187. RTS 隐含寻址

接入隐含寻址的 RTS 操作码 $60。

第 187 / 356 · tests/chapter_01_cpu/test_187_RTS_implied.py

前置条件:第 186 步已提供 `rts。在本步骤中,在 emulator/cpu/opcodes.py` 中添加其导入以及直接的表映射:

from emulator.cpu.instructions import rts

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

操作码

0x60 -> RTS

目标

把操作码 0x60 添加到 OPCODE_TABLE 中。

学习指引

RTS 使用隐含寻址。这意味着该操作码没有操作数字节。该指令所需的全部信息都来自 CPU 栈。

示例

60 -> RTS

执行步骤

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.

常见错误

不要为 RTS 取操作数字节。它是一条隐含寻址指令。

本步骤存在的原因

`CPU.step 已经消费了唯一的操作码字节,而 rts` 从栈上获取其返回字,因此不需要包装函数或寻址方式。不变量:$60 直接映射到该单参数函数;分发过程不消费 $8001;RTS 单独完成两次出栈操作以及 PC 更新。常见误解:并非每个操作码都需要一个具名的处理函数。

本步骤范围之外:标志位辅助函数与 BRK 属于第 188-190 步;RTI 与操作码 $40 属于第 191-192 步。

运行本课

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

示例实现

from emulator.cpu.instructions import rts

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

查看测试代码test_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

测试函数 (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