192. Rti implied

Lesson 192 of 356 · tests/chapter_01_cpu/test_192_RTI_implied.py

Prerequisite: step 191 added `rti. In this step, change only emulator/cpu/opcodes.py by importing that symbol and registering opcode $40 in OPCODE_TABLE`.

Why this step exists

RTI is a one-byte implied instruction. `CPU.step()` has already fetched $40, and all remaining inputs come from the stack, so dispatch must be wired directly to the operation without consuming an operand.

Suggested implementation

from emulator.cpu.instructions import rti

OPCODE_TABLE = {
    # existing entries
    0x40: rti,
}

Invariants: preserve every existing opcode entry; map exactly integer $40 to the same `rti function object; add no operand fetch or wrapper; let rti(cpu)` determine PC from the stack rather than sequential execution.

Misconception: implied does not mean that the byte after $40 is an operand. Fetching one would consume the next instruction before RTI restores PC.

Out of scope: RTI's stack algorithm is step 191. PHA and the remaining stack instructions begin at step 193. NMI entry is later behavior and must not be anticipated here.

Run this lesson

uv run pytest tests/chapter_01_cpu/test_192_RTI_implied.py -v

Example implementation

Suggested implementation

from emulator.cpu.instructions import rti

OPCODE_TABLE = {
    # existing entries
    0x40: rti,
}

Show test codetest_192_RTI_implied.py

import inspect

from emulator.bus.cpu_bus import CpuBus
from emulator.cpu import opcodes
from emulator.cpu.cpu import CPU
from emulator.cpu.instructions import rti
from emulator.memory.fake_rom import FakeROM


CARRY_FLAG = 1 << 0
BREAK_FLAG = 1 << 4
ONE_FLAG = 1 << 5
STACK_BASE = 0x0100


def make_cpu_with_rom():
    rom = FakeROM()
    rom.write(0x7FFC, 0x00)
    rom.write(0x7FFD, 0x80)
    bus = CpuBus(program_rom=rom)
    return CPU(bus), bus, rom


def test_rti_implied_is_in_opcode_table():
    """Objective: opcode 0x40 is the official RTI opcode."""
    assert opcodes.OPCODE_TABLE[0x40] is rti


def test_rti_instruction_signature_takes_only_cpu():
    """Objective: RTI is implied, so rti(cpu) does not need an operand argument."""
    assert list(inspect.signature(rti).parameters) == ["cpu"]


def test_opcode_40_rti_restores_pc_and_status_from_stack():
    """Objective: executing opcode 0x40 restores status and PC from the stack."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x40)

    cpu.reset()
    cpu.s = 0xFA
    bus.write(STACK_BASE | 0xFB, CARRY_FLAG)
    bus.write(STACK_BASE | 0xFC, 0x34)
    bus.write(STACK_BASE | 0xFD, 0x12)

    cpu.step()

    assert cpu.p == CARRY_FLAG
    assert cpu.pc == 0x1234
    assert cpu.s == 0xFD


def test_opcode_40_rti_does_not_add_one_to_pulled_pc():
    """
    Objective:
    RTI must not behave like RTS.

    If the stack contains PC $8002, RTI returns to $8002 exactly.
    """
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x40)

    cpu.reset()
    cpu.s = 0xFA
    bus.write(STACK_BASE | 0xFB, 0x00)
    bus.write(STACK_BASE | 0xFC, 0x02)
    bus.write(STACK_BASE | 0xFD, 0x80)

    cpu.step()

    assert cpu.pc == 0x8002


def test_opcode_40_rti_does_not_fetch_operand_bytes():
    """
    Objective:
    RTI is one byte long. The byte after RTI must not be consumed as an operand.

    The return address comes only from the stack, not from program bytes after
    opcode 0x40.
    """
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x40)
    rom.write(0x0001, 0x99)
    rom.write(0x0002, 0x88)

    cpu.reset()
    cpu.s = 0xFA
    bus.write(STACK_BASE | 0xFB, 0x00)
    bus.write(STACK_BASE | 0xFC, 0xCD)
    bus.write(STACK_BASE | 0xFD, 0xAB)

    cpu.step()

    assert cpu.pc == 0xABCD


def test_opcode_40_rti_clears_break_from_restored_cpu_status():
    """
    Objective:
    If the saved status byte has Break/ONE set, RTI does not keep those bits as
    persistent CPU status in this emulator model.
    """
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x40)

    cpu.reset()
    cpu.s = 0xFA
    bus.write(STACK_BASE | 0xFB, BREAK_FLAG | ONE_FLAG | CARRY_FLAG)
    bus.write(STACK_BASE | 0xFC, 0x34)
    bus.write(STACK_BASE | 0xFD, 0x12)

    cpu.step()

    assert cpu.pc == 0x1234
    assert cpu.p == CARRY_FLAG
    assert cpu.flags.get_break_flag() is False
    assert cpu.flags.get_one_flag() is False

Test functions (6)

test_rti_implied_is_in_opcode_table
test_rti_instruction_signature_takes_only_cpu
test_opcode_40_rti_restores_pc_and_status_from_stack
test_opcode_40_rti_does_not_add_one_to_pulled_pc
test_opcode_40_rti_does_not_fetch_operand_bytes
test_opcode_40_rti_clears_break_from_restored_cpu_status