118. And zero page

add AND zero-page opcode ``0x25``.

Lesson 118 of 356 · tests/chapter_01_cpu/test_118_AND_zero_page.py

In this step, with `and_a imported by lesson 117, add only the zero-page handler and table entry in emulator/cpu/opcodes.py`.

Why this step exists

AND must also consume values from memory; zero-page mode verifies that the operand byte is treated as an address rather than as an immediate value.

Suggested implementation

def and_zero_page(cpu: CPU):
    addr = zero_page(cpu)
    value = cpu.bus.read(addr)
    and_a(cpu, value)

OPCODE_TABLE = {
    ...
    0x25: and_zero_page,
}

`addressing_modes.zero_page fetches the one-byte address; unlike immediate mode, the handler must read the value at that address before calling instructions.and_a. 25 10 uses cpu.bus[$0010]`, changes A and Z/N, preserves Carry/Overflow and memory, and advances PC two bytes.

Misconception: neither pass address `0x10 directly to and_a nor write the result back to $0010`; AND is not a read/modify/write instruction. Out of scope: zero-page,X and wider AND modes (lessons 119-124).

Run this lesson

uv run pytest tests/chapter_01_cpu/test_118_AND_zero_page.py -v

Example implementation

Suggested implementation

def and_zero_page(cpu: CPU):
    addr = zero_page(cpu)
    value = cpu.bus.read(addr)
    and_a(cpu, value)

OPCODE_TABLE = {
    ...
    0x25: and_zero_page,
}

Show test codetest_118_AND_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


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_and_zero_page_handler_exists_and_is_in_opcode_table():
    """Objective: create and_zero_page(cpu) and add 0x25 to OPCODE_TABLE."""
    assert hasattr(opcodes, "and_zero_page")
    assert callable(opcodes.and_zero_page)
    assert list(inspect.signature(opcodes.and_zero_page).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x25] is opcodes.and_zero_page


def test_opcode_25_and_zero_page_reads_memory_value_and_updates_a():
    """Objective: 25 10 means AND value at RAM[$0010]."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x25)
    rom.write(0x0001, 0x10)
    bus.write(0x0010, 0x0F)

    cpu.reset()
    cpu.a = 0xF3
    cpu.step()

    assert cpu.a == 0x03
    assert cpu.pc == 0x8002

Test functions (2)

test_and_zero_page_handler_exists_and_is_in_opcode_table
test_opcode_25_and_zero_page_reads_memory_value_and_updates_a