144. Bit 零页寻址

添加 BIT zero-page 操作码 ``0x24``。

第 144 / 356 · tests/chapter_01_cpu/test_144_BIT_zero_page.py

此步骤存在的原因

第一个 BIT 操作码必须从内存获取操作数而不是修改 A,然后将其独特的 Z/N/V 规则委托给指令原语。

在此步骤中,第 143 课已经添加了 `bit。作为 BIT 的第一个操作码课程,请添加其导入,并在 emulator/cpu/opcodes.py` 中准确添加以下处理函数/表项:

from emulator.cpu.instructions import (..., bit)

def bit_zero_page(cpu: CPU):
    addr = zero_page(cpu)
    value = cpu.bus.read(addr)
    bit(cpu, value)

OPCODE_TABLE = {
    ...
    0x24: bit_zero_page,
}

`addressing_modes.zero_page` 将操作数当作地址来使用;总线读取提供该值,而 BIT 测试的正是它的各个位。 Z/N/V 可能改变,而 A、Carry、X、Y 和内存保持不变;操作码加操作数使 PC 前进两个字节。

常见误解:操作数字节本身并不会被直接测试,并且 BIT 不会写入被寻址的字节。超出范围:BIT absolute 属于第 145 课。

运行本课

uv run pytest tests/chapter_01_cpu/test_144_BIT_zero_page.py -v

示例实现

此步骤存在的原因

from emulator.cpu.instructions import (..., bit)

def bit_zero_page(cpu: CPU):
    addr = zero_page(cpu)
    value = cpu.bus.read(addr)
    bit(cpu, value)

OPCODE_TABLE = {
    ...
    0x24: bit_zero_page,
}

查看测试代码test_144_BIT_zero_page.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
from tests.helpers import NEGATIVE_FLAG, ZERO_FLAG


OVERFLOW_FLAG = 1 << 6


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_bit_zero_page_handler_exists_and_is_in_opcode_table():
    """Objective: create bit_zero_page(cpu) and add 0x24 to OPCODE_TABLE."""
    assert hasattr(opcodes, "bit_zero_page")
    assert callable(opcodes.bit_zero_page)
    assert list(inspect.signature(opcodes.bit_zero_page).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x24] is opcodes.bit_zero_page


def test_opcode_24_bit_zero_page_sets_zero_from_a_and_memory_value():
    """Objective: 24 10 means BIT value at RAM[$0010]."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x24)
    rom.write(0x0001, 0x10)
    bus.write(0x0010, 0b1111_0000)

    cpu.reset()
    cpu.a = 0b0000_1111
    cpu.step()

    assert (cpu.p & ZERO_FLAG) != 0
    assert cpu.a == 0b0000_1111
    assert bus.read(0x0010) == 0b1111_0000
    assert cpu.pc == 0x8002


def test_opcode_24_bit_zero_page_copies_memory_bits_7_and_6_to_n_and_v():
    """Objective: memory bits 7 and 6 are copied into Negative and Overflow."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x24)
    rom.write(0x0001, 0x10)
    bus.write(0x0010, 0b1100_0000)

    cpu.reset()
    cpu.a = 0x00
    cpu.step()

    assert (cpu.p & NEGATIVE_FLAG) != 0
    assert (cpu.p & OVERFLOW_FLAG) != 0

测试函数 (3)

test_bit_zero_page_handler_exists_and_is_in_opcode_table
test_opcode_24_bit_zero_page_sets_zero_from_a_and_memory_value
test_opcode_24_bit_zero_page_copies_memory_bits_7_and_6_to_n_and_v