Plotting a pixel on the GameBoy Advance with the GVASM assembler for ARM

plotpixel.gvasm

// ==================================================================
// ============== Pixel Plotting on GBA with GVASM ==================
// ==================================================================
.stdlib

.def MODE_3 = 0x3
.def BG2_ENABLE = 0x400
.def VRAM = 0x06000000
.def COLOUR = rgb(0, 15, 31)

// Header: Required for ROM to be executed
.begin
  .arm
  b main
  .logo
  .title "PLOTPIXEL"
  .str "CUNE77"
  .i16 150, 0, 0, 0, 0
  .i8 0 // version
  .crc
  .i16 0
.end

.begin main
  .arm
  .regs r0-r3, x, y, r6-r11

  // Set display mode 3 and enable BG2
  ldr r0, =REG_DISPCNT
  ldr r1, =(MODE_3 | BG2_ENABLE)
  strh r1, [r0]

  // Set coordinates (x = 120, y = 80)
  ldr x, =80
  ldr y, =80

  // Calculate pixel address: VRAM + (y * 240 + x) * 2
  mov r2, #240         // Screen width in pixels
  mul r3, y, r2        // y * 240
  add r3, r3, x        // y * 240 + x
  lsl r3, r3, #1       // Multiply by 2 (16 bits per pixel)
  ldr x, =VRAM
  add r3, x, r3       // VRAM + offset

  // Plot the pixel
  ldr y, =COLOUR
  strh y, [r3]

  // Plot some pixels in a line
  mov r7, #0
plot:
  add r3, #240
  add r7, #1
  strh y, [r3]
  cmp r7, #0x10
  blt plot

  // Infinite loop
- b -
  .pool
.end