Reset Sequence

Overview

Every NES program needs this exact startup sequence. The reset handler disables interrupts, waits for the PPU to stabilise (two VBlank waits), and clears all RAM. Only after this is it safe to configure the PPU and start your game.

Code

; =============================================================================
; RESET SEQUENCE - NES
; Standard NES initialisation boilerplate
; Taught: Game 1 (Neon Nexus), Unit 1
; CPU: ~30,000 cycles | Memory: ~60 bytes
; =============================================================================

PPUCTRL   = $2000
PPUMASK   = $2001
PPUSTATUS = $2002

.segment "CODE"

reset:
    sei                     ; Disable interrupts
    cld                     ; Clear decimal mode (not used on NES, but safe)

    ldx #$40
    stx $4017               ; Disable APU frame IRQ

    ldx #$FF
    txs                     ; Set up stack at $01FF

    inx                     ; X = 0
    stx PPUCTRL             ; Disable NMI
    stx PPUMASK             ; Disable rendering
    stx $4010               ; Disable DMC IRQs

    ; === First VBlank wait ===
    ; PPU needs time to warm up after power-on
@vblank1:
    bit PPUSTATUS           ; Read status (bit 7 = in VBlank)
    bpl @vblank1            ; Loop until VBlank flag set

    ; === Clear RAM while we wait ===
    ; RAM contents are undefined at power-on
    lda #0
@clear_ram:
    sta $0000, x
    sta $0100, x
    sta $0200, x            ; OAM buffer
    sta $0300, x
    sta $0400, x
    sta $0500, x
    sta $0600, x
    sta $0700, x
    inx
    bne @clear_ram          ; Loop 256 times (X wraps from $FF to $00)

    ; === Second VBlank wait ===
    ; PPU is now fully ready
@vblank2:
    bit PPUSTATUS
    bpl @vblank2

    ; === PPU is ready - continue with game setup ===
    ; Load palettes, set up sprites, etc.
    jsr setup_game

    ; Enable NMI and rendering
    lda #%10010000          ; NMI enable, sprites from pattern table 1
    sta PPUCTRL
    lda #%00011110          ; Show sprites and background
    sta PPUMASK

    ; Fall through to main loop
main_loop:
    jmp main_loop           ; NMI does the work

setup_game:
    ; Your initialisation code here
    rts

Trade-offs

Aspect	Cost
CPU	~30,000 cycles (two VBlank waits)
Memory	~60 bytes
Limitation	None - this is mandatory

When to use: Every NES program. Copy this exactly.

When to avoid: Never - you always need this.

Why Two VBlank Waits?

The PPU takes time to stabilise after power-on:

First wait: PPU internal state is settling
Clear RAM: Use this time productively
Second wait: PPU is now fully ready

Skipping this can cause graphics glitches or crashes on real hardware.

What Each Step Does

Step	Purpose
`sei`	Prevent interrupts during setup
`cld`	Clear decimal flag (6502 habit, harmless)
`stx $4017`	Disable APU frame counter IRQ
`txs`	Set stack pointer to $01FF
`stx PPUCTRL`	Disable NMI until ready
`stx PPUMASK`	Disable rendering until ready
`stx $4010`	Disable DMC channel IRQ
VBlank waits	Let PPU stabilise
Clear RAM	Initialise all variables to 0

Vector Table

Don’t forget to set up your vectors:

.segment "VECTORS"
    .word nmi               ; $FFFA - NMI vector
    .word reset             ; $FFFC - Reset vector
    .word irq               ; $FFFE - IRQ vector

Patterns: NMI Game Loop, Palette Loading

Vault: NES