-time boot section. Only state that resets between games belongs in the subroutine: ship position, enemy positions, score, bullet state, and game_state itself.
The Complete Code
; Starfield - Unit 11: Game Over
; Assemble with: acme -f cbm -o starfield.prg starfield.asm
; ------------------------------------------------
; Zero-page variables
; ------------------------------------------------
bullet_active = $02 ; 0 = no bullet, 1 = active
bullet_y = $03 ; Bullet Y position
score = $07 ; Score (0-99, BCD format)
enemy_x_tbl = $08 ; 3 bytes ($08, $09, $0a)
enemy_y_tbl = $0b ; 3 bytes ($0b, $0c, $0d)
flash_tbl = $0e ; 3 bytes ($0e, $0f, $10)
game_state = $11 ; 0 = playing, 1 = game over
; ------------------------------------------------
; BASIC stub
; ------------------------------------------------
*= $0801
!byte $0c,$08,$0a,$00,$9e,$32,$30,$36,$31,$00,$00,$00
; ------------------------------------------------
; One-time hardware setup
; ------------------------------------------------
*= $080d
; Black screen
lda #$00
sta $d020 ; Border colour
sta $d021 ; Background colour
; Sprite 1 colour (yellow, never changes)
lda #$07
sta $d028
; Set score colour to white (persists across restarts)
lda #$01
sta $d800
sta $d801
; SID setup — voice 1 laser sound
lda #$0f
sta $d418 ; Volume to maximum
lda #$00
sta $d400 ; Frequency low byte
lda #$10
sta $d401 ; Frequency high byte ($1000 = mid-high pitch)
lda #$09
sta $d405 ; Attack=0, Decay=9
lda #$00
sta $d406 ; Sustain=0, Release=0
; First-time init
jsr clear_screen
jsr init_game
!ifdef SCREENSHOT_MODE {
; Set a visible score
lda #$07
sta score
lda #$30
sta $0400
lda #$37
sta $0401
; Set game over state
lda #$01
sta game_state
; Ship is dead — turn red
lda #$02
sta $d027
; Show GAME OVER text
jsr show_game_over
; Enable sprites (ship + 3 enemies, no bullet)
lda #%00011101
sta $d015
}
; ------------------------------------------------
; Game loop — runs once per frame
; ------------------------------------------------
game_loop:
; Wait for the raster beam to reach line 255
- lda $d012
cmp #$ff
bne -
; --- Check game state ---
lda game_state
beq game_active
; --- Game over: poll fire button ---
lda $dc00
and #%00010000
bne game_loop ; Not pressed, keep waiting
; Fire pressed — restart the game
jsr clear_screen
jsr init_game
jmp game_loop
game_active:
; --- Read joystick and move ship ---
; UP (bit 0)
lda $dc00
and #%00000001
bne not_up
dec $d001
dec $d001
not_up:
; DOWN (bit 1)
lda $dc00
and #%00000010
bne not_down
inc $d001
inc $d001
not_down:
; LEFT (bit 2)
lda $dc00
and #%00000100
bne not_left
dec $d000
dec $d000
not_left:
; RIGHT (bit 3)
lda $dc00
and #%00001000
bne not_right
inc $d000
inc $d000
not_right:
; --- Fire button (bit 4) ---
lda $dc00
and #%00010000
bne no_fire ; Bit is 1 = NOT pressed
; Fire is pressed — only spawn if no bullet active
lda bullet_active
bne no_fire ; Already active, skip
; Spawn bullet at ship position
lda $d000 ; Ship X → bullet X
sta $d002
lda $d001 ; Ship Y → bullet Y
sta bullet_y
; Enable sprite 1 (keep other sprites enabled)
lda $d015
ora #%00000010
sta $d015
; Mark bullet active
lda #$01
sta bullet_active
; Trigger laser sound (gate off then on to retrigger)
lda #$20
