145. Bit absolute

add BIT absolute opcode ``0x2C``.

Lesson 145 of 356 · tests/chapter_01_cpu/test_145_BIT_absolute.py

Why this step exists

Absolute BIT extends the flag-only test to the full address space and verifies that its two-byte operand is decoded as a little-endian memory address.

In this step, with BIT semantics and its opcode import established in lessons 143-144, add exactly the following to `emulator/cpu/opcodes.py`:

def bit_absolute(cpu: CPU):
    addr = absolute(cpu)
    value = cpu.bus.read(addr)
    bit(cpu, value)

OPCODE_TABLE = {
    ...
    0x2C: bit_absolute,
}

`emulator/cpu/addressing_modes.py::absolute uses CPU.fetch_word` to decode the little-endian address. The handler performs one bus read and BIT only consumes that value. Z/N/V may change; A, Carry, X, Y, and memory remain invariant; opcode plus word advances PC three bytes.

Misconception: `2C 02 20 does not test literal $2002`; it reads that address, with any bus-owned side effects remaining bus behavior. Out of scope: CMP and its compare opcodes begin in lesson 146.

Run this lesson

uv run pytest tests/chapter_01_cpu/test_145_BIT_absolute.py -v

Example implementation

Why this step exists

def bit_absolute(cpu: CPU):
    addr = absolute(cpu)
    value = cpu.bus.read(addr)
    bit(cpu, value)

OPCODE_TABLE = {
    ...
    0x2C: bit_absolute,
}

Show test codetest_145_BIT_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
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_absolute_handler_exists_and_is_in_opcode_table():
    """Objective: create bit_absolute(cpu) and add 0x2C to OPCODE_TABLE."""
    assert hasattr(opcodes, "bit_absolute")
    assert callable(opcodes.bit_absolute)
    assert list(inspect.signature(opcodes.bit_absolute).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x2C] is opcodes.bit_absolute


def test_opcode_2C_bit_absolute_sets_zero_from_a_and_memory_value():
    """Objective: 2C 00 02 means BIT value at RAM[$0200]."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x2C)
    rom.write(0x0001, 0x00)
    rom.write(0x0002, 0x02)
    bus.write(0x0200, 0b1111_0000)

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

    assert (cpu.p & ZERO_FLAG) != 0
    assert cpu.a == 0b0000_1111
    assert bus.read(0x0200) == 0b1111_0000
    assert cpu.pc == 0x8003


def test_opcode_2C_bit_absolute_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, 0x2C)
    rom.write(0x0001, 0x00)
    rom.write(0x0002, 0x02)
    bus.write(0x0200, 0b1100_0000)

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

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

Test functions (3)

test_bit_absolute_handler_exists_and_is_in_opcode_table
test_opcode_2C_bit_absolute_sets_zero_from_a_and_memory_value
test_opcode_2C_bit_absolute_copies_memory_bits_7_and_6_to_n_and_v