Game 1 Unit 9 of 64 1 hr learning time

Collision Detection

Detect when player touches enemies with visual feedback.

14% of Neon Nexus

Mark as complete Completed!

What You’re Building

Collision detection. Touch an enemy and you flash - a warning that danger is real.

Collision Detection

Actions now have consequences.

Bounding Box Collision

The simplest collision test: do two rectangles overlap?

For 8×8 sprites, we check if the distance between centres is less than a threshold:

COLLISION_DIST = 6       ; Pixels of overlap needed

check_collisions:
    ldx #0
@check_enemy:
    ; Check X distance: |player_x - enemy_x| < COLLISION_DIST
    lda player_x
    sec
    sbc enemy_x, x
    bpl @check_x_positive
    ; Negative result - make it positive
    eor #$FF
    clc
    adc #1
@check_x_positive:
    cmp #COLLISION_DIST
    bcs @next_enemy      ; Too far apart in X

    ; Same check for Y...

Absolute Value

The 6502 has no absolute value instruction. We compute it manually:

Subtract to get difference (may be negative)
If result is negative (bit 7 set), negate it
Negate: EOR #$FF then ADC #1 (two’s complement)

Visual Feedback

When collision is detected, the player flashes:

collision_flag: .res 1     ; Non-zero if hit
flash_timer:    .res 1     ; Frames remaining

; On collision:
    lda #1
    sta collision_flag
    lda #30              ; Flash for 30 frames
    sta flash_timer

The Flash Effect

Toggle sprite visibility every few frames:

update_player_sprite:
    lda flash_timer
    beq @no_flash

    dec flash_timer

    ; Flash every 4 frames
    and #%00000100
    beq @show_player

    ; Hide: move Y off screen
    lda #$FF
    sta oam_buffer+0
    rts

@show_player:
    lda player_y
    sta oam_buffer+0
    rts

Setting Y to $FF moves the sprite below the visible screen.

The Code

; =============================================================================
; NEON NEXUS - Unit 9: Collision Detection
; =============================================================================
; Detect when player touches enemies. Visual feedback on collision.
; =============================================================================

; -----------------------------------------------------------------------------
; NES Hardware Addresses
; -----------------------------------------------------------------------------
PPUCTRL   = $2000
PPUMASK   = $2001
PPUSTATUS = $2002
OAMADDR   = $2003
PPUSCROLL = $2005
PPUADDR   = $2006
PPUDATA   = $2007
OAMDMA    = $4014

JOYPAD1   = $4016
JOYPAD2   = $4017

; Controller buttons
BTN_A      = %10000000
BTN_B      = %01000000
BTN_SELECT = %00100000
BTN_START  = %00010000
BTN_UP     = %00001000
BTN_DOWN   = %00000100
BTN_LEFT   = %00000010
BTN_RIGHT  = %00000001

; -----------------------------------------------------------------------------
; Game Constants
; -----------------------------------------------------------------------------
PLAYER_START_X = 124
PLAYER_START_Y = 116
PLAYER_SPEED   = 2
ENEMY_SPEED    = 1

NUM_ENEMIES    = 4
SPRITE_SIZE    = 8       ; 8x8 pixel sprites
COLLISION_DIST = 6       ; Overlap distance for collision

; Tile indices
TILE_EMPTY     = 0
TILE_BORDER    = 1
TILE_FLOOR     = 2
TILE_CORNER_TL = 3
TILE_CORNER_TR = 4
TILE_CORNER_BL = 5
TILE_CORNER_BR = 6

SPRITE_PLAYER  = 7
SPRITE_ENEMY   = 8

; Arena boundaries
ARENA_LEFT   = 16
ARENA_RIGHT  = 232
ARENA_TOP    = 16
ARENA_BOTTOM = 208

DIR_RIGHT = 1
DIR_LEFT  = $FF
DIR_DOWN  = 1
DIR_UP    = $FF

BG_COLOUR = $0F

; -----------------------------------------------------------------------------
; Memory Layout
; -----------------------------------------------------------------------------
.segment "ZEROPAGE"
player_x:      .res 1
player_y:      .res 1
buttons:       .res 1
temp:          .res 1
temp2:         .res 1
row_counter:   .res 1
frame_count:   .res 1
collision_flag: .res 1     ; Non-zero if collision detected
flash_timer:   .res 1      ; Timer for visual feedback

enemy_x:       .res NUM_ENEMIES
enemy_y:       .res NUM_ENEMIES
enemy_dir_x:   .res NUM_ENEMIES
enemy_dir_y:   .res NUM_ENEMIES