sta $d404 ; Sawtooth, gate OFF (reset envelope)
lda #$21
sta $d404 ; Sawtooth, gate ON (start sound)
no_fire:
; --- Update bullet ---
lda bullet_active
beq no_bullet ; Not active, skip
; Move bullet up (4 pixels per frame)
lda bullet_y
sec
sbc #$04
sta bullet_y
sta $d003 ; Update sprite 1 Y position
; Check if bullet has left the screen (Y < 30)
cmp #$1e
bcs no_bullet ; Y >= 30, still on screen
; Deactivate bullet
lda #$00
sta bullet_active
; Disable sprite 1 (keep other sprites enabled)
lda $d015
and #%11111101
sta $d015
no_bullet:
; --- Check bullet-enemy collision ---
lda bullet_active
bne check_collision
jmp no_hit
check_collision:
ldx #$00
collision_loop:
lda flash_tbl,x
bne next_collision ; Skip flashing enemies
; Check Y distance
lda bullet_y
sec
sbc enemy_y_tbl,x
cmp #$10
bcc check_x
cmp #$f0
bcc next_collision
check_x:
; Check X distance
lda $d002 ; Bullet X (sprite 1)
sec
sbc enemy_x_tbl,x
cmp #$10
bcc hit_enemy
cmp #$f0
bcc next_collision
jmp hit_enemy ; >= $F0 = close (negative)
next_collision:
inx
cpx #$03
bne collision_loop
jmp no_hit
hit_enemy:
; X = index of hit enemy
; Deactivate bullet
lda #$00
sta bullet_active
lda $d015
and #%11111101
sta $d015
; Flash this enemy white
lda #$08
sta flash_tbl,x
ldy sprite_colour_off,x
lda #$01
sta $d000,y ; White
; Explosion sound — SID voice 2 (noise)
lda #$00
sta $d407 ; Frequency low
lda #$08
sta $d408 ; Frequency high (low rumble)
lda #$09
sta $d40c ; Attack=0, Decay=9
lda #$00
sta $d40d ; Sustain=0, Release=0
lda #$80
sta $d40b ; Noise, gate OFF (reset envelope)
lda #$81
sta $d40b ; Noise, gate ON (trigger)
; Increment score (BCD)
sed ; Decimal mode
lda score
clc
adc #$01
sta score
cld ; Back to binary mode
; Update score display — tens digit
lda score
lsr
lsr
lsr
lsr ; High nybble in A (0-9)
clc
adc #$30 ; Convert to screen code
sta $0400 ; Write tens digit
; Update score display — ones digit
lda score
and #$0f ; Low nybble in A (0-9)
clc
adc #$30 ; Convert to screen code
sta $0401 ; Write ones digit
no_hit:
; --- Update all enemies ---
ldx #$00
enemy_loop:
lda flash_tbl,x
bne do_flash
; Move down 1 pixel per frame
lda enemy_y_tbl,x
clc
adc #$01
sta enemy_y_tbl,x
; Off-screen check (Y >= 248)
cmp #$f8
bcc update_sprite
; Respawn at top
lda #$32
jsr spawn_enemy
jmp next_enemy
do_flash:
dec flash_tbl,x
bne update_sprite ; Still flashing
; Flash done — respawn
lda #$32
jsr spawn_enemy
jmp next_enemy
update_sprite:
; Copy position to VIC-II
ldy sprite_pos_off,x
lda enemy_x_tbl,x
sta $d000,y
lda enemy_y_tbl,x
sta $d001,y
next_enemy:
inx
cpx #$03
bne enemy_loop
; --- Check ship-enemy collision ---
ldx #$00
ship_collision_loop:
lda flash_tbl,x
bne next_ship_check ; Skip flashing enemies
; Check Y distance (ship Y vs enemy Y)
lda $d001 ; Ship Y
sec
sbc enemy_y_tbl,x
cmp #$10
bcc check_ship_x
cmp #$f0
bcc next_ship_check
check_ship_x:
; Check X distance (ship X vs enemy X)
lda $d000 ; Ship X
sec
sbc enemy_x_tbl,x
cmp #$10
bcc ship_hit
cmp #$f0
bcc next_ship_check
jmp ship_hit ; >= $F0 = close (negative)
next_ship_check:
inx
cpx #$03
bne ship_collision_loop
jmp game_loop
ship_hit:
; Ship is destroyed
lda #$01
sta game_state
; Turn ship red
lda #$02
sta $d027
; Show GAME OVER text
jsr show_game_over
; Death sound — SID voice 3 (descending sawtooth)
