059. Adc absolute y

Connect ADC absolute,Y (opcode $79) to CPU dispatch.

Lesson 59 of 356 · tests/chapter_01_cpu/test_059_ADC_absolute_y.py

File to update

emulator/cpu/opcodes.py

Symbols to create/update

opcodes.adc_absolute_y
OPCODE_TABLE[$79]

Why this step exists

This is the Y-indexed counterpart to test 058. absolute_y consumes the two-byte base address and applies Y; the handler reads the resulting location and delegates the value to adc.

Complete example implementation

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

OPCODE_TABLE = {
    # ...existing entries...
    0x79: adc_absolute_y,
}

Important invariants

Y is applied exactly once by absolute_y
the effective address is not constrained to page zero
PC advances by three bytes total
the addressed value is passed to adc

Common misconception

Do not copy the absolute,X handler and leave it using X; opcode $79 indexes the base address with Y.

Out of scope

indirect ADC modes
page-crossing timing penalties
changes to the addressing or arithmetic helpers

Run this lesson

uv run pytest tests/chapter_01_cpu/test_059_ADC_absolute_y.py -v

Example implementation

Complete example implementation

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

OPCODE_TABLE = {
    # ...existing entries...
    0x79: adc_absolute_y,
}

Show test codetest_059_ADC_absolute_y.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_y_handler_exists_and_is_in_opcode_table():
    """Objective: create adc_absolute_y(cpu) and add 0x79 to OPCODE_TABLE."""
    assert hasattr(opcodes, "adc_absolute_y")
    assert callable(opcodes.adc_absolute_y)
    assert list(inspect.signature(opcodes.adc_absolute_y).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x79] is opcodes.adc_absolute_y


def test_opcode_79_adc_absolute_y_adds_indexed_value():
    """Objective: 79 00 02 with Y=0x04 adds RAM[$0204] to A."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x79)
    rom.write(0x0001, 0x00)
    rom.write(0x0002, 0x02)
    bus.write(0x0204, 0x05)

    cpu.reset()
    cpu.a = 0x10
    cpu.y = 0x04
    cpu.flags.set_carry_flag(False)
    cpu.step()

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

Test functions (2)

test_adc_absolute_y_handler_exists_and_is_in_opcode_table
test_opcode_79_adc_absolute_y_adds_indexed_value