Now the machine comes alive - Step fetches, decodes, and dispatches on the top nibble, refusing anything it does not know. Today you wire that loop and give it its first real instruction, 00E0, which clears the screen.
Implement a Step that fetches, decodes, and executes 00E0 (clear), returning an error on an unknown opcode.
This is the engine every instruction plugs into. Step performs one full cycle: fetch the opcode (which advances PC), then decide what to do by switching on the high nibble (op & 0xF000). Most instruction families are chosen by that top nibble alone; a few, like the 0x0 family, need the whole opcode to disambiguate. Today only one instruction exists - 00E0, “clear the display” - so the switch has a single real arm.
The other half of a good dispatcher is what it does with an opcode it does not recognise: it must fail loudly, not silently do nothing and march on. Returning an error that names the exact opcode and the address it came from turns a missing instruction into a clear report instead of a mysterious hang. That unimplemented opcode 0xXXXX at 0xNNN message is the safety net the whole emulator leans on as you fill the switch in one arm at a time.
func (v *VM) Step() error {op := v.Fetch()switch op & 0xF000 {case 0x0000:if op == 0x00E0 { v.ClearDisplay(); return nil }// ... other high nibbles land here as you add opcodes ...}return fmt.Errorf("unimplemented opcode 0x%04X at 0x%03X", op, v.pc-2)}