.segment "OAM"
oam_buffer:    .res 256

.segment "BSS"

; -----------------------------------------------------------------------------
; iNES Header
; -----------------------------------------------------------------------------
.segment "HEADER"
    .byte "NES", $1A
    .byte 2
    .byte 1
    .byte $01
    .byte $00
    .byte 0,0,0,0,0,0,0,0

; -----------------------------------------------------------------------------
; Code
; -----------------------------------------------------------------------------
.segment "CODE"

reset:
    sei
    cld
    ldx #$40
    stx $4017
    ldx #$FF
    txs
    inx
    stx PPUCTRL
    stx PPUMASK
    stx $4010

@vblank1:
    bit PPUSTATUS
    bpl @vblank1

    lda #0
@clear_ram:
    sta $0000, x
    sta $0100, x
    sta $0200, x
    sta $0300, x
    sta $0400, x
    sta $0500, x
    sta $0600, x
    sta $0700, x
    inx
    bne @clear_ram

@vblank2:
    bit PPUSTATUS
    bpl @vblank2

    jsr load_palette
    jsr draw_arena
    jsr set_attributes
    jsr init_enemies

    lda #PLAYER_START_X
    sta player_x
    lda #PLAYER_START_Y
    sta player_y

    lda #0
    sta collision_flag
    sta flash_timer

    ; Player sprite
    lda player_y
    sta oam_buffer+0
    lda #SPRITE_PLAYER
    sta oam_buffer+1
    lda #0
    sta oam_buffer+2
    lda player_x
    sta oam_buffer+3

    jsr update_enemy_sprites

    ; Hide remaining sprites
    lda #$FF
    ldx #20
@hide_sprites:
    sta oam_buffer, x
    inx
    bne @hide_sprites

    lda #0
    sta PPUSCROLL
    sta PPUSCROLL
    sta frame_count

    lda #%10000000
    sta PPUCTRL
    lda #%00011110
    sta PPUMASK

main_loop:
    jsr read_controller
    jsr move_player
    jsr move_enemies
    jsr check_collisions
    jsr update_player_sprite
    jsr update_enemy_sprites
    jmp main_loop

; -----------------------------------------------------------------------------
; Check Collisions - Player vs all enemies
; -----------------------------------------------------------------------------
check_collisions:
    lda #0
    sta collision_flag

    ldx #0
@check_enemy:
    ; Check X overlap
    ; |player_x - enemy_x| < COLLISION_DIST
    lda player_x
    sec
    sbc enemy_x, x
    bpl @check_x_positive
    ; Negative - negate it
    eor #$FF
    clc
    adc #1
@check_x_positive:
    cmp #COLLISION_DIST
    bcs @next_enemy      ; No X overlap

    ; Check Y overlap
    lda player_y
    sec
    sbc enemy_y, x
    bpl @check_y_positive
    eor #$FF
    clc
    adc #1
@check_y_positive:
    cmp #COLLISION_DIST
    bcs @next_enemy      ; No Y overlap

    ; Collision detected!
    lda #1
    sta collision_flag
    lda #30              ; Flash for 30 frames
    sta flash_timer
    rts                  ; Exit early on first collision

@next_enemy:
    inx
    cpx #NUM_ENEMIES
    bne @check_enemy
    rts

; -----------------------------------------------------------------------------
; Update Player Sprite - Flash when hit
; -----------------------------------------------------------------------------
update_player_sprite:
    ; Update position
    lda player_y
    sta oam_buffer+0
    lda player_x
    sta oam_buffer+3

    ; Check if flashing
    lda flash_timer
    beq @no_flash

    ; Decrement timer
    dec flash_timer

    ; Flash by toggling visibility every 4 frames
    and #%00000100
    beq @show_player
    ; Hide player (move off screen)
    lda #$FF
    sta oam_buffer+0
    rts

