055. Adc zero page

Connect ADC zero page (opcode $65) to CPU dispatch.

Lesson 55 of 356 · tests/chapter_01_cpu/test_055_ADC_zero_page.py

File to update

emulator/cpu/opcodes.py

Symbols to create/update

opcodes.adc_zero_page
OPCODE_TABLE[$65]

Why this step exists

Unlike immediate mode, zero_page(cpu) resolves the operand byte to an address. The handler must read the value at that address before handing the value to the existing adc instruction.

Complete example implementation

# emulator/cpu/opcodes.py
def adc_zero_page(cpu):
    addr = zero_page(cpu)
    value = cpu.bus.read(addr)
    adc(cpu, value)

OPCODE_TABLE = {
    # ...existing entries...
    0x65: adc_zero_page,
}

Important invariants

the one-byte operand is an address in $0000-$00FF
exactly one data value is read and passed to adc
PC advances by two bytes total
address resolution remains in the opcode layer

Common misconception

Do not pass the zero-page address itself to adc; ADC adds the byte stored at that address.

Out of scope

indexed, absolute, and indirect ADC modes
changes to zero_page or adc
cycle accounting

Run this lesson

uv run pytest tests/chapter_01_cpu/test_055_ADC_zero_page.py -v

Example implementation

Complete example implementation

# emulator/cpu/opcodes.py
def adc_zero_page(cpu):
    addr = zero_page(cpu)
    value = cpu.bus.read(addr)
    adc(cpu, value)

OPCODE_TABLE = {
    # ...existing entries...
    0x65: adc_zero_page,
}

Show test codetest_055_ADC_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_adc_zero_page_handler_exists_and_is_in_opcode_table():
    """Objective: create adc_zero_page(cpu) and add 0x65 to OPCODE_TABLE."""
    assert hasattr(opcodes, "adc_zero_page")
    assert callable(opcodes.adc_zero_page)
    assert list(inspect.signature(opcodes.adc_zero_page).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x65] is opcodes.adc_zero_page


def test_opcode_65_adc_zero_page_adds_value_from_memory():
    """Objective: 65 10 means ADC $10, so add RAM[$0010] to A."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x65)
    rom.write(0x0001, 0x10)
    bus.write(0x0010, 0x05)

    cpu.reset()
    cpu.a = 0x10
    cpu.flags.set_carry_flag(False)
    cpu.step()

    assert cpu.a == 0x15
    assert cpu.pc == 0x8002

Test functions (2)

test_adc_zero_page_handler_exists_and_is_in_opcode_table
test_opcode_65_adc_zero_page_adds_value_from_memory