045. Ldy absolute

Add LDY absolute ($AC).

Lesson 45 of 356 · tests/chapter_01_cpu/test_045_LDY_absolute.py

File to update

emulator/cpu/opcodes.py

Locations

opcodes imports of absolute and ldy
opcodes.ldy_absolute
opcodes.OPCODE_TABLE[$AC]

Why this step exists

Absolute addressing extends LDY beyond page zero. The handler decodes the existing little-endian 16-bit operand, reads that address, and delegates the loaded value to ldy.

Complete example implementation

# emulator/cpu/opcodes.py
from emulator.cpu.addressing_modes import absolute
from emulator.cpu.instructions import ldy


def ldy_absolute(cpu: CPU):
    addr = absolute(cpu)
    value = cpu.bus.read(addr)
    ldy(cpu, value)


OPCODE_TABLE = {
    # Preserve existing entries.
    0xAC: ldy_absolute,
}

Important invariants

$AC maps to ldy_absolute and consumes two operand bytes
absolute combines the low byte first and high byte second
the handler reads the effective address and passes the resulting value to ldy
execution advances three bytes total, including the opcode

Common misconception

Do not reverse AC 00 02; the established absolute helper resolves those operand bytes to $0200, not $0002.

Out of scope

absolute,X LDY
changing the absolute addressing helper
cycle timing

Run this lesson

uv run pytest tests/chapter_01_cpu/test_045_LDY_absolute.py -v

Example implementation

Complete example implementation

# emulator/cpu/opcodes.py
from emulator.cpu.addressing_modes import absolute
from emulator.cpu.instructions import ldy


def ldy_absolute(cpu: CPU):
    addr = absolute(cpu)
    value = cpu.bus.read(addr)
    ldy(cpu, value)


OPCODE_TABLE = {
    # Preserve existing entries.
    0xAC: ldy_absolute,
}

Show test codetest_045_LDY_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_ldy_absolute_handler_exists_and_is_in_opcode_table():
    """Objective: create ldy_absolute(cpu) and add 0xAC to OPCODE_TABLE."""
    assert hasattr(opcodes, "ldy_absolute")
    assert callable(opcodes.ldy_absolute)
    assert list(inspect.signature(opcodes.ldy_absolute).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0xAC] is opcodes.ldy_absolute


def test_opcode_AC_ldy_absolute_loads_register_y():
    """Objective: AC 00 02 means LDY $0200, so Y loads RAM[$0200]."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xAC)
    rom.write(0x0001, 0x00)
    rom.write(0x0002, 0x02)
    bus.write(0x0200, 0x42)

    cpu.reset()
    cpu.step()

    assert cpu.y == 0x42
    assert cpu.pc == 0x8003

Test functions (2)

test_ldy_absolute_handler_exists_and_is_in_opcode_table
test_opcode_AC_ldy_absolute_loads_register_y