@show_player:
    lda player_y
    sta oam_buffer+0
    rts

@no_flash:
    rts

; -----------------------------------------------------------------------------
; Initialise Enemies
; -----------------------------------------------------------------------------
init_enemies:
    lda #48
    sta enemy_x+0
    lda #48
    sta enemy_y+0
    lda #DIR_RIGHT
    sta enemy_dir_x+0
    lda #DIR_DOWN
    sta enemy_dir_y+0

    lda #200
    sta enemy_x+1
    lda #48
    sta enemy_y+1
    lda #DIR_LEFT
    sta enemy_dir_x+1
    lda #DIR_DOWN
    sta enemy_dir_y+1

    lda #48
    sta enemy_x+2
    lda #176
    sta enemy_y+2
    lda #DIR_RIGHT
    sta enemy_dir_x+2
    lda #DIR_UP
    sta enemy_dir_y+2

    lda #200
    sta enemy_x+3
    lda #176
    sta enemy_y+3
    lda #DIR_LEFT
    sta enemy_dir_x+3
    lda #DIR_UP
    sta enemy_dir_y+3

    rts

; -----------------------------------------------------------------------------
; Move Enemies
; -----------------------------------------------------------------------------
move_enemies:
    ldx #0

@enemy_loop:
    lda enemy_x, x
    clc
    adc enemy_dir_x, x
    sta enemy_x, x

    cmp #ARENA_LEFT
    bcs @check_right
    lda #DIR_RIGHT
    sta enemy_dir_x, x
    lda #ARENA_LEFT
    sta enemy_x, x
    jmp @move_y

@check_right:
    cmp #ARENA_RIGHT
    bcc @move_y
    lda #DIR_LEFT
    sta enemy_dir_x, x
    lda #ARENA_RIGHT
    sec
    sbc #1
    sta enemy_x, x

@move_y:
    lda enemy_y, x
    clc
    adc enemy_dir_y, x
    sta enemy_y, x

    cmp #ARENA_TOP
    bcs @check_bottom
    lda #DIR_DOWN
    sta enemy_dir_y, x
    lda #ARENA_TOP
    sta enemy_y, x
    jmp @next_enemy

@check_bottom:
    cmp #ARENA_BOTTOM
    bcc @next_enemy
    lda #DIR_UP
    sta enemy_dir_y, x
    lda #ARENA_BOTTOM
    sec
    sbc #1
    sta enemy_y, x

@next_enemy:
    inx
    cpx #NUM_ENEMIES
    bne @enemy_loop
    rts

; -----------------------------------------------------------------------------
; Update Enemy Sprites
; -----------------------------------------------------------------------------
update_enemy_sprites:
    ldx #0
    ldy #4

@loop:
    lda enemy_y, x
    sta oam_buffer, y
    iny
    lda #SPRITE_ENEMY
    sta oam_buffer, y
    iny
    lda #%00000001
    sta oam_buffer, y
    iny
    lda enemy_x, x
    sta oam_buffer, y
    iny
    inx
    cpx #NUM_ENEMIES
    bne @loop
    rts

; -----------------------------------------------------------------------------
; Load Palette
; -----------------------------------------------------------------------------
load_palette:
    bit PPUSTATUS
    lda #$3F
    sta PPUADDR
    lda #$00
    sta PPUADDR
    ldx #0
@loop:
    lda palette_data, x
    sta PPUDATA
    inx
    cpx #32
    bne @loop
    rts

; -----------------------------------------------------------------------------
; Draw Arena
; -----------------------------------------------------------------------------
draw_arena:
    bit PPUSTATUS
    lda #$20
    sta PPUADDR
    lda #$00
    sta PPUADDR

    lda #0
    sta row_counter

@draw_row:
    lda row_counter
    cmp #0
    beq @top_row
    cmp #1
    beq @top_row
    cmp #28
    beq @bottom_row
    cmp #29
    beq @bottom_row
    jmp @middle_row

@top_row:
    lda row_counter
    cmp #0
    bne @top_row_inner
    lda #TILE_CORNER_TL
    sta PPUDATA
    lda #TILE_BORDER
    ldx #30
@top_fill:
    sta PPUDATA
    dex
    bne @top_fill
    lda #TILE_CORNER_TR
    sta PPUDATA
    jmp @next_row

