Minimal Famicom assembly example using cc65 which (in PAL mode only, apparently?) flips the screen colour every 256 frames

game.s

PPUMASK     = $2001
PPUSTATUS   = $2002

; The iNES header which makes this into an actual “rom”
.segment "INESHDR"
  .byt "NES", $1A   ; magic number!
  .byt 1            ; program ROM size in multiples of 16384
  .byt 1            ; character ROM size in multiples of 8192
  .byt 0, 0         ; flags including mapper (which in this case is 0 for NROM)
  .byt 2            ; program ROM size in multiples of 8192
  .byt 1            ; PAL flag

; Interrupt addresses
.segment "VECTORS"
    .addr nmi, reset, irq

; Self-explanatory :p
.segment "CODE"
.proc reset
    ; Init code time! I'll admit this is very much “cargo cult”.
    ; Following some of http://wiki.nesdev.com/w/index.php/Init_code
    sei             ; ignore IRQs (why?) 
    cld             ; disable decimal mode (which is redundant actually)
    ldx #$40
    stx $4017       ; disable APU frame IRQ (…so we don't get interrupted?)
    ldx #$ff
    txs             ; set up stack
    stx $2000       ; disable NMI
    stx $2001       ; disable rendering
    stx $4010       ; disable DMC IRQs
    ; We want to use the PPU, so we can draw stuff! Let's wait for it.
    ; http://wiki.nesdev.com/w/index.php/PPU_power_up_state#Best_practice
    bit PPUSTATUS   ; clears the VBL flag if it was set at reset time
vwait1:
    bit PPUSTATUS
    bpl vwait1      ; at this point, about 27384 cycles have passed
vwait2:
    bit PPUSTATUS
    bpl vwait2      ; at this point, about 57165 cycles have passed
    
    ; video works now! let's do an effect or something

    lda #$00        ; initial value of toggle
    sta $0100

    lda #%11111000  ; set PPU state to something less silly
    sta PPUMASK

forever:
    jmp forever
.endproc

.proc nmi
    lda $0100
    tay
    bne skip
    lda PPUMASK     ; flip emphasise RGB bits
    eor #%1110000
    sta PPUMASK
skip:
    iny
    sty $0100
    rti
.endproc

.proc irq
    rti
.endproc

Makefile

game.nes: game.o
	ld65 -C nrom128.x game.o -o game.nes

game.o: game.s
	ca65 game.s -o game.o

clean:
	rm *.o *.nes

nrom128.x

# Taken from http://bin.smwcentral.net/u/1780/ex.zip

MEMORY {
  ZP:     start = $10, size = $f0, type = rw;
  # use first $10 zeropage locations as locals
  HEADER: start = $7f00, size = $0010, type = ro, file = %O, fill = yes, fillval = $00;
  RAM:    start = $0300, size = $0500, type = rw;
  PRG0:   start = $C000, size = $4000, type = ro, file = %O, fill = yes, fillval = $FF;
  CHR0:   start = $0000, size = $2000;
}

SEGMENTS {
  INESHDR:  load = HEADER, type = ro, align = $10;
  ZEROPAGE: load = ZP, type = zp;
  BSS:      load = RAM, type = bss, define = yes, align = $100;
  CODE:     load = PRG0, type = ro, align = $100;
  RODATA:   load = PRG0, type = ro, align = $100;
  DMC:      load = PRG0, type = ro, align = $40, optional = yes;
  VECTORS:  load = PRG0, type = ro, start = $FFFA;
  CHR:      load = CHR0, type = ro;
}

FILES {
  %O: format = bin;
}