046. Ldy absolute x

Add LDY absolute,X ($BC).

Lesson 46 of 356 · tests/chapter_01_cpu/test_046_LDY_absolute_x.py

File to update

emulator/cpu/opcodes.py

Locations

opcodes imports of absolute_x and ldy
opcodes.ldy_absolute_x
opcodes.OPCODE_TABLE[$BC]

Why this step exists

This lesson completes the LDY opcode family by reusing absolute,X address resolution, then reading the effective address and applying the existing ldy behavior.

Complete example implementation

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


def ldy_absolute_x(cpu: CPU):
    addr = absolute_x(cpu)
    value = cpu.bus.read(addr)
    ldy(cpu, value)


OPCODE_TABLE = {
    # Preserve existing entries.
    0xBC: ldy_absolute_x,
}

Important invariants

$BC maps to ldy_absolute_x and consumes two operand bytes
X, not Y, is added to the decoded 16-bit base address
the effective address is read before its value is passed to ldy
ldy updates Zero and Negative from the loaded byte

Common misconception

LDY absolute,X does not index with the destination register Y; the encoding explicitly uses X.

Out of scope

STY opcode handlers
page-crossing cycle penalties
changes to absolute_x

Run this lesson

uv run pytest tests/chapter_01_cpu/test_046_LDY_absolute_x.py -v

Example implementation

Complete example implementation

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


def ldy_absolute_x(cpu: CPU):
    addr = absolute_x(cpu)
    value = cpu.bus.read(addr)
    ldy(cpu, value)


OPCODE_TABLE = {
    # Preserve existing entries.
    0xBC: ldy_absolute_x,
}

Show test codetest_046_LDY_absolute_x.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_x_handler_exists_and_is_in_opcode_table():
    """Objective: create ldy_absolute_x(cpu) and add 0xBC to OPCODE_TABLE."""
    assert hasattr(opcodes, "ldy_absolute_x")
    assert callable(opcodes.ldy_absolute_x)
    assert list(inspect.signature(opcodes.ldy_absolute_x).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0xBC] is opcodes.ldy_absolute_x


def test_opcode_BC_ldy_absolute_x_loads_register_y():
    """Objective: BC 00 02 with X=0x04 reads RAM[$0204] into Y."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xBC)
    rom.write(0x0001, 0x00)
    rom.write(0x0002, 0x02)
    bus.write(0x0204, 0x42)

    cpu.reset()
    cpu.x = 0x04
    cpu.step()

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

Test functions (2)

test_ldy_absolute_x_handler_exists_and_is_in_opcode_table
test_opcode_BC_ldy_absolute_x_loads_register_y