043. Ldy zero page

Add LDY zero page ($A4).

Lesson 43 of 356 · tests/chapter_01_cpu/test_043_LDY_zero_page.py

File to update

emulator/cpu/opcodes.py

Locations

opcodes imports of zero_page and ldy
opcodes.ldy_zero_page
opcodes.OPCODE_TABLE[$A4]

Why this step exists

Unlike immediate LDY, zero-page LDY resolves the operand to an address, reads the byte at that address, and then delegates register and flag behavior to ldy.

Complete example implementation

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


def ldy_zero_page(cpu: CPU):
    addr = zero_page(cpu)
    value = cpu.bus.read(addr)
    ldy(cpu, value)


OPCODE_TABLE = {
    # Preserve existing entries.
    0xA4: ldy_zero_page,
}

Important invariants

$A4 maps to ldy_zero_page and consumes one operand byte
zero_page returns an address in $0000-$00FF
the handler performs one data read and passes that value, not its address, to ldy
ldy updates Zero and Negative

Common misconception

Passing addr directly to ldy would load the zero-page location number rather than the byte stored there.

Out of scope

zero-page,X and absolute LDY encodings
new addressing-mode helpers
cycle timing

Run this lesson

uv run pytest tests/chapter_01_cpu/test_043_LDY_zero_page.py -v

Example implementation

Complete example implementation

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


def ldy_zero_page(cpu: CPU):
    addr = zero_page(cpu)
    value = cpu.bus.read(addr)
    ldy(cpu, value)


OPCODE_TABLE = {
    # Preserve existing entries.
    0xA4: ldy_zero_page,
}

Show test codetest_043_LDY_zero_page.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_zero_page_handler_exists_and_is_in_opcode_table():
    """Objective: create ldy_zero_page(cpu) and add 0xA4 to OPCODE_TABLE."""
    assert hasattr(opcodes, "ldy_zero_page")
    assert callable(opcodes.ldy_zero_page)
    assert list(inspect.signature(opcodes.ldy_zero_page).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0xA4] is opcodes.ldy_zero_page


def test_opcode_A4_ldy_zero_page_loads_register_y():
    """Objective: A4 10 means LDY $10, so Y loads RAM[$0010]."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xA4)
    rom.write(0x0001, 0x10)
    bus.write(0x0010, 0x42)

    cpu.reset()
    cpu.step()

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

Test functions (2)

test_ldy_zero_page_handler_exists_and_is_in_opcode_table
test_opcode_A4_ldy_zero_page_loads_register_y