Game 1 Unit 8 of 64 1 hr learning time

Enemy Movement

Make enemies patrol the arena with bounce patterns.

13% of Neon Nexus

Mark as complete Completed!

What You’re Building

Enemies that patrol. They bounce off walls, constantly hunting.

Enemy Movement

The arena is now alive with danger.

Direction Variables

Each enemy needs a direction for X and Y movement:

enemy_dir_x:  .res NUM_ENEMIES   ; +1 (right) or -1 (left)
enemy_dir_y:  .res NUM_ENEMIES   ; +1 (down) or -1 (up)

We use $FF for -1 (two’s complement). When added to a position, it subtracts 1.

Initialising Directions

Each enemy starts moving in a different diagonal:

init_enemies:
    ; Enemy 0: Top-left, moving right and down
    lda #1
    sta enemy_dir_x+0
    sta enemy_dir_y+0

    ; Enemy 1: Top-right, moving left and down
    lda #$FF            ; -1
    sta enemy_dir_x+1
    lda #1
    sta enemy_dir_y+1
    ; ...

Movement Logic

Each frame, move each enemy and check for wall collisions:

move_enemies:
    ldx #0

@enemy_loop:
    ; Move X
    lda enemy_x, x
    clc
    adc enemy_dir_x, x
    sta enemy_x, x

    ; Check bounds
    cmp #ARENA_LEFT
    bcs @check_right
    ; Hit left wall - reverse
    lda #1
    sta enemy_dir_x, x
    lda #ARENA_LEFT
    sta enemy_x, x
    jmp @move_y

@check_right:
    cmp #ARENA_RIGHT
    bcc @move_y
    ; Hit right wall - reverse
    lda #$FF
    sta enemy_dir_x, x
    ; ... same pattern for Y

The Bounce

When an enemy hits a wall:

Reverse its direction for that axis
Clamp its position to the boundary

This creates the classic “bounce” behaviour seen in many early games.

Calling Movement

Add enemy movement to the main loop:

main_loop:
    jsr read_controller
    jsr move_player
    jsr move_enemies        ; NEW
    jsr update_enemy_sprites
    jmp main_loop

The Code

; =============================================================================
; NEON NEXUS - Unit 8: Enemy Movement
; =============================================================================
; Make enemies patrol the arena with bounce patterns.
; =============================================================================

; -----------------------------------------------------------------------------
; 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

; 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

; Direction flags
DIR_RIGHT = 1
DIR_LEFT  = $FF     ; -1 in two's complement
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
row_counter:  .res 1
frame_count:  .res 1

; Enemy data
enemy_x:      .res NUM_ENEMIES
enemy_y:      .res NUM_ENEMIES
enemy_dir_x:  .res NUM_ENEMIES   ; +1 or -1
enemy_dir_y:  .res NUM_ENEMIES   ; +1 or -1

.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

    ; 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 update_enemy_sprites
    jmp main_loop

; -----------------------------------------------------------------------------
; Initialise Enemies with positions and directions
; -----------------------------------------------------------------------------
init_enemies:
    ; Enemy 0: Top-left, moving right and down
    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

    ; Enemy 1: Top-right, moving left and down
    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

    ; Enemy 2: Bottom-left, moving right and up
    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

    ; Enemy 3: Bottom-right, moving left and up
    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 - Bounce off arena walls
; -----------------------------------------------------------------------------
move_enemies:
    ldx #0

@enemy_loop:
    ; Move X
    lda enemy_x, x
    clc
    adc enemy_dir_x, x
    sta enemy_x, x

    ; Check X bounds
    cmp #ARENA_LEFT
    bcs @check_right
    ; Hit left wall - reverse X direction
    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
    ; Hit right wall - reverse X direction
    lda #DIR_LEFT
    sta enemy_dir_x, x
    lda #ARENA_RIGHT
    sec
    sbc #1
    sta enemy_x, x

@move_y:
    ; Move Y
    lda enemy_y, x
    clc
    adc enemy_dir_y, x
    sta enemy_y, x

    ; Check Y bounds
    cmp #ARENA_TOP
    bcs @check_bottom
    ; Hit top wall - reverse Y direction
    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
    ; Hit bottom wall - reverse Y direction
    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 in OAM
; -----------------------------------------------------------------------------
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

    lda player_y
    sta oam_buffer+0
    lda player_x
    sta oam_buffer+3

    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

Enemies now patrol endlessly. The arena feels dangerous.