lda #$00
sta $d40e ; Frequency low
lda #$10
sta $d40f ; Frequency high
lda #$0a
sta $d412 ; Attack=0, Decay=10 (long decay)
lda #$00
sta $d413 ; Sustain=0, Release=0
lda #$20
sta $d411 ; Sawtooth, gate OFF (reset envelope)
lda #$21
sta $d411 ; Sawtooth, gate ON (trigger)
jmp game_loop
; ------------------------------------------------
; Subroutine: clear_screen
; Fills screen RAM with spaces ($20)
; ------------------------------------------------
clear_screen:
ldx #$00
- lda #$20
sta $0400,x
sta $0500,x
sta $0600,x
sta $0700,x
inx
bne -
rts
; ------------------------------------------------
; Subroutine: init_game
; Resets all game state for a new game
; ------------------------------------------------
init_game:
; Sprite data pointers (must be set after clear_screen)
lda #128
sta $07f8 ; Ship (block 128 = $2000)
lda #129
sta $07f9 ; Bullet (block 129 = $2040)
lda #130
sta $07fa ; Enemy 0 (block 130 = $2080)
sta $07fb ; Enemy 1
sta $07fc ; Enemy 2
; Ship position and colour
lda #172
sta $d000 ; X position
lda #220
sta $d001 ; Y position
lda #$01
sta $d027 ; Colour (white)
; Enemy colours
lda #$05
sta $d029
sta $d02a
sta $d02b
; Spawn three enemies at staggered heights
lda #$32 ; Top
ldx #$00
jsr spawn_enemy
lda #$82 ; Middle
ldx #$01
jsr spawn_enemy
lda #$d2 ; Lower
ldx #$02
jsr spawn_enemy
; Enable sprites 0, 2, 3, 4 (bullet starts disabled)
lda #%00011101
sta $d015
; Reset state
lda #$00
sta bullet_active
sta game_state
sta score
; Score display
lda #$30
sta $0400
sta $0401
rts
; ------------------------------------------------
; Subroutine: show_game_over
; Writes "GAME OVER" to screen RAM, row 12, col 16
; ------------------------------------------------
show_game_over:
; Screen codes: G=7, A=1, M=13, E=5, space=32, O=15, V=22, E=5, R=18
; Position: row 12 × 40 + col 16 = 496 → $0400 + $01F0 = $05F0
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 to white
lda #$01
sta $d9f0
sta $d9f1
sta $d9f2
sta $d9f3
sta $d9f4
sta $d9f5
sta $d9f6
sta $d9f7
sta $d9f8
rts
; ------------------------------------------------
; Subroutine: spawn_enemy
; A = starting Y position, X = enemy index
; ------------------------------------------------
spawn_enemy:
sta enemy_y_tbl,x
; Random X from raster
lda $d012
and #$7f
clc
adc #$30
sta enemy_x_tbl,x
; Clear flash timer
lda #$00
sta flash_tbl,x
; Restore sprite colour to green
ldy sprite_colour_off,x
lda #$05
sta $d000,y
; Update VIC-II sprite position
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 (offset + 1)
rts
; ------------------------------------------------
; Lookup tables
; ------------------------------------------------
sprite_pos_off:
!byte $04, $06, $08 ; VIC-II X-position offsets for sprites 2, 3, 4
sprite_colour_off:
!byte $29, $2a, $2b ; VIC-II colour register offsets
; ------------------------------------------------
; Sprite data at $2000 (block 128) — ship
; ------------------------------------------------
*= $2000
!byte $00,$18,$00 ; ##
!byte $00,$3c,$00 ; ####
!byte $00,$3c,$00 ; ####
!byte $00,$7e,$00 ; ######
!byte $00,$7e,$00 ; ######
!byte $00,$ff,$00 ; ########
!byte $00,$ff,$00 ; ########
!byte $01,$ff,$80 ; ##########
!byte $03,$ff,$c0 ; ############
