Lesson 24 · DXYN - draw a sprite - Build a CHIP-8 Emulator

DXYN - draw a sprite

This is the instruction that makes CHIP-8 visible - it XORs a sprite onto the screen and reports collisions. Today you implement the core of DXYN: draw N rows from memory at (VX, VY) by XOR, setting VF when a lit pixel is erased.

The goal

Implement DXYN so it XOR-draws an N-row sprite at (VX, VY) and sets VF on collision.

Start here - the target

TO DO

Scenario: A sprite draws by XOR and flags an erase

GivenI points at a byte 0xFF in memory, V0 = 0 (x), V1 = 0 (y), on a blank screen, about to execute 0xD011 (draw 1 row)

WhenStep runs it

Thenthe pixels at columns 0 through 7 of row 0 turn on, and VF is 0 (nothing was erased)

Andrunning the same 0xD011 again XORs those eight pixels back off and sets VF to 1 (a collision - lit pixels were erased)

Background

DXYN is the heart of CHIP-8 graphics. It draws an N-row sprite whose bytes live in memory starting at I, placing its top-left corner at pixel (VX, VY). Each byte is one row of eight pixels, most-significant bit on the left, and drawing is done by XOR: a 1 bit flips the pixel it lands on. XOR drawing is what lets a program erase a sprite by simply drawing it a second time in the same place, which is how CHIP-8 animates.

That same XOR is why the machine needs a collision flag. Before drawing, VF is cleared to 0; then, if drawing ever turns an already-lit pixel off (a 1 sprite bit meeting a 1 screen pixel), VF is set to 1. Games read VF right after a draw to detect that a sprite overlapped something - a ball hitting a paddle, a missile hitting an invader. Today assume the sprite sits fully on screen; the test pins both halves of the behaviour by drawing once (pixels on, VF = 0) and again (pixels off, VF = 1). Wrapping and clipping at the edges come next.

Make it work

case 0xD000:
  x, y, n := byte(op>>8&0x0F), byte(op>>4&0x0F), byte(op&0x0F)
  px, py := int(v.V[x]), int(v.V[y])
  v.V[0xF] = 0
  for row := 0; row < int(n); row++ {
    b := v.mem[v.i+uint16(row)]
    for col := 0; col < 8; col++ {
      if b&(0x80>>col) != 0 { /* XOR this pixel; if it was on, set VF=1 */ }
    }
  }

CheckpointDONE

DXYN draws a sprite by XOR and reports a collision in VF. Commit and stop here.