Movement creates threat, but there’s no consequence for touching enemies yet. Unit 9 adds collision detection.

What Changed

Unit 7 → Unit 8

+166-172

1	1		`; =============================================================================`
2		-	`; NEON NEXUS - Unit 7: Enemies Appear`
	2	+	`; NEON NEXUS - Unit 8: Enemy Movement`
3	3		`; =============================================================================`
4		-	`; Add enemy sprites to the arena.`
	4	+	`; Make enemies patrol the arena with bounce patterns.`
5	5		`; =============================================================================`
6	6
7	7		`; -----------------------------------------------------------------------------`
			...
35	35		`PLAYER_START_X = 124`
36	36		`PLAYER_START_Y = 116`
37	37		`PLAYER_SPEED = 2`
	38	+	`ENEMY_SPEED = 1`
38	39
39	40		`NUM_ENEMIES = 4`
40	41
41		-	`; Tile indices (background)`
	42	+	`; Tile indices`
42	43		`TILE_EMPTY = 0`
43	44		`TILE_BORDER = 1`
44	45		`TILE_FLOOR = 2`
			...
47	48		`TILE_CORNER_BL = 5`
48	49		`TILE_CORNER_BR = 6`
49	50
50		-	`; Sprite tiles`
51	51		`SPRITE_PLAYER = 7`
52	52		`SPRITE_ENEMY = 8`
53	53
			...
56	56		`ARENA_RIGHT = 232`
57	57		`ARENA_TOP = 16`
58	58		`ARENA_BOTTOM = 208`
	59	+
	60	+	`; Direction flags`
	61	+	`DIR_RIGHT = 1`
	62	+	`DIR_LEFT = $FF ; -1 in two's complement`
	63	+	`DIR_DOWN = 1`
	64	+	`DIR_UP = $FF`
59	65
60	66		`BG_COLOUR = $0F`
61	67
			...
63	69		`; Memory Layout`
64	70		`; -----------------------------------------------------------------------------`
65	71		`.segment "ZEROPAGE"`
66		-	`player_x: .res 1`
67		-	`player_y: .res 1`
68		-	`buttons: .res 1`
69		-	`temp: .res 1`
70		-	`row_counter: .res 1`
	72	+	`player_x: .res 1`
	73	+	`player_y: .res 1`
	74	+	`buttons: .res 1`
	75	+	`temp: .res 1`
	76	+	`row_counter: .res 1`
	77	+	`frame_count: .res 1`
71	78
72		-	`; Enemy positions (4 enemies)`
73		-	`enemy_x: .res NUM_ENEMIES`
74		-	`enemy_y: .res NUM_ENEMIES`
	79	+	`; Enemy data`
	80	+	`enemy_x: .res NUM_ENEMIES`
	81	+	`enemy_y: .res NUM_ENEMIES`
	82	+	`enemy_dir_x: .res NUM_ENEMIES ; +1 or -1`
	83	+	`enemy_dir_y: .res NUM_ENEMIES ; +1 or -1`
75	84
76	85		`.segment "OAM"`
77		-	`oam_buffer: .res 256`
	86	+	`oam_buffer: .res 256`
78	87
79	88		`.segment "BSS"`
80	89
			...
132	141		`jsr set_attributes`
133	142		`jsr init_enemies`
134	143
135		-	`; Set up player`
136	144		`lda #PLAYER_START_X`
137	145		`sta player_x`
138	146		`lda #PLAYER_START_Y`
139	147		`sta player_y`
140	148
141		-	`; Initialise player sprite (OAM slot 0)`
	149	+	`; Player sprite`
142	150		`lda player_y`
143	151		`sta oam_buffer+0`
144	152		`lda #SPRITE_PLAYER`
145	153		`sta oam_buffer+1`
146		-	`lda #0 ; Attributes: palette 0, no flip`
	154	+	`lda #0`
147	155		`sta oam_buffer+2`
148	156		`lda player_x`
149	157		`sta oam_buffer+3`
150	158
151		-	`; Set up enemy sprites in OAM`
