201. 指令 txs

实现 TXS 操作。

第 201 / 356 · tests/chapter_01_cpu/test_201_instruction_txs.py

本步骤存在的原因

在本步骤中，添加 `emulator/cpu/instructions.py::txs`。之所以现在引入这个操作，是因为栈传送是剩余的栈指令行为：将 X 复制到栈指针字节，且不产生内存访问。

建议实现

def txs(cpu: CPU):
    cpu.s = cpu.x

不变式：X、P、PC、A、Y 以及内存均保持不变；S 仍然是已建立的 $0100-$01FF 栈页内的低字节偏移量。一个常见的误解是像其他寄存器传送指令那样更新 Zero 和 Negative 标志；TXS 不改变任何标志，即使 X 为 $00 或第 7 位被置位也是如此。

范围之外：在 `emulator/cpu/opcodes.py::OPCODE_TABLE` 中注册隐含寻址操作码 $9A 属于步骤 202；TSX 及其操作码属于步骤 203-204。此处不要添加那些后续的 API。

运行本课

uv run pytest tests/chapter_01_cpu/test_201_instruction_txs.py -v

示例实现

建议实现

def txs(cpu: CPU):
    cpu.s = cpu.x

查看测试代码test_201_instruction_txs.py

from emulator.cpu.instructions import txs
from tests.helpers import NEGATIVE_FLAG, ZERO_FLAG, make_cpu


CARRY_FLAG = 1 << 0
OVERFLOW_FLAG = 1 << 6


def test_txs_copies_x_to_stack_pointer():
    """Objective: TXS sets S to the current value of X."""
    cpu = make_cpu()
    cpu.x = 0x80
    cpu.s = 0xFD

    txs(cpu)

    assert cpu.s == 0x80


def test_txs_does_not_modify_x():
    """Objective: TXS copies X but leaves X unchanged."""
    cpu = make_cpu()
    cpu.x = 0x42
    cpu.s = 0xFD

    txs(cpu)

    assert cpu.x == 0x42


def test_txs_can_set_stack_pointer_to_zero_without_setting_zero_flag():
    """
    Objective:
    TXS does not update Zero, even if X is $00.
    """
    cpu = make_cpu()
    cpu.x = 0x00
    cpu.s = 0xFD
    cpu.flags.set_zero_flag(False)

    txs(cpu)

    assert cpu.s == 0x00
    assert cpu.flags.get_zero_flag() is False


def test_txs_can_set_stack_pointer_to_negative_value_without_setting_negative_flag():
    """
    Objective:
    TXS does not update Negative, even if bit 7 of X is set.
    """
    cpu = make_cpu()
    cpu.x = 0x80
    cpu.s = 0xFD
    cpu.flags.set_negative_flag(False)

    txs(cpu)

    assert cpu.s == 0x80
    assert cpu.flags.get_negative_flag() is False


def test_txs_preserves_all_status_flags():
    """Objective: TXS is a register transfer to S and does not change flags."""
    cpu = make_cpu()
    cpu.x = 0x12
    cpu.s = 0xFD
    cpu.p = CARRY_FLAG | ZERO_FLAG | OVERFLOW_FLAG | NEGATIVE_FLAG

    txs(cpu)

    assert cpu.p == (CARRY_FLAG | ZERO_FLAG | OVERFLOW_FLAG | NEGATIVE_FLAG)

测试函数 (5)

test_txs_copies_x_to_stack_pointer
test_txs_does_not_modify_x
test_txs_can_set_stack_pointer_to_zero_without_setting_zero_flag
test_txs_can_set_stack_pointer_to_negative_value_without_setting_negative_flag
test_txs_preserves_all_status_flags