@top_row_inner:
    lda #TILE_BORDER
    ldx #32
@top_inner_fill:
    sta PPUDATA
    dex
    bne @top_inner_fill
    jmp @next_row

@bottom_row:
    lda row_counter
    cmp #29
    bne @bottom_row_inner
    lda #TILE_CORNER_BL
    sta PPUDATA
    lda #TILE_BORDER
    ldx #30
@bottom_fill:
    sta PPUDATA
    dex
    bne @bottom_fill
    lda #TILE_CORNER_BR
    sta PPUDATA
    jmp @next_row

@bottom_row_inner:
    lda #TILE_BORDER
    ldx #32
@bottom_inner_fill:
    sta PPUDATA
    dex
    bne @bottom_inner_fill
    jmp @next_row

@middle_row:
    lda #TILE_BORDER
    sta PPUDATA
    sta PPUDATA
    lda #TILE_FLOOR
    ldx #28
@floor_fill:
    sta PPUDATA
    dex
    bne @floor_fill
    lda #TILE_BORDER
    sta PPUDATA
    sta PPUDATA

@next_row:
    inc row_counter
    lda row_counter
    cmp #30
    beq @done_drawing
    jmp @draw_row

@done_drawing:
    rts

; -----------------------------------------------------------------------------
; Set Attribute Table
; -----------------------------------------------------------------------------
set_attributes:
    bit PPUSTATUS
    lda #$23
    sta PPUADDR
    lda #$C0
    sta PPUADDR

    ldx #8
    lda #$00
@attr_top:
    sta PPUDATA
    dex
    bne @attr_top

    ldx #6
@attr_floor_rows:
    lda #$00
    sta PPUDATA
    lda #%01010101
    sta PPUDATA
    sta PPUDATA
    sta PPUDATA
    sta PPUDATA
    sta PPUDATA
    sta PPUDATA
    lda #$00
    sta PPUDATA
    dex
    bne @attr_floor_rows

    ldx #8
    lda #$00
@attr_bottom:
    sta PPUDATA
    dex
    bne @attr_bottom
    rts

; -----------------------------------------------------------------------------
; Read Controller
; -----------------------------------------------------------------------------
read_controller:
    lda #1
    sta JOYPAD1
    lda #0
    sta JOYPAD1
    ldx #8
@read_loop:
    lda JOYPAD1
    lsr a
    rol buttons
    dex
    bne @read_loop
    rts

; -----------------------------------------------------------------------------
; Move Player
; -----------------------------------------------------------------------------
move_player:
    lda buttons
    and #BTN_UP
    beq @check_down
    lda player_y
    sec
    sbc #PLAYER_SPEED
    cmp #ARENA_TOP
    bcc @check_down
    sta player_y

@check_down:
    lda buttons
    and #BTN_DOWN
    beq @check_left
    lda player_y
    clc
    adc #PLAYER_SPEED
    cmp #ARENA_BOTTOM
    bcs @check_left
    sta player_y

@check_left:
    lda buttons
    and #BTN_LEFT
    beq @check_right
    lda player_x
    sec
    sbc #PLAYER_SPEED
    cmp #ARENA_LEFT
    bcc @check_right
    sta player_x

@check_right:
    lda buttons
    and #BTN_RIGHT
    beq @done
    lda player_x
    clc
    adc #PLAYER_SPEED
    cmp #ARENA_RIGHT
    bcs @done
    sta player_x

@done:
    rts

; === NMI ===
nmi:
    pha
    txa
    pha
    tya
    pha

    lda #0
    sta OAMADDR
    lda #>oam_buffer
    sta OAMDMA

    inc frame_count

    lda #0
    sta PPUSCROLL
    sta PPUSCROLL

    pla
    tay
    pla
    tax
    pla
    rti

irq:
    rti

; -----------------------------------------------------------------------------
; Data
; -----------------------------------------------------------------------------
palette_data:
    .byte BG_COLOUR, $11, $21, $31
    .byte BG_COLOUR, $13, $23, $33
    .byte BG_COLOUR, $19, $29, $39
    .byte BG_COLOUR, $16, $26, $36
    .byte BG_COLOUR, $30, $27, $17
    .byte BG_COLOUR, $16, $26, $36
    .byte BG_COLOUR, $30, $27, $17
    .byte BG_COLOUR, $30, $27, $17