152	159		`jsr update_enemy_sprites`
153	160
154		-	`; Hide remaining sprites (after player + 4 enemies = 5 sprites used)`
	161	+	`; Hide remaining sprites`
155	162		`lda #$FF`
156		-	`ldx #20 ; Start after 5 sprites (5*4=20)`
	163	+	`ldx #20`
157	164		`@hide_sprites:`
158	165		`sta oam_buffer, x`
159	166		`inx`
			...
162	169		`lda #0`
163	170		`sta PPUSCROLL`
164	171		`sta PPUSCROLL`
	172	+	`sta frame_count`
165	173
166	174		`lda #%10000000`
167	175		`sta PPUCTRL`
			...
171	179		`main_loop:`
172	180		`jsr read_controller`
173	181		`jsr move_player`
	182	+	`jsr move_enemies`
	183	+	`jsr update_enemy_sprites`
174	184		`jmp main_loop`
175	185
176	186		`; -----------------------------------------------------------------------------`
177		-	`; Initialise Enemies - Place 4 enemies at corners of play area`
	187	+	`; Initialise Enemies with positions and directions`
178	188		`; -----------------------------------------------------------------------------`
179	189		`init_enemies:`
180		-	`; Enemy 0: Top-left area`
	190	+	`; Enemy 0: Top-left, moving right and down`
181	191		`lda #48`
182	192		`sta enemy_x+0`
183	193		`lda #48`
184	194		`sta enemy_y+0`
	195	+	`lda #DIR_RIGHT`
	196	+	`sta enemy_dir_x+0`
	197	+	`lda #DIR_DOWN`
	198	+	`sta enemy_dir_y+0`
185	199
186		-	`; Enemy 1: Top-right area`
	200	+	`; Enemy 1: Top-right, moving left and down`
187	201		`lda #200`
188	202		`sta enemy_x+1`
189	203		`lda #48`
190	204		`sta enemy_y+1`
	205	+	`lda #DIR_LEFT`
	206	+	`sta enemy_dir_x+1`
	207	+	`lda #DIR_DOWN`
	208	+	`sta enemy_dir_y+1`
191	209
192		-	`; Enemy 2: Bottom-left area`
	210	+	`; Enemy 2: Bottom-left, moving right and up`
193	211		`lda #48`
194	212		`sta enemy_x+2`
195	213		`lda #176`
196	214		`sta enemy_y+2`
	215	+	`lda #DIR_RIGHT`
	216	+	`sta enemy_dir_x+2`
	217	+	`lda #DIR_UP`
	218	+	`sta enemy_dir_y+2`
197	219
198		-	`; Enemy 3: Bottom-right area`
	220	+	`; Enemy 3: Bottom-right, moving left and up`
199	221		`lda #200`
200	222		`sta enemy_x+3`
201	223		`lda #176`
202	224		`sta enemy_y+3`
	225	+	`lda #DIR_LEFT`
	226	+	`sta enemy_dir_x+3`
	227	+	`lda #DIR_UP`
	228	+	`sta enemy_dir_y+3`
	229	+
	230	+	`rts`
	231	+
	232	+	`; -----------------------------------------------------------------------------`
	233	+	`; Move Enemies - Bounce off arena walls`
	234	+	`; -----------------------------------------------------------------------------`
	235	+	`move_enemies:`
	236	+	`ldx #0`
	237	+
	238	+	`@enemy_loop:`
	239	+	`; Move X`
	240	+	`lda enemy_x, x`
	241	+	`clc`
	242	+	`adc enemy_dir_x, x`
	243	+	`sta enemy_x, x`
	244	+
	245	+	`; Check X bounds`
	246	+	`cmp #ARENA_LEFT`
	247	+	`bcs @check_right`
	248	+	`; Hit left wall - reverse X direction`
	249	+	`lda #DIR_RIGHT`
	250	+	`sta enemy_dir_x, x`
	251	+	`lda #ARENA_LEFT`
	252	+	`sta enemy_x, x`
	253	+	`jmp @move_y`
	254	+
	255	+	`@check_right:`
	256	+	`cmp #ARENA_RIGHT`
