054. Adc immediate

Connect ADC immediate (opcode $69) to CPU dispatch.

Lesson 54 of 356 · tests/chapter_01_cpu/test_054_ADC_immediate.py

File to update

emulator/cpu/opcodes.py

Symbols to create/update

opcodes.adc_immediate
OPCODE_TABLE[$69]
the `adc` instruction import

Why this step exists

Test 053 supplied arithmetic on an already-resolved value. Immediate mode supplies the next program byte itself, so this handler passes the return value of immediate(cpu) directly to adc.

Complete example implementation

# emulator/cpu/opcodes.py
from emulator.cpu.instructions import ..., adc

def adc_immediate(cpu):
    value = immediate(cpu)
    adc(cpu, value)

OPCODE_TABLE = {
    # ...existing entries...
    0x69: adc_immediate,
}

Important invariants

the handler accepts only cpu
immediate mode consumes exactly one operand byte
PC advances by two bytes total: opcode plus operand
the handler delegates all arithmetic and flag changes to adc

Common misconception

Do not treat the immediate byte as an address and read the bus a second time; immediate(cpu) already returns the operand value.

Out of scope

all memory-addressed ADC opcodes
changes to adc or the addressing helpers
cycle accounting

Run this lesson

uv run pytest tests/chapter_01_cpu/test_054_ADC_immediate.py -v

Example implementation

Complete example implementation

# emulator/cpu/opcodes.py
from emulator.cpu.instructions import ..., adc

def adc_immediate(cpu):
    value = immediate(cpu)
    adc(cpu, value)

OPCODE_TABLE = {
    # ...existing entries...
    0x69: adc_immediate,
}

Show test codetest_054_ADC_immediate.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_immediate_handler_exists_and_is_in_opcode_table():
    """
    Objective:
    Create adc_immediate(cpu) and add 0x69 to OPCODE_TABLE.

    Important:
    immediate(cpu) returns the value directly.
    Do not read from cpu.bus again for immediate mode.
    """
    assert hasattr(opcodes, "adc_immediate")
    assert callable(opcodes.adc_immediate)
    assert list(inspect.signature(opcodes.adc_immediate).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x69] is opcodes.adc_immediate


def test_opcode_69_adc_immediate_adds_value_to_register_a():
    """Objective: 69 05 means ADC #$05, so A = A + 0x05 + Carry."""
    cpu, _, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x69)
    rom.write(0x0001, 0x05)

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

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


def test_opcode_69_adc_immediate_uses_carry_flag():
    """Objective: ADC includes the old Carry flag in the addition."""
    cpu, _, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x69)
    rom.write(0x0001, 0x05)

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

    assert cpu.a == 0x16
    assert cpu.pc == 0x8002

Test functions (3)

test_adc_immediate_handler_exists_and_is_in_opcode_table
test_opcode_69_adc_immediate_adds_value_to_register_a
test_opcode_69_adc_immediate_uses_carry_flag