074. Inc absolute

wire INC Absolute (`0xEE`).

Lesson 74 of 356 · tests/chapter_01_cpu/test_074_INC_absolute.py

In this step, with inc already imported, add emulator/cpu/opcodes.py:inc_absolute and OPCODE_TABLE[0xEE] only.

Why this step exists

addressing_modes.absolute centralizes little-endian decoding of the two-byte address while the existing INC primitive performs the read-modify-write.

Suggested implementation in emulator/cpu/opcodes.py:

def inc_absolute(cpu: CPU):
    addr = absolute(cpu)
    inc(cpu, addr)

Add this exact entry to the existing OPCODE_TABLE:

0xEE: inc_absolute,

Invariants: the low operand byte precedes the high byte; absolute(cpu) consumes both and advances PC twice; EE 00 02 targets $0200; total instruction length is three bytes; INC updates memory and Z/N, not registers, Carry, or Overflow.

Misconception: do not reverse the operand bytes or pass the fetched byte value to inc; the helper returns the effective 16-bit address.

Out of scope: inc_absolute_x and OPCODE_TABLE[0xFE] are lesson 075.

Run this lesson

uv run pytest tests/chapter_01_cpu/test_074_INC_absolute.py -v

Example implementation

Why this step exists

def inc_absolute(cpu: CPU):
    addr = absolute(cpu)
    inc(cpu, addr)

Show test codetest_074_INC_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_inc_absolute_handler_exists_and_is_in_opcode_table():
    """Objective: create inc_absolute(cpu) and add 0xEE to OPCODE_TABLE."""
    assert hasattr(opcodes, "inc_absolute")
    assert callable(opcodes.inc_absolute)
    assert list(inspect.signature(opcodes.inc_absolute).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0xEE] is opcodes.inc_absolute


def test_opcode_EE_inc_absolute_increments_memory():
    """Objective: EE 00 02 means INC $0200, so RAM[$0200] is incremented."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xEE)
    rom.write(0x0001, 0x00)
    rom.write(0x0002, 0x02)
    bus.write(0x0200, 0x41)

    cpu.reset()
    cpu.step()

    assert bus.read(0x0200) == 0x42
    assert cpu.pc == 0x8003

Test functions (2)

test_inc_absolute_handler_exists_and_is_in_opcode_table
test_opcode_EE_inc_absolute_increments_memory