Game 1 Unit 12 of 64 1 hr learning time

Scoring

Add a score that increases when collecting items.

19% of Neon Nexus

Mark as complete Completed!

What You’re Building

A score. Collect items, earn points.

Scoring

Score Variables

A 16-bit score allows values up to 65,535:

score_lo:  .res 1     ; Low byte
score_hi:  .res 1     ; High byte

Adding Points

When an item is collected, add 100 points:

; Add 100 to score
lda score_lo
clc
adc #100
sta score_lo
lda score_hi
adc #0           ; Add carry to high byte
sta score_hi

The adc #0 adds the carry flag from the low byte addition. If score_lo overflowed (went past 255), the carry is set and score_hi increments.

16-Bit Arithmetic

The 6502 is an 8-bit processor. For numbers larger than 255:

Add low bytes, storing the result
Add high bytes with carry (adc #0 uses the carry from step 1)

This extends to any number of bytes.

The Code

; =============================================================================
; NEON NEXUS - Unit 12: Scoring
; =============================================================================
; Add a score that increases when collecting items.
; =============================================================================

PPUCTRL   = $2000
PPUMASK   = $2001
PPUSTATUS = $2002
OAMADDR   = $2003
PPUSCROLL = $2005
PPUADDR   = $2006
PPUDATA   = $2007
OAMDMA    = $4014
JOYPAD1   = $4016

BTN_UP     = %00001000
BTN_DOWN   = %00000100
BTN_LEFT   = %00000010
BTN_RIGHT  = %00000001

PLAYER_START_X  = 124
PLAYER_START_Y  = 116
PLAYER_SPEED    = 2
ENEMY_SPEED     = 1
NUM_ENEMIES     = 4
NUM_ITEMS       = 4
STARTING_LIVES  = 3
COLLISION_DIST  = 6
COLLECT_DIST    = 8
INVULN_TIME     = 90

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
SPRITE_LIFE    = 9
SPRITE_ITEM    = 10

ARENA_LEFT   = 16
ARENA_RIGHT  = 232
ARENA_TOP    = 24
ARENA_BOTTOM = 208

DIR_RIGHT = 1
DIR_LEFT  = $FF
DIR_DOWN  = 1
DIR_UP    = $FF
BG_COLOUR = $0F

.segment "ZEROPAGE"
player_x:       .res 1
player_y:       .res 1
buttons:        .res 1
temp:           .res 1
row_counter:    .res 1
frame_count:    .res 1
lives:          .res 1
invuln_timer:   .res 1
game_over:      .res 1
items_collected: .res 1
score_lo:       .res 1     ; Score low byte
score_hi:       .res 1     ; Score high byte (for scores > 255)

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

item_x:         .res NUM_ITEMS
item_y:         .res NUM_ITEMS
item_active:    .res NUM_ITEMS   ; Non-zero if item exists

.segment "OAM"
oam_buffer:     .res 256

.segment "BSS"

.segment "HEADER"
    .byte "NES", $1A, 2, 1, $01, $00, 0,0,0,0,0,0,0,0

.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_game

    lda #$FF
    ldx #0
@hide_all:
    sta oam_buffer, x
    inx
    bne @hide_all

    jsr update_all_sprites

    lda #0
    sta PPUSCROLL
    sta PPUSCROLL

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

main_loop:
    lda game_over
    bne @game_over_loop
    jsr read_controller
    jsr move_player
    jsr move_enemies
    jsr check_enemy_collisions
    jsr check_item_collisions
    jsr update_all_sprites
    jmp main_loop

@game_over_loop:
    jmp @game_over_loop

init_game:
    lda #STARTING_LIVES
    sta lives
    lda #0
    sta game_over
    sta invuln_timer
    sta items_collected
    sta score_lo
    sta score_hi

    lda #PLAYER_START_X
    sta player_x
    lda #PLAYER_START_Y
    sta player_y

    jsr init_enemies
    jsr init_items
    rts

init_enemies:
    lda #48
    sta enemy_x+0
    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

init_items:
    ; Place 4 items around the arena
    lda #80
    sta item_x+0
    lda #64
    sta item_y+0
    lda #1
    sta item_active+0

    lda #168
    sta item_x+1
    lda #64
    sta item_y+1
    lda #1
    sta item_active+1

    lda #80
    sta item_x+2
    lda #160
    sta item_y+2
    lda #1
    sta item_active+2

    lda #168
    sta item_x+3
    lda #160
    sta item_y+3
    lda #1
    sta item_active+3
    rts

check_enemy_collisions:
    lda invuln_timer
    beq @check
    dec invuln_timer
    rts

