035. Ldx absolute y

Add LDX absolute,Y ($BE).

Lesson 35 of 356 · tests/chapter_01_cpu/test_035_LDX_absolute_y.py

File to update

emulator/cpu/opcodes.py

Locations

opcodes.ldx_absolute_y
opcodes.OPCODE_TABLE[$BE]

Why this step exists

This lesson completes the LDX opcode family available at this point by reusing the existing absolute,Y helper. As with the other memory forms, the handler resolves and reads the operand before passing its value to ldx.

Complete example implementation

# emulator/cpu/opcodes.py
def ldx_absolute_y(cpu: CPU):
    addr = absolute_y(cpu)
    value = cpu.bus.read(addr)
    ldx(cpu, value)


OPCODE_TABLE = {
    # Preserve existing entries.
    0xBE: ldx_absolute_y,
}

Important invariants

$BE maps to ldx_absolute_y
the little-endian base address is indexed with Y, not X
the byte at the indexed address is passed to ldx
two operand bytes are consumed, so the full instruction advances PC by three

Common misconception

Do not wrap absolute,Y within page $00; unlike zero-page,Y, its indexed address is a 16-bit result.

Out of scope

store-X and load-Y instructions
changes to absolute_y or ldx
cycle timing and page-cross penalties

Run this lesson

uv run pytest tests/chapter_01_cpu/test_035_LDX_absolute_y.py -v

Example implementation

Complete example implementation

# emulator/cpu/opcodes.py
def ldx_absolute_y(cpu: CPU):
    addr = absolute_y(cpu)
    value = cpu.bus.read(addr)
    ldx(cpu, value)


OPCODE_TABLE = {
    # Preserve existing entries.
    0xBE: ldx_absolute_y,
}

Show test codetest_035_LDX_absolute_y.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_ldx_absolute_y_handler_exists_and_is_in_opcode_table():
    """Objective: create ldx_absolute_y(cpu) and add 0xBE to OPCODE_TABLE."""
    assert hasattr(opcodes, "ldx_absolute_y")
    assert callable(opcodes.ldx_absolute_y)
    assert list(inspect.signature(opcodes.ldx_absolute_y).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0xBE] is opcodes.ldx_absolute_y


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

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

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

Test functions (2)

test_ldx_absolute_y_handler_exists_and_is_in_opcode_table
test_opcode_BE_ldx_absolute_y_loads_register_x