162. Cpy absolute

add CPY absolute opcode ``0xCC``.

Lesson 162 of 356 · tests/chapter_01_cpu/test_162_CPY_absolute.py

Why this step exists

Absolute CPY compares Y with a byte anywhere in CPU memory and verifies the two-byte operand is decoded as a little-endian address.

In this step, after lessons 159-161 provide `cpy and its immediate and zero-page handlers, add exactly the following to emulator/cpu/opcodes.py`:

def cpy_absolute(cpu: CPU):
    addr = absolute(cpu)
    value = cpu.bus.read(addr)
    cpy(cpu, value)

OPCODE_TABLE = {
    ...
    0xCC: cpy_absolute,
}

`emulator/cpu/addressing_modes.py::absolute consumes a little- endian address through CPU.fetch_word; CC 00 02 therefore compares Y with the byte read from $0200 via instructions.cpy`.

Invariants: Y and memory are unchanged; only C/Z/N change, and opcode plus the two-byte operand advances PC by three bytes. Misconception: `00 02 is not the literal value 0x0002` to compare and is not a big-endian address.

Out of scope: relative addressing and branches begin at lesson 163.

Run this lesson

uv run pytest tests/chapter_01_cpu/test_162_CPY_absolute.py -v

Example implementation

Why this step exists

def cpy_absolute(cpu: CPU):
    addr = absolute(cpu)
    value = cpu.bus.read(addr)
    cpy(cpu, value)

OPCODE_TABLE = {
    ...
    0xCC: cpy_absolute,
}

Show test codetest_162_CPY_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


CARRY_FLAG = 1 << 0


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


def test_opcode_CC_cpy_absolute_reads_memory_value():
    """Objective: CC 00 02 means compare Y with RAM[$0200]."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xCC)
    rom.write(0x0001, 0x00)
    rom.write(0x0002, 0x02)
    bus.write(0x0200, 0x10)

    cpu.reset()
    cpu.y = 0x20
    cpu.step()

    assert (cpu.p & CARRY_FLAG) != 0
    assert cpu.pc == 0x8003

Test functions (2)

test_cpy_absolute_handler_exists_and_is_in_opcode_table
test_opcode_CC_cpy_absolute_reads_memory_value