057. Adc absolute

Connect ADC absolute (opcode $6D) to CPU dispatch.

Lesson 57 of 356 · tests/chapter_01_cpu/test_057_ADC_absolute.py

File to update

emulator/cpu/opcodes.py

Symbols to create/update

opcodes.adc_absolute
OPCODE_TABLE[$6D]

Why this step exists

Absolute mode expands ADC to a little-endian 16-bit operand address. The existing absolute helper consumes that word; the opcode handler reads the addressed byte and delegates arithmetic to adc.

Complete example implementation

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

OPCODE_TABLE = {
    # ...existing entries...
    0x6D: adc_absolute,
}

Important invariants

absolute combines the low operand byte before the high operand byte
the handler reads from the resulting 16-bit address
PC advances by three bytes total
adc remains the sole owner of arithmetic and flag behavior

Common misconception

Do not pass the fetched 16-bit address to adc; first read the byte stored at that address.

Out of scope

absolute,X and absolute,Y ADC modes
page-crossing timing
changes to absolute or adc

Run this lesson

uv run pytest tests/chapter_01_cpu/test_057_ADC_absolute.py -v

Example implementation

Complete example implementation

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

OPCODE_TABLE = {
    # ...existing entries...
    0x6D: adc_absolute,
}

Show test codetest_057_ADC_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


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_absolute_handler_exists_and_is_in_opcode_table():
    """Objective: create adc_absolute(cpu) and add 0x6D to OPCODE_TABLE."""
    assert hasattr(opcodes, "adc_absolute")
    assert callable(opcodes.adc_absolute)
    assert list(inspect.signature(opcodes.adc_absolute).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x6D] is opcodes.adc_absolute


def test_opcode_6D_adc_absolute_adds_value_from_memory():
    """Objective: 6D 00 02 means ADC $0200, so add RAM[$0200] to A."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x6D)
    rom.write(0x0001, 0x00)
    rom.write(0x0002, 0x02)
    bus.write(0x0200, 0x05)

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

    assert cpu.a == 0x15
    assert cpu.pc == 0x8003

Test functions (2)

test_adc_absolute_handler_exists_and_is_in_opcode_table
test_opcode_6D_adc_absolute_adds_value_from_memory