; -----------------------------------------------------------------------------
; Vectors
; -----------------------------------------------------------------------------
.segment "VECTORS"
    .word nmi
    .word reset
    .word irq

; -----------------------------------------------------------------------------
; CHR-ROM
; -----------------------------------------------------------------------------
.segment "CHARS"

; Tile 0: Empty
.byte $00,$00,$00,$00,$00,$00,$00,$00
.byte $00,$00,$00,$00,$00,$00,$00,$00

; Tile 1: Border
.byte %11111111,%10000001,%10000001,%11111111
.byte %11111111,%00010001,%00010001,%11111111
.byte %00000000,%01111110,%01111110,%00000000
.byte %00000000,%11101110,%11101110,%00000000

; Tile 2: Floor
.byte %00000000,%00000000,%00000000,%00000000
.byte %00000000,%00000000,%00000000,%10000001
.byte %00000000,%00000000,%00000000,%00000000
.byte %00000000,%00000000,%00000000,%00000000

; Tile 3: Corner TL
.byte %11111111,%11000000,%10100000,%10010000
.byte %10001000,%10000100,%10000010,%10000001
.byte %00000000,%00111111,%01011111,%01101111
.byte %01110111,%01111011,%01111101,%01111110

; Tile 4: Corner TR
.byte %11111111,%00000011,%00000101,%00001001
.byte %00010001,%00100001,%01000001,%10000001
.byte %00000000,%11111100,%11111010,%11110110
.byte %11101110,%11011110,%10111110,%01111110

; Tile 5: Corner BL
.byte %10000001,%10000010,%10000100,%10001000
.byte %10010000,%10100000,%11000000,%11111111
.byte %01111110,%01111101,%01111011,%01110111
.byte %01101111,%01011111,%00111111,%00000000

; Tile 6: Corner BR
.byte %10000001,%01000001,%00100001,%00010001
.byte %00001001,%00000101,%00000011,%11111111
.byte %01111110,%10111110,%11011110,%11101110
.byte %11110110,%11111010,%11111100,%00000000

; Tile 7: Player
.byte %00011000,%00011000,%00111100,%01111110
.byte %11111111,%10111101,%00100100,%00100100
.byte %00000000,%00011000,%00011000,%00111100
.byte %01000010,%01000010,%00011000,%00000000

; Tile 8: Enemy
.byte %00011000,%00111100,%01111110,%11111111
.byte %11111111,%01111110,%00111100,%00011000
.byte %00000000,%00011000,%00100100,%01000010
.byte %01000010,%00100100,%00011000,%00000000

; Fill rest
.res 8192 - 144, $00

Build It

ca65 nexus.asm -o nexus.o
ld65 -C nes.cfg nexus.o -o nexus.nes

Touch an enemy and the player flashes. The game now reacts to your mistakes.

Flashing is a warning. Unit 10 adds real consequences: lives and death.