@check:
    ldx #0
@check_enemy:
    lda player_x
    sec
    sbc enemy_x, x
    bpl @check_x_pos
    eor #$FF
    clc
    adc #1
@check_x_pos:
    cmp #COLLISION_DIST
    bcs @next_enemy

    lda player_y
    sec
    sbc enemy_y, x
    bpl @check_y_pos
    eor #$FF
    clc
    adc #1
@check_y_pos:
    cmp #COLLISION_DIST
    bcs @next_enemy

    jsr player_hit
    rts

@next_enemy:
    inx
    cpx #NUM_ENEMIES
    bne @check_enemy
    rts

check_item_collisions:
    ldx #0
@check_item:
    lda item_active, x
    beq @next_item

    lda player_x
    sec
    sbc item_x, x
    bpl @item_x_pos
    eor #$FF
    clc
    adc #1
@item_x_pos:
    cmp #COLLECT_DIST
    bcs @next_item

    lda player_y
    sec
    sbc item_y, x
    bpl @item_y_pos
    eor #$FF
    clc
    adc #1
@item_y_pos:
    cmp #COLLECT_DIST
    bcs @next_item

    ; Collected! Add 100 points
    lda #0
    sta item_active, x
    inc items_collected
    ; Add 100 to score (100 = $64)
    lda score_lo
    clc
    adc #100
    sta score_lo
    lda score_hi
    adc #0
    sta score_hi

@next_item:
    inx
    cpx #NUM_ITEMS
    bne @check_item
    rts

player_hit:
    dec lives
    lda lives
    beq @game_over

    lda #PLAYER_START_X
    sta player_x
    lda #PLAYER_START_Y
    sta player_y

    lda #INVULN_TIME
    sta invuln_timer
    rts

@game_over:
    lda #1
    sta game_over
    rts

update_all_sprites:
    jsr update_player_sprite
    jsr update_enemy_sprites
    jsr update_item_sprites
    jsr update_lives_display
    rts

update_player_sprite:
    lda game_over
    beq @alive
    lda #$FF
    sta oam_buffer+0
    rts

@alive:
    lda invuln_timer
    beq @show
    and #%00000100
    beq @show
    lda #$FF
    sta oam_buffer+0
    rts

@show:
    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
    rts

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

update_item_sprites:
    ldx #0
    ldy #20             ; OAM offset after enemies
@loop:
    lda item_active, x
    beq @hide_item

    lda item_y, x
    sta oam_buffer, y
    iny
    lda #SPRITE_ITEM
    sta oam_buffer, y
    iny
    lda #%00000010      ; Palette 2 (green)
    sta oam_buffer, y
    iny
    lda item_x, x
    sta oam_buffer, y
    iny
    jmp @next_item

@hide_item:
    lda #$FF
    sta oam_buffer, y
    iny
    iny
    iny
    iny

@next_item:
    inx
    cpx #NUM_ITEMS
    bne @loop
    rts

update_lives_display:
    ldy #36             ; OAM offset for lives
    ldx lives
    beq @hide_all

    lda #8
    sta oam_buffer, y
    iny
    lda #SPRITE_LIFE
    sta oam_buffer, y
    iny
    lda #0
    sta oam_buffer, y
    iny
    lda #16
    sta oam_buffer, y
    iny

    cpx #1
    beq @hide_rest

    lda #8
    sta oam_buffer, y
    iny
    lda #SPRITE_LIFE
    sta oam_buffer, y
    iny
    lda #0
    sta oam_buffer, y
    iny
    lda #26
    sta oam_buffer, y
    iny

    cpx #2
    beq @hide_rest

    lda #8
    sta oam_buffer, y
    iny
    lda #SPRITE_LIFE
    sta oam_buffer, y
    iny
    lda #0
    sta oam_buffer, y
    iny
    lda #36
    sta oam_buffer, y
    rts

@hide_rest:
@hide_all:
    rts

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

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

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:
    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:
    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

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    ; Player
    .byte BG_COLOUR, $16, $26, $36    ; Enemies
    .byte BG_COLOUR, $1A, $2A, $3A    ; Items (green)
    .byte BG_COLOUR, $30, $27, $17