	257	+	`bcc @move_y`
	258	+	`; Hit right wall - reverse X direction`
	259	+	`lda #DIR_LEFT`
	260	+	`sta enemy_dir_x, x`
	261	+	`lda #ARENA_RIGHT`
	262	+	`sec`
	263	+	`sbc #1`
	264	+	`sta enemy_x, x`
	265	+
	266	+	`@move_y:`
	267	+	`; Move Y`
	268	+	`lda enemy_y, x`
	269	+	`clc`
	270	+	`adc enemy_dir_y, x`
	271	+	`sta enemy_y, x`
	272	+
	273	+	`; Check Y bounds`
	274	+	`cmp #ARENA_TOP`
	275	+	`bcs @check_bottom`
	276	+	`; Hit top wall - reverse Y direction`
	277	+	`lda #DIR_DOWN`
	278	+	`sta enemy_dir_y, x`
	279	+	`lda #ARENA_TOP`
	280	+	`sta enemy_y, x`
	281	+	`jmp @next_enemy`
	282	+
	283	+	`@check_bottom:`
	284	+	`cmp #ARENA_BOTTOM`
	285	+	`bcc @next_enemy`
	286	+	`; Hit bottom wall - reverse Y direction`
	287	+	`lda #DIR_UP`
	288	+	`sta enemy_dir_y, x`
	289	+	`lda #ARENA_BOTTOM`
	290	+	`sec`
	291	+	`sbc #1`
	292	+	`sta enemy_y, x`
	293	+
	294	+	`@next_enemy:`
	295	+	`inx`
	296	+	`cpx #NUM_ENEMIES`
	297	+	`bne @enemy_loop`
203	298
204	299		`rts`
205	300
			...
207	302		`; Update Enemy Sprites in OAM`
208	303		`; -----------------------------------------------------------------------------`
209	304		`update_enemy_sprites:`
210		-	`ldx #0 ; Enemy index`
211		-	`ldy #4 ; OAM offset (after player sprite)`
	305	+	`ldx #0`
	306	+	`ldy #4`
212	307
213	308		`@loop:`
214		-	`; Y position`
215	309		`lda enemy_y, x`
216	310		`sta oam_buffer, y`
217	311		`iny`
218	312
219		-	`; Tile`
220	313		`lda #SPRITE_ENEMY`
221	314		`sta oam_buffer, y`
222	315		`iny`
223	316
224		-	`; Attributes: palette 1 (different colour from player)`
225	317		`lda #%00000001`
226	318		`sta oam_buffer, y`
227	319		`iny`
228	320
229		-	`; X position`
230	321		`lda enemy_x, x`
231	322		`sta oam_buffer, y`
232	323		`iny`
			...
485	576		`lda #>oam_buffer`
486	577		`sta OAMDMA`
487	578
488		-	`; Update player sprite position`
489	579		`lda player_y`
490	580		`sta oam_buffer+0`
491	581		`lda player_x`
492	582		`sta oam_buffer+3`
	583	+
	584	+	`inc frame_count`
493	585
494	586		`lda #0`
495	587		`sta PPUSCROLL`
			...
509	601		`; Data`
510	602		`; -----------------------------------------------------------------------------`
511	603		`palette_data:`
512		-	`; Background palettes`
513		-	`.byte BG_COLOUR, $11, $21, $31 ; Palette 0: Blue`
514		-	`.byte BG_COLOUR, $13, $23, $33 ; Palette 1: Purple`
515		-	`.byte BG_COLOUR, $19, $29, $39 ; Palette 2: Green`
516		-	`.byte BG_COLOUR, $16, $26, $36 ; Palette 3: Red`
517		-	`; Sprite palettes`
518		-	`.byte BG_COLOUR, $30, $27, $17 ; Palette 0: Player (white/orange)`
519		-	`.byte BG_COLOUR, $16, $26, $36 ; Palette 1: Enemy (red)`
	604	+	`.byte BG_COLOUR, $11, $21, $31`
	605	+	`.byte BG_COLOUR, $13, $23, $33`
	606	+	`.byte BG_COLOUR, $19, $29, $39`
	607	+	`.byte BG_COLOUR, $16, $26, $36`
	608	+	`.byte BG_COLOUR, $30, $27, $17`
	609	+	`.byte BG_COLOUR, $16, $26, $36`
520	610		`.byte BG_COLOUR, $30, $27, $17`
