078. Dec absolute

wire DEC Absolute (`0xCE`).

Lesson 78 of 356 · tests/chapter_01_cpu/test_078_DEC_absolute.py

In this step, after the DEC primitive and zero-page forms from lessons 076-077, add emulator/cpu/opcodes.py:dec_absolute and OPCODE_TABLE[0xCE].

Why this step exists

Let addressing_modes.absolute decode the little-endian effective address, then reuse instructions.dec for mutation and flags.

Suggested implementation in emulator/cpu/opcodes.py:

def dec_absolute(cpu: CPU):
    addr = absolute(cpu)
    dec(cpu, addr)

Add this exact entry to the existing OPCODE_TABLE:

0xCE: dec_absolute,

Invariants: two operand bytes are consumed low-byte first; CE 00 02 resolves to $0200; PC advances to the next three-byte instruction; DEC wraps the memory byte to eight bits and updates only Zero and Negative.

Misconception: $00 $02 is not address $0002; 6502 absolute operands are little-endian and therefore identify $0200.

Out of scope: absolute-X DEC (dec_absolute_x, opcode 0xDE) belongs to lesson 079.

Run this lesson

uv run pytest tests/chapter_01_cpu/test_078_DEC_absolute.py -v

Example implementation

Why this step exists

def dec_absolute(cpu: CPU):
    addr = absolute(cpu)
    dec(cpu, addr)

Show test codetest_078_DEC_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_dec_absolute_handler_exists_and_is_in_opcode_table():
    """Objective: create dec_absolute(cpu) and add 0xCE to OPCODE_TABLE."""
    assert hasattr(opcodes, "dec_absolute")
    assert callable(opcodes.dec_absolute)
    assert list(inspect.signature(opcodes.dec_absolute).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0xCE] is opcodes.dec_absolute


def test_opcode_CE_dec_absolute_decrements_memory():
    """Objective: CE 00 02 means DEC $0200, so RAM[$0200] is decremented."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xCE)
    rom.write(0x0001, 0x00)
    rom.write(0x0002, 0x02)
    bus.write(0x0200, 0x42)

    cpu.reset()
    cpu.step()

    assert bus.read(0x0200) == 0x41
    assert cpu.pc == 0x8003

Test functions (2)

test_dec_absolute_handler_exists_and_is_in_opcode_table
test_opcode_CE_dec_absolute_decrements_memory