processor 6502
   ;; This is a simple BIOS for a 6502 machine.
   ;; The BIOS code assumes the following:
   ;;    Random Number generator lives at $fe
   ;;    ASCII keypress is stored at $ff (pseudo-readonly, value only changes when new keypress occurs.
   ;;                 You can write to the location but it changes with next keypress.)
   ;;    Stack will live at $100-$1ff
   ;;    BIOS starts at $600 and ends at $fff (2560 bytes)
   ;;    Screen pixels mapped at $200-$5ff with following color info:
   ;;       $0: Black
   ;;       $1: White
   ;;       $2: Red
   ;;       $3: Cyan
   ;;       $4: Purple
   ;;       $5: Green
   ;;       $6: Blue
   ;;       $7: Yellow
   ;;       $8: Orange
   ;;       $9: Brown
   ;;       $a: Light red
   ;;       $b: Dark gray
   ;;       $c: Gray
   ;;       $d: Light green
   ;;       $e: Light blue
   ;;       $f: Light gray
   ;;    The BIOS also assumes that color info gets ANDed with $0f (not by the BIOS) to always get a valid color.
   ;;    To read the color of a pixel, the BIOS will read from the pixel address. (e.g., to read $200: lda $200)
   ;;    NMI means serious hardware issue
   ;;    IRQ resets processor
   ;;    Interrupt vectors at $fffa
   ;; Anything not mentioned is RAM space.
   ;; Assemble with DASM.

   ;; Let's begin some code, shall we?
   ORG $600
Reset
   ;; Register Immediate addressing mode check
   lda #$ff
   ldx #$ff
   ldy #$ff
   cmp #$ff
   bne die         ;Stop on error
   cpx #$ff
   bne die
   cpy #$ff
   bne die
   ;; Register transfer test
   and #$00
   tax
   tay
   cpx #$00
   bne die
   cpy #$00
   bne die
   ;; store to RAM test
   lda #$ea
   sta $00
   ldx $00
   cpx #$ea
   bne die
   and #$00
   tax
   sta $00
   ;; 'White Dot' test
   lda #$01
   sta $200
   ldx $200
   cpx #$01
   bne die
   and #$00
   tax
   sta $200
   ;; Your code goes here
   jmp Reset      ;Simple filler instruction. Remove if you wish.
die
   ;; Red screen of death
   lda #$02
   ldx #$00
loop
   sta $200, x
   inx
   cpx #$ff
   bne loop
loop2
   sta $2ff, x
   inx
   cpx #$ff
   bne loop2
loop3
   sta $3ff, x
   inx
   cpx #$ff
   bne loop3
loop4
   sta $4ff,x
   inx
   cpx #$ff
   bne loop4
   jmp loop

   ORG $fffa
InterruptVectors
   .word die      ;NMI
   .word Reset      ;RESET
   .word Reset      ;IRQ
END
   

dasm bios.asm -obios.bin -lbios.txt -v5

processor 6502
ORG $F000
; Code goes here
ORG $FFFA
; Interrupt vectors here