521	611		`.byte BG_COLOUR, $30, $27, $17`
522	612
			...
537	627		`.byte $00,$00,$00,$00,$00,$00,$00,$00`
538	628		`.byte $00,$00,$00,$00,$00,$00,$00,$00`
539	629
540		-	`; Tile 1: Border (brick pattern)`
541		-	`.byte %11111111`
542		-	`.byte %10000001`
543		-	`.byte %10000001`
544		-	`.byte %11111111`
545		-	`.byte %11111111`
546		-	`.byte %00010001`
547		-	`.byte %00010001`
548		-	`.byte %11111111`
549		-	`.byte %00000000`
550		-	`.byte %01111110`
551		-	`.byte %01111110`
552		-	`.byte %00000000`
553		-	`.byte %00000000`
554		-	`.byte %11101110`
555		-	`.byte %11101110`
556		-	`.byte %00000000`
	630	+	`; Tile 1: Border`
	631	+	`.byte %11111111,%10000001,%10000001,%11111111`
	632	+	`.byte %11111111,%00010001,%00010001,%11111111`
	633	+	`.byte %00000000,%01111110,%01111110,%00000000`
	634	+	`.byte %00000000,%11101110,%11101110,%00000000`
557	635
558		-	`; Tile 2: Floor (subtle grid)`
559		-	`.byte %00000000`
560		-	`.byte %00000000`
561		-	`.byte %00000000`
562		-	`.byte %00000000`
563		-	`.byte %00000000`
564		-	`.byte %00000000`
565		-	`.byte %00000000`
566		-	`.byte %10000001`
567		-	`.byte %00000000`
568		-	`.byte %00000000`
569		-	`.byte %00000000`
570		-	`.byte %00000000`
571		-	`.byte %00000000`
572		-	`.byte %00000000`
573		-	`.byte %00000000`
574		-	`.byte %00000000`
	636	+	`; Tile 2: Floor`
	637	+	`.byte %00000000,%00000000,%00000000,%00000000`
	638	+	`.byte %00000000,%00000000,%00000000,%10000001`
	639	+	`.byte %00000000,%00000000,%00000000,%00000000`
	640	+	`.byte %00000000,%00000000,%00000000,%00000000`
575	641
576	642		`; Tile 3: Corner TL`
577		-	`.byte %11111111`
578		-	`.byte %11000000`
579		-	`.byte %10100000`
580		-	`.byte %10010000`
581		-	`.byte %10001000`
582		-	`.byte %10000100`
583		-	`.byte %10000010`
584		-	`.byte %10000001`
585		-	`.byte %00000000`
586		-	`.byte %00111111`
587		-	`.byte %01011111`
588		-	`.byte %01101111`
589		-	`.byte %01110111`
590		-	`.byte %01111011`
591		-	`.byte %01111101`
592		-	`.byte %01111110`
	643	+	`.byte %11111111,%11000000,%10100000,%10010000`
	644	+	`.byte %10001000,%10000100,%10000010,%10000001`
	645	+	`.byte %00000000,%00111111,%01011111,%01101111`
	646	+	`.byte %01110111,%01111011,%01111101,%01111110`
593	647
594	648		`; Tile 4: Corner TR`
595		-	`.byte %11111111`
596		-	`.byte %00000011`
597		-	`.byte %00000101`
598		-	`.byte %00001001`
599		-	`.byte %00010001`
600		-	`.byte %00100001`
601		-	`.byte %01000001`
602		-	`.byte %10000001`
603		-	`.byte %00000000`
604		-	`.byte %11111100`
605		-	`.byte %11111010`
606		-	`.byte %11110110`
607		-	`.byte %11101110`
608		-	`.byte %11011110`
609		-	`.byte %10111110`
610		-	`.byte %01111110`
	649	+	`.byte %11111111,%00000011,%00000101,%00001001`
	650	+	`.byte %00010001,%00100001,%01000001,%10000001`
	651	+	`.byte %00000000,%11111100,%11111010,%11110110`
