187. Rts implied

wire implied RTS opcode $60.

Lesson 187 of 356 · tests/chapter_01_cpu/test_187_RTS_implied.py

Prerequisite: step 186 supplied `rts. In this step, add its import and direct table mapping in emulator/cpu/opcodes.py`:

from emulator.cpu.instructions import rts

OPCODE_TABLE = {
    # existing entries...
    0x60: rts,
}

Opcode

0x60 -> RTS

Goal

add opcode 0x60 to OPCODE_TABLE.

Student guidance

RTS uses implied addressing. That means the opcode has no operand bytes. The instruction gets all information it needs from the CPU stack.

Example

60 -> RTS

Execution steps

1. CPU.step() fetches opcode 0x60.
2. OPCODE_TABLE dispatches directly to rts(cpu).
3. RTS pulls the return address from the stack.
4. RTS sets PC to pulled_address + 1.

Common mistake

Do not fetch an operand byte for RTS. It is an implied instruction.

Why this step exists

`CPU.step has already consumed the sole opcode byte, and rts` gets its return word from the stack, so no wrapper or addressing mode is needed. Invariants: $60 maps directly to the one-argument function; dispatch does not consume $8001; RTS alone performs the two pulls and PC update. Misconception: every opcode does not require a named handler.

Out of scope: flag helpers and BRK are steps 188-190; RTI and opcode $40 are steps 191-192.

Run this lesson

uv run pytest tests/chapter_01_cpu/test_187_RTS_implied.py -v

Example implementation

from emulator.cpu.instructions import rts

OPCODE_TABLE = {
    # existing entries...
    0x60: rts,
}

Show test codetest_187_RTS_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 rts
from emulator.memory.fake_rom import FakeROM


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_rts_implied_is_in_opcode_table():
    """Objective: opcode 0x60 is the official RTS opcode."""
    assert opcodes.OPCODE_TABLE[0x60] is rts


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


def test_opcode_60_rts_restores_program_counter_from_stack():
    """Objective: executing opcode 0x60 returns to the address saved on the stack."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x60)

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

    cpu.step()

    assert cpu.pc == 0x8003
    assert cpu.s == 0xFD


def test_opcode_60_rts_does_not_fetch_operand_bytes():
    """
    Objective:
    RTS is one byte long. The byte after RTS must not be consumed as an operand.

    If RTS incorrectly fetched an operand byte, stack behavior or PC behavior could
    accidentally depend on the byte at $8001. This test makes the return target
    come only from the stack.
    """
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x60)
    rom.write(0x0001, 0x99)

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

    cpu.step()

    assert cpu.pc == 0x1235
    assert cpu.s == 0xFD


def test_opcode_60_rts_can_return_after_jsr_style_stack_contents():
    """
    Objective:
    JSR to a subroutine at $9000 would push return address $8002.
    RTS then returns to $8003, the instruction after JSR.
    """
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x1000, 0x60)

    cpu.reset()
    cpu.pc = 0x9000
    cpu.s = 0xFB
    bus.write(STACK_BASE | 0xFC, 0x02)
    bus.write(STACK_BASE | 0xFD, 0x80)

    cpu.step()

    assert cpu.pc == 0x8003
    assert cpu.s == 0xFD

Test functions (5)

test_rts_implied_is_in_opcode_table
test_rts_instruction_signature_takes_only_cpu
test_opcode_60_rts_restores_program_counter_from_stack
test_opcode_60_rts_does_not_fetch_operand_bytes
test_opcode_60_rts_can_return_after_jsr_style_stack_contents