185. JSR 绝对寻址

接入 JSR 绝对寻址操作码 $20。

第 185 / 356 · tests/chapter_01_cpu/test_185_JSR_absolute.py

前置条件:第 184 步已添加 `jsr。在本步骤中,向 emulator/cpu/opcodes.py` 添加以下改动:

from emulator.cpu.instructions import jsr
from emulator.cpu.addressing_modes import absolute

def jsr_absolute(cpu: CPU):
    addr = absolute(cpu)
    jsr(cpu, addr)

OPCODE_TABLE = {
    # existing entries...
    0x20: jsr_absolute,
}

操作码

0x20 -> JSR $hhhh

目标

创建 jsr_absolute(cpu),使用 absolute(cpu),然后调用 jsr(cpu, addr)。

学习指引

JSR 绝对寻址使用一个 16 位小端序操作数作为子程序地址。

示例

20 34 12 -> JSR $1234

执行步骤

1. CPU.step() fetches opcode 0x20.
2. absolute(cpu) fetches operand bytes 34 12 and returns $1234.
3. PC now points to the next instruction, $8003 in these tests.
4. jsr(cpu, $1234) pushes return address $8002.
5. PC becomes $1234.

常见错误

不要调用 jmp(cpu, addr)。JSR 必须先压入返回地址。

本步骤存在的原因

`absolute 消费两字节的目标地址,使 PC 停留在下一条指令处,从而让 jsr` 能够压入 PC 减一后的值并跳转。不变量:该步骤消费操作码加操作数,先压入高字节再压入低字节,S 递减两次,PC 变为目标地址,并保持状态不变。常见误解:与 JMP 共用同一种寻址方式,并不意味着 JSR 只是一次普通跳转。

本步骤范围之外:`rts` 与操作码 $60 属于第 186-187 步。BRK、RTI 以及后续通用的栈辅助函数不属于本次改动。

运行本课

uv run pytest tests/chapter_01_cpu/test_185_JSR_absolute.py -v

示例实现

from emulator.cpu.instructions import jsr
from emulator.cpu.addressing_modes import absolute

def jsr_absolute(cpu: CPU):
    addr = absolute(cpu)
    jsr(cpu, addr)

OPCODE_TABLE = {
    # existing entries...
    0x20: jsr_absolute,
}

查看测试代码test_185_JSR_absolute.py

import inspect

from emulator.bus.cpu_bus import CpuBus
from emulator.cpu import opcodes
from emulator.cpu.cpu import CPU
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_jsr_absolute_handler_exists_and_is_in_opcode_table():
    """Objective: create jsr_absolute(cpu) and add 0x20 to OPCODE_TABLE."""
    assert hasattr(opcodes, "jsr_absolute")
    assert callable(opcodes.jsr_absolute)
    assert list(inspect.signature(opcodes.jsr_absolute).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x20] is opcodes.jsr_absolute


def test_opcode_20_jsr_absolute_jumps_to_subroutine_address():
    """Objective: 20 34 12 means JSR $1234, so PC becomes $1234."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x20)
    rom.write(0x0001, 0x34)
    rom.write(0x0002, 0x12)

    cpu.reset()
    cpu.step()

    assert cpu.pc == 0x1234


def test_opcode_20_jsr_absolute_pushes_return_address_to_stack():
    """
    Objective:
    After fetching opcode + 2-byte operand, PC is $8003.
    JSR pushes $8002, high byte first, low byte second.
    """
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x20)
    rom.write(0x0001, 0x34)
    rom.write(0x0002, 0x12)

    cpu.reset()
    cpu.step()

    assert bus.read(STACK_BASE | 0xFD) == 0x80
    assert bus.read(STACK_BASE | 0xFC) == 0x02
    assert cpu.s == 0xFB


def test_opcode_20_jsr_absolute_does_not_behave_like_jmp():
    """
    Objective:
    JSR must save a return address. If jsr_absolute accidentally calls jmp,
    PC may be correct, but the stack will not contain the return address.
    """
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x20)
    rom.write(0x0001, 0x34)
    rom.write(0x0002, 0x12)

    cpu.reset()
    cpu.step()

    assert cpu.pc == 0x1234
    assert bus.read(STACK_BASE | 0xFD) == 0x80
    assert bus.read(STACK_BASE | 0xFC) == 0x02

测试函数 (4)

test_jsr_absolute_handler_exists_and_is_in_opcode_table
test_opcode_20_jsr_absolute_jumps_to_subroutine_address
test_opcode_20_jsr_absolute_pushes_return_address_to_stack
test_opcode_20_jsr_absolute_does_not_behave_like_jmp