042. Ldy immediate

Add LDY immediate ($A0).

Lesson 42 of 356 · tests/chapter_01_cpu/test_042_LDY_immediate.py

File to update

emulator/cpu/opcodes.py

Locations

opcodes import of ldy
opcodes.ldy_immediate
opcodes.OPCODE_TABLE[$A0]

Why this step exists

The core ldy instruction already owns the register and flag behavior. This lesson connects its first encoding by fetching one literal operand and passing that value to ldy.

Complete example implementation

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


def ldy_immediate(cpu: CPU):
    ldy(cpu, immediate(cpu))


OPCODE_TABLE = {
    # Preserve existing entries.
    0xA0: ldy_immediate,
}

Important invariants

$A0 maps to ldy_immediate
immediate fetches exactly one operand byte, advancing PC by one after the opcode
the operand itself is loaded; it is not treated as a memory address
ldy remains responsible for updating Zero and Negative

Common misconception

Do not read the bus at the immediate byte's numeric value. For A0 42, Y receives $42 directly.

Out of scope

LDY zero-page, indexed, and absolute encodings
STY opcode handlers
cycle timing

Run this lesson

uv run pytest tests/chapter_01_cpu/test_042_LDY_immediate.py -v

Example implementation

Complete example implementation

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


def ldy_immediate(cpu: CPU):
    ldy(cpu, immediate(cpu))


OPCODE_TABLE = {
    # Preserve existing entries.
    0xA0: ldy_immediate,
}

Show test codetest_042_LDY_immediate.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
from tests.helpers import NEGATIVE_FLAG, ZERO_FLAG


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_immediate_handler_exists_and_is_in_opcode_table():
    """Objective: create ldy_immediate(cpu) and add 0xA0 to OPCODE_TABLE."""
    assert hasattr(opcodes, "ldy_immediate")
    assert callable(opcodes.ldy_immediate)
    assert list(inspect.signature(opcodes.ldy_immediate).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0xA0] is opcodes.ldy_immediate


def test_opcode_A0_ldy_immediate_loads_register_y():
    """Objective: A0 42 means LDY #$42, so Y becomes 0x42."""
    cpu, _, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xA0)
    rom.write(0x0001, 0x42)

    cpu.reset()
    cpu.step()

    assert cpu.y == 0x42
    assert cpu.pc == 0x8002


def test_opcode_A0_ldy_immediate_updates_flags():
    """Objective: LDY Immediate updates Zero and Negative flags."""
    cpu, _, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xA0)
    rom.write(0x0001, 0x80)

    cpu.reset()
    cpu.step()

    assert cpu.y == 0x80
    assert (cpu.p & ZERO_FLAG) == 0
    assert (cpu.p & NEGATIVE_FLAG) != 0

Test functions (3)

test_ldy_immediate_handler_exists_and_is_in_opcode_table
test_opcode_A0_ldy_immediate_loads_register_y
test_opcode_A0_ldy_immediate_updates_flags