193. 指令 pha

实现 PHA 行为。

第 193 / 356 · tests/chapter_01_cpu/test_193_instruction_pha.py

在本步骤中，只修改 `emulator/cpu/instructions.py，添加 pha(cpu)。栈页起始地址为 STACK_BASE = 0x0100`。

本步骤存在的原因

PHA 把 A 复制到 6502 的硬件栈上。入栈操作先写入当前的 $0100 | S 槽位，然后再递减这个 8 位栈指针。

建议实现

cpu.bus.write(0x0100 | cpu.s, cpu.a)
cpu.s = (cpu.s - 1) & 0xFF

把这些语句放入 `def pha(cpu: CPU) 中；用模块级常量 STACK_BASE = 0x0100` 代替字面量效果是等价的。

不变量：只压入 A；先写入再递减；把 S 按 8 位环绕处理；A 和所有状态标志保持不变。

常见误解：先递减 S 并不能实现 6502 的入栈操作；这样写入的是下一个空闲槽位，而不是当前的栈槽位。

范围之外：导入/注册操作码 $48 属于第 194 步。PLA、PHP、PLP、TXS 和 TSX 属于后续的第 195-204 步，这里不得添加。

运行本课

uv run pytest tests/chapter_01_cpu/test_193_instruction_pha.py -v

示例实现

建议实现

cpu.bus.write(0x0100 | cpu.s, cpu.a)
cpu.s = (cpu.s - 1) & 0xFF

查看测试代码test_193_instruction_pha.py

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


CARRY_FLAG = 1 << 0
INTERRUPT_DISABLE_FLAG = 1 << 2
OVERFLOW_FLAG = 1 << 6
STACK_BASE = 0x0100


def test_pha_pushes_accumulator_to_current_stack_address():
    """Objective: PHA writes A to $0100 | S before changing S."""
    cpu = make_cpu()
    cpu.a = 0x42
    cpu.s = 0xFD

    pha(cpu)

    assert cpu.bus.read(STACK_BASE | 0xFD) == 0x42


def test_pha_decrements_stack_pointer_after_push():
    """Objective: after one push, S decreases by 1."""
    cpu = make_cpu()
    cpu.a = 0x42
    cpu.s = 0xFD

    pha(cpu)

    assert cpu.s == 0xFC


def test_pha_does_not_modify_accumulator():
    """Objective: PHA copies A to the stack but leaves A unchanged."""
    cpu = make_cpu()
    cpu.a = 0x99
    cpu.s = 0xFD

    pha(cpu)

    assert cpu.a == 0x99


def test_pha_stack_pointer_wraps_to_8_bits():
    """Objective: S is an 8-bit stack pointer and wraps after decrementing."""
    cpu = make_cpu()
    cpu.a = 0xAB
    cpu.s = 0x00

    pha(cpu)

    assert cpu.bus.read(STACK_BASE | 0x00) == 0xAB
    assert cpu.s == 0xFF


def test_pha_can_push_zero_value_without_setting_zero_flag():
    """
    Objective:
    PHA is a stack transfer, not an arithmetic/load instruction.

    Even if A is $00, PHA must not update the Zero flag.
    """
    cpu = make_cpu()
    cpu.a = 0x00
    cpu.s = 0xFD
    cpu.flags.set_zero_flag(False)

    pha(cpu)

    assert cpu.bus.read(STACK_BASE | 0xFD) == 0x00
    assert cpu.flags.get_zero_flag() is False


def test_pha_can_push_negative_value_without_setting_negative_flag():
    """
    Objective:
    PHA must not update Negative based on bit 7 of A.

    It only saves A to stack.
    """
    cpu = make_cpu()
    cpu.a = 0x80
    cpu.s = 0xFD
    cpu.flags.set_negative_flag(False)

    pha(cpu)

    assert cpu.bus.read(STACK_BASE | 0xFD) == 0x80
    assert cpu.flags.get_negative_flag() is False


def test_pha_preserves_status_flags():
    """Objective: PHA does not modify any processor status flags."""
    cpu = make_cpu()
    cpu.a = 0x42
    cpu.s = 0xFD
    cpu.p = CARRY_FLAG | ZERO_FLAG | INTERRUPT_DISABLE_FLAG | OVERFLOW_FLAG | NEGATIVE_FLAG

    pha(cpu)

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

测试函数 (7)

test_pha_pushes_accumulator_to_current_stack_address
test_pha_decrements_stack_pointer_after_push
test_pha_does_not_modify_accumulator
test_pha_stack_pointer_wraps_to_8_bits
test_pha_can_push_zero_value_without_setting_zero_flag
test_pha_can_push_negative_value_without_setting_negative_flag
test_pha_preserves_status_flags