What Changed

Unit 8 → Unit 9

+109-52

1	1		`; =============================================================================`
2		-	`; NEON NEXUS - Unit 8: Enemy Movement`
	2	+	`; NEON NEXUS - Unit 9: Collision Detection`
3	3		`; =============================================================================`
4		-	`; Make enemies patrol the arena with bounce patterns.`
	4	+	`; Detect when player touches enemies. Visual feedback on collision.`
5	5		`; =============================================================================`
6	6
7	7		`; -----------------------------------------------------------------------------`
			...
38	38		`ENEMY_SPEED = 1`
39	39
40	40		`NUM_ENEMIES = 4`
	41	+	`SPRITE_SIZE = 8 ; 8x8 pixel sprites`
	42	+	`COLLISION_DIST = 6 ; Overlap distance for collision`
41	43
42	44		`; Tile indices`
43	45		`TILE_EMPTY = 0`
			...
57	59		`ARENA_TOP = 16`
58	60		`ARENA_BOTTOM = 208`
59	61
60		-	`; Direction flags`
61	62		`DIR_RIGHT = 1`
62		-	`DIR_LEFT = $FF ; -1 in two's complement`
	63	+	`DIR_LEFT = $FF`
63	64		`DIR_DOWN = 1`
64	65		`DIR_UP = $FF`
65	66
			...
69	70		`; Memory Layout`
70	71		`; -----------------------------------------------------------------------------`
71	72		`.segment "ZEROPAGE"`
72		-	`player_x: .res 1`
73		-	`player_y: .res 1`
74		-	`buttons: .res 1`
75		-	`temp: .res 1`
76		-	`row_counter: .res 1`
77		-	`frame_count: .res 1`
	73	+	`player_x: .res 1`
	74	+	`player_y: .res 1`
	75	+	`buttons: .res 1`
	76	+	`temp: .res 1`
	77	+	`temp2: .res 1`
	78	+	`row_counter: .res 1`
	79	+	`frame_count: .res 1`
	80	+	`collision_flag: .res 1 ; Non-zero if collision detected`
	81	+	`flash_timer: .res 1 ; Timer for visual feedback`
78	82
79		-	`; Enemy data`
80		-	`enemy_x: .res NUM_ENEMIES`
81		-	`enemy_y: .res NUM_ENEMIES`
82		-	`enemy_dir_x: .res NUM_ENEMIES ; +1 or -1`
83		-	`enemy_dir_y: .res NUM_ENEMIES ; +1 or -1`
	83	+	`enemy_x: .res NUM_ENEMIES`
	84	+	`enemy_y: .res NUM_ENEMIES`
	85	+	`enemy_dir_x: .res NUM_ENEMIES`
	86	+	`enemy_dir_y: .res NUM_ENEMIES`
84	87
85	88		`.segment "OAM"`
86		-	`oam_buffer: .res 256`
	89	+	`oam_buffer: .res 256`
87	90
88	91		`.segment "BSS"`
89	92
			...
145	148		`sta player_x`
146	149		`lda #PLAYER_START_Y`
147	150		`sta player_y`
	151	+
	152	+	`lda #0`
	153	+	`sta collision_flag`
	154	+	`sta flash_timer`
148	155
149	156		`; Player sprite`
150	157		`lda player_y`
			...
180	187		`jsr read_controller`
181	188		`jsr move_player`
182	189		`jsr move_enemies`
	190	+	`jsr check_collisions`
	191	+	`jsr update_player_sprite`
183	192		`jsr update_enemy_sprites`
184	193		`jmp main_loop`
185	194
186	195		`; -----------------------------------------------------------------------------`
187		-	`; Initialise Enemies with positions and directions`
	196	+	`; Check Collisions - Player vs all enemies`
	197	+	`; -----------------------------------------------------------------------------`
	198	+	`check_collisions:`
	199	+	`lda #0`
	200	+	`sta collision_flag`
	201	+
	202	+	`ldx #0`
	203	+	`@check_enemy:`
	204	+	`; Check X overlap`
	205	+	`; \|player_x - enemy_x\| < COLLISION_DIST`
	206	+	`lda player_x`
	207	+	`sec`
	208	+	`sbc enemy_x, x`
	209	+	`bpl @check_x_positive`
	210	+	`; Negative - negate it`
	211	+	`eor #$FF`
	212	+	`clc`
	213	+	`adc #1`
	214	+	`@check_x_positive:`