.segment "VECTORS"
    .word nmi
    .word reset
    .word irq

.segment "CHARS"
.byte $00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00
.byte %11111111,%10000001,%10000001,%11111111,%11111111,%00010001,%00010001,%11111111
.byte %00000000,%01111110,%01111110,%00000000,%00000000,%11101110,%11101110,%00000000
.byte %00000000,%00000000,%00000000,%00000000,%00000000,%00000000,%00000000,%10000001
.byte %00000000,%00000000,%00000000,%00000000,%00000000,%00000000,%00000000,%00000000
.byte %11111111,%11000000,%10100000,%10010000,%10001000,%10000100,%10000010,%10000001
.byte %00000000,%00111111,%01011111,%01101111,%01110111,%01111011,%01111101,%01111110
.byte %11111111,%00000011,%00000101,%00001001,%00010001,%00100001,%01000001,%10000001
.byte %00000000,%11111100,%11111010,%11110110,%11101110,%11011110,%10111110,%01111110
.byte %10000001,%10000010,%10000100,%10001000,%10010000,%10100000,%11000000,%11111111
.byte %01111110,%01111101,%01111011,%01110111,%01101111,%01011111,%00111111,%00000000
.byte %10000001,%01000001,%00100001,%00010001,%00001001,%00000101,%00000011,%11111111
.byte %01111110,%10111110,%11011110,%11101110,%11110110,%11111010,%11111100,%00000000
.byte %00011000,%00011000,%00111100,%01111110,%11111111,%10111101,%00100100,%00100100
.byte %00000000,%00011000,%00011000,%00111100,%01000010,%01000010,%00011000,%00000000
.byte %00011000,%00111100,%01111110,%11111111,%11111111,%01111110,%00111100,%00011000
.byte %00000000,%00011000,%00100100,%01000010,%01000010,%00100100,%00011000,%00000000
.byte %00000000,%00100100,%01111110,%01111110,%00111100,%00011000,%00000000,%00000000
.byte %00000000,%00000000,%00000000,%00100100,%00011000,%00000000,%00000000,%00000000
; Tile 10: Item (data core - diamond)
.byte %00000000,%00011000,%00111100,%01111110,%01111110,%00111100,%00011000,%00000000
.byte %00000000,%00000000,%00011000,%00100100,%00100100,%00011000,%00000000,%00000000

.res 8192 - 176, $00

Build It

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

Score increases when you collect items. The number is tracked internally - a visible HUD display comes later.

The game is silent. Unit 13 adds sound effects to bring it to life.

What Changed

Unit 11 → Unit 12

+15-3

1	1		`; =============================================================================`
2		-	`; NEON NEXUS - Unit 11: Collectibles`
	2	+	`; NEON NEXUS - Unit 12: Scoring`
3	3		`; =============================================================================`
4		-	`; Add data cores that the player can collect.`
	4	+	`; Add a score that increases when collecting items.`
5	5		`; =============================================================================`
6	6
7	7		`PPUCTRL = $2000`
			...
63	63		`invuln_timer: .res 1`
64	64		`game_over: .res 1`
65	65		`items_collected: .res 1`
	66	+	`score_lo: .res 1 ; Score low byte`
	67	+	`score_hi: .res 1 ; Score high byte (for scores > 255)`
66	68
67	69		`enemy_x: .res NUM_ENEMIES`
68	70		`enemy_y: .res NUM_ENEMIES`
			...
160	162		`sta game_over`
161	163		`sta invuln_timer`
162	164		`sta items_collected`
	165	+	`sta score_lo`
	166	+	`sta score_hi`
163	167
164	168		`lda #PLAYER_START_X`
165	169		`sta player_x`
			...
306	310		`cmp #COLLECT_DIST`
307	311		`bcs @next_item`
308	312
309		-	`; Collected!`
	313	+	`; Collected! Add 100 points`
310	314		`lda #0`
311	315		`sta item_active, x`
312	316		`inc items_collected`
	317	+	`; Add 100 to score (100 = $64)`
	318	+	`lda score_lo`
	319	+	`clc`
	320	+	`adc #100`
	321	+	`sta score_lo`
	322	+	`lda score_hi`
	323	+	`adc #0`
	324	+	`sta score_hi`
313	325
314	326		`@next_item:`
315	327		`inx`