	652	+	`.byte %11101110,%11011110,%10111110,%01111110`
611	653
612	654		`; Tile 5: Corner BL`
613		-	`.byte %10000001`
614		-	`.byte %10000010`
615		-	`.byte %10000100`
616		-	`.byte %10001000`
617		-	`.byte %10010000`
618		-	`.byte %10100000`
619		-	`.byte %11000000`
620		-	`.byte %11111111`
621		-	`.byte %01111110`
622		-	`.byte %01111101`
623		-	`.byte %01111011`
624		-	`.byte %01110111`
625		-	`.byte %01101111`
626		-	`.byte %01011111`
627		-	`.byte %00111111`
628		-	`.byte %00000000`
	655	+	`.byte %10000001,%10000010,%10000100,%10001000`
	656	+	`.byte %10010000,%10100000,%11000000,%11111111`
	657	+	`.byte %01111110,%01111101,%01111011,%01110111`
	658	+	`.byte %01101111,%01011111,%00111111,%00000000`
629	659
630	660		`; Tile 6: Corner BR`
631		-	`.byte %10000001`
632		-	`.byte %01000001`
633		-	`.byte %00100001`
634		-	`.byte %00010001`
635		-	`.byte %00001001`
636		-	`.byte %00000101`
637		-	`.byte %00000011`
638		-	`.byte %11111111`
639		-	`.byte %01111110`
640		-	`.byte %10111110`
641		-	`.byte %11011110`
642		-	`.byte %11101110`
643		-	`.byte %11110110`
644		-	`.byte %11111010`
645		-	`.byte %11111100`
646		-	`.byte %00000000`
	661	+	`.byte %10000001,%01000001,%00100001,%00010001`
	662	+	`.byte %00001001,%00000101,%00000011,%11111111`
	663	+	`.byte %01111110,%10111110,%11011110,%11101110`
	664	+	`.byte %11110110,%11111010,%11111100,%00000000`
647	665
648		-	`; Tile 7: Player sprite`
649		-	`.byte %00011000`
650		-	`.byte %00011000`
651		-	`.byte %00111100`
652		-	`.byte %01111110`
653		-	`.byte %11111111`
654		-	`.byte %10111101`
655		-	`.byte %00100100`
656		-	`.byte %00100100`
657		-	`.byte %00000000`
658		-	`.byte %00011000`
659		-	`.byte %00011000`
660		-	`.byte %00111100`
661		-	`.byte %01000010`
662		-	`.byte %01000010`
663		-	`.byte %00011000`
664		-	`.byte %00000000`
	666	+	`; Tile 7: Player`
	667	+	`.byte %00011000,%00011000,%00111100,%01111110`
	668	+	`.byte %11111111,%10111101,%00100100,%00100100`
	669	+	`.byte %00000000,%00011000,%00011000,%00111100`
	670	+	`.byte %01000010,%01000010,%00011000,%00000000`
665	671
666		-	`; Tile 8: Enemy sprite (hostile diamond shape)`
667		-	`.byte %00011000`
668		-	`.byte %00111100`
669		-	`.byte %01111110`
670		-	`.byte %11111111`
671		-	`.byte %11111111`
672		-	`.byte %01111110`
673		-	`.byte %00111100`
674		-	`.byte %00011000`
675		-	`.byte %00000000`
676		-	`.byte %00011000`
677		-	`.byte %00100100`
678		-	`.byte %01000010`
679		-	`.byte %01000010`
680		-	`.byte %00100100`
681		-	`.byte %00011000`
682		-	`.byte %00000000`
	672	+	`; Tile 8: Enemy`
	673	+	`.byte %00011000,%00111100,%01111110,%11111111`
	674	+	`.byte %11111111,%01111110,%00111100,%00011000`
	675	+	`.byte %00000000,%00011000,%00100100,%01000010`
	676	+	`.byte %01000010,%00100100,%00011000,%00000000`
683	677
684		-	`; Fill rest of CHR-ROM`
	678	+	`; Fill rest`
685	679		`.res 8192 - 144, $00`
686	680