	215	+	`cmp #COLLISION_DIST`
	216	+	`bcs @next_enemy ; No X overlap`
	217	+
	218	+	`; Check Y overlap`
	219	+	`lda player_y`
	220	+	`sec`
	221	+	`sbc enemy_y, x`
	222	+	`bpl @check_y_positive`
	223	+	`eor #$FF`
	224	+	`clc`
	225	+	`adc #1`
	226	+	`@check_y_positive:`
	227	+	`cmp #COLLISION_DIST`
	228	+	`bcs @next_enemy ; No Y overlap`
	229	+
	230	+	`; Collision detected!`
	231	+	`lda #1`
	232	+	`sta collision_flag`
	233	+	`lda #30 ; Flash for 30 frames`
	234	+	`sta flash_timer`
	235	+	`rts ; Exit early on first collision`
	236	+
	237	+	`@next_enemy:`
	238	+	`inx`
	239	+	`cpx #NUM_ENEMIES`
	240	+	`bne @check_enemy`
	241	+	`rts`
	242	+
	243	+	`; -----------------------------------------------------------------------------`
	244	+	`; Update Player Sprite - Flash when hit`
	245	+	`; -----------------------------------------------------------------------------`
	246	+	`update_player_sprite:`
	247	+	`; Update position`
	248	+	`lda player_y`
	249	+	`sta oam_buffer+0`
	250	+	`lda player_x`
	251	+	`sta oam_buffer+3`
	252	+
	253	+	`; Check if flashing`
	254	+	`lda flash_timer`
	255	+	`beq @no_flash`
	256	+
	257	+	`; Decrement timer`
	258	+	`dec flash_timer`
	259	+
	260	+	`; Flash by toggling visibility every 4 frames`
	261	+	`and #%00000100`
	262	+	`beq @show_player`
	263	+	`; Hide player (move off screen)`
	264	+	`lda #$FF`
	265	+	`sta oam_buffer+0`
	266	+	`rts`
	267	+
	268	+	`@show_player:`
	269	+	`lda player_y`
	270	+	`sta oam_buffer+0`
	271	+	`rts`
	272	+
	273	+	`@no_flash:`
	274	+	`rts`
	275	+
	276	+	`; -----------------------------------------------------------------------------`
	277	+	`; Initialise Enemies`
188	278		`; -----------------------------------------------------------------------------`
189	279		`init_enemies:`
190		-	`; Enemy 0: Top-left, moving right and down`
191	280		`lda #48`
192	281		`sta enemy_x+0`
193	282		`lda #48`
			...
197	286		`lda #DIR_DOWN`
198	287		`sta enemy_dir_y+0`
199	288
200		-	`; Enemy 1: Top-right, moving left and down`
201	289		`lda #200`
202	290		`sta enemy_x+1`
203	291		`lda #48`
			...
207	295		`lda #DIR_DOWN`
208	296		`sta enemy_dir_y+1`
209	297
210		-	`; Enemy 2: Bottom-left, moving right and up`
211	298		`lda #48`
212	299		`sta enemy_x+2`
213	300		`lda #176`
			...
217	304		`lda #DIR_UP`
218	305		`sta enemy_dir_y+2`
219	306
220		-	`; Enemy 3: Bottom-right, moving left and up`
221	307		`lda #200`
222	308		`sta enemy_x+3`
223	309		`lda #176`
			...
230	316		`rts`
231	317
232	318		`; -----------------------------------------------------------------------------`
233		-	`; Move Enemies - Bounce off arena walls`
	319	+	`; Move Enemies`
234	320		`; -----------------------------------------------------------------------------`
235	321		`move_enemies:`
236	322		`ldx #0`
237	323
238	324		`@enemy_loop:`
239		-	`; Move X`
240	325		`lda enemy_x, x`
241	326		`clc`
242	327		`adc enemy_dir_x, x`
243	328		`sta enemy_x, x`
244	329
245		-	`; Check X bounds`
246	330		`cmp #ARENA_LEFT`
247	331		`bcs @check_right`
248		-	`; Hit left wall - reverse X direction`
249	332		`lda #DIR_RIGHT`
250	333		`sta enemy_dir_x, x`
251	334		`lda #ARENA_LEFT`
			...
255	338		`@check_right:`
256	339		`cmp #ARENA_RIGHT`
257	340		`bcc @move_y`
258		-	`; Hit right wall - reverse X direction`
259	341		`lda #DIR_LEFT`
