Lesson 02 · The V registers - Build a CHIP-8 Emulator

The V registers

CHIP-8 does its arithmetic in sixteen fast 8-bit registers named V0 through VF. Today you add that register file to the VM and read and write it, the scratchpad every opcode from here on will use.

The goal

Add sixteen 8-bit general registers V0-VF to the VM with read and write access.

Start here - the target

TO DO

Scenario: The register file holds sixteen independent bytes

Givena new VM

When0x2A is written to register 0 (V0) and 0xFF to register 15 (VF)

Thenreading V0 returns 0x2A and reading VF returns 0xFF

Andall fourteen other registers still read 0x00, and there are exactly sixteen registers

Background

A CHIP-8 program can not compute directly in memory - it moves values into registers first. There are sixteen of them, each one byte wide, named V0 through VF (that is hexadecimal, so VF is register 15). Almost every opcode names a register by its single hex digit, which is why there are exactly sixteen.

One of them is special by convention: VF is the flag register. Later opcodes use it to report a carry, a borrow, or a sprite collision, so a program is not supposed to keep important data there. You do not need any of that flag behaviour today - just build the plain array of sixteen bytes and confirm each one stores and returns a value on its own. The flag semantics arrive when the arithmetic does.

Make it work

type VM struct {
  mem [4096]byte
  V   [16]byte // general registers V0..VF; VF doubles as a flag register
}
func (v *VM) SetV(x byte, val byte) { v.V[x] = val }
func (v *VM) GetV(x byte) byte { return v.V[x] }

CheckpointDONE

The VM has sixteen general registers that store and return bytes. Commit and stop here.