!byte $07,$ff,$e0 ; ##############
!byte $07,$ff,$e0 ; ##############
!byte $07,$e7,$e0 ; ###..####..###
!byte $03,$c3,$c0 ; ##....##....##
!byte $01,$ff,$80 ; ##########
!byte $00,$ff,$00 ; ########
!byte $00,$ff,$00 ; ########
!byte $00,$db,$00 ; ##.##.##
!byte $00,$db,$00 ; ##.##.##
!byte $00,$66,$00 ; ##..##
!byte $00,$24,$00 ; #..#
!byte $00,$00,$00 ;
; ------------------------------------------------
; Sprite data at $2040 (block 129) — bullet
; ------------------------------------------------
*= $2040
!byte $00,$00,$00 ;
!byte $00,$00,$00 ;
!byte $00,$00,$00 ;
!byte $00,$00,$00 ;
!byte $00,$00,$00 ;
!byte $00,$00,$00 ;
!byte $00,$00,$00 ;
!byte $00,$18,$00 ; ##
!byte $00,$18,$00 ; ##
!byte $00,$18,$00 ; ##
!byte $00,$18,$00 ; ##
!byte $00,$18,$00 ; ##
!byte $00,$18,$00 ; ##
!byte $00,$00,$00 ;
!byte $00,$00,$00 ;
!byte $00,$00,$00 ;
!byte $00,$00,$00 ;
!byte $00,$00,$00 ;
!byte $00,$00,$00 ;
!byte $00,$00,$00 ;
!byte $00,$00,$00 ;
; ------------------------------------------------
; Sprite data at $2080 (block 130) — enemy
; ------------------------------------------------
*= $2080
!byte $00,$66,$00 ; ##..##
!byte $00,$3c,$00 ; ####
!byte $00,$7e,$00 ; ######
!byte $00,$db,$00 ; ##.##.##
!byte $00,$ff,$00 ; ########
!byte $01,$ff,$80 ; ##########
!byte $01,$7e,$80 ; #.######.#
!byte $01,$3c,$80 ; #..####..#
!byte $00,$a5,$00 ; #.#..#.#
!byte $01,$81,$80 ; ##......##
!byte $00,$00,$00 ;
!byte $00,$00,$00 ;
!byte $00,$00,$00 ;
!byte $00,$00,$00 ;
!byte $00,$00,$00 ;
!byte $00,$00,$00 ;
!byte $00,$00,$00 ;
!byte $00,$00,$00 ;
!byte $00,$00,$00 ;
!byte $00,$00,$00 ;
!byte $00,$00,$00 ;
If It Doesn’t Work
- No text appears? Check the screen code values — G is
$07, not$47. Screen codes differ from PETSCII. Also check colour RAM writes at$D9F0+. - Text in the wrong position? Row 12, column 16 = offset 496 =
$01F0. The address is$05F0, not$04F0. - Fire doesn’t restart? The game over state must poll
$DC00bit 4. Check theAND #%00010000andBNEback togame_loop. - Score persists after restart?
init_gamemust write$30to both$0400and$0401, and setscoreto 0. - Enemies don’t reset?
init_gamemust callspawn_enemyfor all three enemies with staggered Y values. - “GAME OVER” still visible after restart?
clear_screenmust run beforeinit_game. The screen clear wipes the text;init_gamethen writes the fresh “00” score.
Try This: Coloured Text
Write different colour values for each letter. Red for “GAME”, white for the space, green for “OVER”. Nine colour bytes — nine chances to experiment.
Try This: Flash the Text
Toggle the colour between white and another colour every 8 frames. Read a frame counter, AND #$08, and branch to set either colour. The text blinks, demanding attention.
What You’ve Learnt
- Screen codes — the VIC-II’s character mapping (A=1, B=2, not the same as PETSCII).
- Calculated screen positions — row × 40 + column +
$0400for any cell on the 40×25 grid. - Subroutine extraction —
init_gameandclear_screencalled from both boot and restart. - Game state with restart — polling fire during game over, resetting all state cleanly.
What’s Next
One death ends the game. In Unit 12, the player gets three lives — each death costs one, and game over waits until the last life is spent.