260	342		`sta enemy_dir_x, x`
261	343		`lda #ARENA_RIGHT`
			...
264	346		`sta enemy_x, x`
265	347
266	348		`@move_y:`
267		-	`; Move Y`
268	349		`lda enemy_y, x`
269	350		`clc`
270	351		`adc enemy_dir_y, x`
271	352		`sta enemy_y, x`
272	353
273		-	`; Check Y bounds`
274	354		`cmp #ARENA_TOP`
275	355		`bcs @check_bottom`
276		-	`; Hit top wall - reverse Y direction`
277	356		`lda #DIR_DOWN`
278	357		`sta enemy_dir_y, x`
279	358		`lda #ARENA_TOP`
			...
283	362		`@check_bottom:`
284	363		`cmp #ARENA_BOTTOM`
285	364		`bcc @next_enemy`
286		-	`; Hit bottom wall - reverse Y direction`
287	365		`lda #DIR_UP`
288	366		`sta enemy_dir_y, x`
289	367		`lda #ARENA_BOTTOM`
			...
295	373		`inx`
296	374		`cpx #NUM_ENEMIES`
297	375		`bne @enemy_loop`
298		-
299	376		`rts`
300	377
301	378		`; -----------------------------------------------------------------------------`
302		-	`; Update Enemy Sprites in OAM`
	379	+	`; Update Enemy Sprites`
303	380		`; -----------------------------------------------------------------------------`
304	381		`update_enemy_sprites:`
305	382		`ldx #0`
			...
309	386		`lda enemy_y, x`
310	387		`sta oam_buffer, y`
311	388		`iny`
312		-
313	389		`lda #SPRITE_ENEMY`
314	390		`sta oam_buffer, y`
315	391		`iny`
316		-
317	392		`lda #%00000001`
318	393		`sta oam_buffer, y`
319	394		`iny`
320		-
321	395		`lda enemy_x, x`
322	396		`sta oam_buffer, y`
323	397		`iny`
324		-
325	398		`inx`
326	399		`cpx #NUM_ENEMIES`
327	400		`bne @loop`
328		-
329	401		`rts`
330	402
331	403		`; -----------------------------------------------------------------------------`
			...
337	409		`sta PPUADDR`
338	410		`lda #$00`
339	411		`sta PPUADDR`
340		-
341	412		`ldx #0`
342	413		`@loop:`
343	414		`lda palette_data, x`
			...
362	433
363	434		`@draw_row:`
364	435		`lda row_counter`
365		-
366	436		`cmp #0`
367	437		`beq @top_row`
368	438		`cmp #1`
369	439		`beq @top_row`
370		-
371	440		`cmp #28`
372	441		`beq @bottom_row`
373	442		`cmp #29`
374	443		`beq @bottom_row`
375		-
376	444		`jmp @middle_row`
377	445
378	446		`@top_row:`
379	447		`lda row_counter`
380	448		`cmp #0`
381	449		`bne @top_row_inner`
382		-
383	450		`lda #TILE_CORNER_TL`
384	451		`sta PPUDATA`
385	452		`lda #TILE_BORDER`
			...
405	472		`lda row_counter`
406	473		`cmp #29`
407	474		`bne @bottom_row_inner`
408		-
409	475		`lda #TILE_CORNER_BL`
410	476		`sta PPUDATA`
411	477		`lda #TILE_BORDER`
			...
431	497		`lda #TILE_BORDER`
432	498		`sta PPUDATA`
433	499		`sta PPUDATA`
434		-
435	500		`lda #TILE_FLOOR`
436	501		`ldx #28`
437	502		`@floor_fill:`
438	503		`sta PPUDATA`
439	504		`dex`
440	505		`bne @floor_fill`
441		-
442	506		`lda #TILE_BORDER`
443	507		`sta PPUDATA`
444	508		`sta PPUDATA`
			...
492	556		`sta PPUDATA`
493	557		`dex`
494	558		`bne @attr_bottom`
495		-
496	559		`rts`
497	560
498	561		`; -----------------------------------------------------------------------------`
			...
503	566		`sta JOYPAD1`
504	567		`lda #0`
505	568		`sta JOYPAD1`
506		-
507	569		`ldx #8`
508	570		`@read_loop:`
509	571		`lda JOYPAD1`
			...
575	637		`sta OAMADDR`
576	638		`lda #>oam_buffer`
577	639		`sta OAMDMA`
578		-
579		-	`lda player_y`
580		-	`sta oam_buffer+0`
581		-	`lda player_x`
582		-	`sta oam_buffer+3`
583	640
584	641		`inc frame_count`
585	642