i6502/cpu/instruction.go

package cpu

import (
	"fmt"
	"github.com/ariejan/i6502/bus"
)

// Instruction is a self-contained, variable length instruction,
// it includes the the operation type and a possible 8 or 16 bit operand.
type Instruction struct {
	OpType

	// 2 byte instructions have a single byte operand
	Op8 uint8

	// 3 byte instruction have a double byte operand
	Op16 uint16
}

func ReadInstruction(pc uint16, bus *bus.Bus) Instruction {
	// Read the opcode
	opcode := bus.Read(pc)

	// Do we know this opcode in our optypes table?
	optype, ok := optypes[opcode]
	if !ok {
		panic(fmt.Sprintf("Unknown opcode $%02X at $04X", opcode, pc))
	}

	instruction := Instruction{OpType: optype}
	switch instruction.Bytes {
	case 1: // No operand.
	case 2:
		instruction.Op8 = bus.Read(pc + 1)
	case 3:
		instruction.Op16 = bus.Read16(pc + 1)
	default:
		panic(fmt.Sprintf("Unknown instruction length (%d) for $%02X at $04X", instruction.Bytes, opcode, pc))
	}

	return instruction
}