091. Asl zero page

Add ASL Zero Page.

Lesson 91 of 356 · tests/chapter_01_cpu/test_091_ASL_zero_page.py

In this step, add the first addressed ASL form. Tests 088-090 are prerequisites for memory and accumulator behavior and accumulator dispatch.

File and symbols

emulator/cpu/opcodes.py: asl_zero_page, OPCODE_TABLE[0x06]

Why this step exists

Tests 088-090 already established instructions.asl, instructions.asl_a, and the accumulator opcode. This transition exposes the memory implementation through the first addressed form without duplicating shift or flag behavior in the handler.

Suggested implementation for this step

# emulator/cpu/opcodes.py
def asl_zero_page(cpu: CPU):
    addr = zero_page(cpu)
    asl(cpu, addr)

OPCODE_TABLE = {
    # existing entries unchanged
    0x06: asl_zero_page,
}

Important invariants

zero_page(cpu) consumes one operand byte, so the full instruction is two bytes
the operand is an address; asl performs the read/modify/write at that address
old bit 7 sets Carry; Zero and Negative come from the masked result
A is unchanged

Common misconception

For 06 10, 0x10 is not shifted directly. It selects RAM[$0010].

Out of scope

ASL Zero Page,X and absolute forms in Tests 092-094
LSR beginning in Test 095
cycle timing and read/modify/write bus-cycle accuracy

Run this lesson

uv run pytest tests/chapter_01_cpu/test_091_ASL_zero_page.py -v

Example implementation

Suggested implementation for this step

# emulator/cpu/opcodes.py
def asl_zero_page(cpu: CPU):
    addr = zero_page(cpu)
    asl(cpu, addr)

OPCODE_TABLE = {
    # existing entries unchanged
    0x06: asl_zero_page,
}

Show test codetest_091_ASL_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
from tests.helpers import NEGATIVE_FLAG, ZERO_FLAG


CARRY_FLAG = 1 << 0


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_asl_zero_page_handler_exists_and_is_in_opcode_table():
    """Objective: create asl_zero_page(cpu) and add 0x06 to OPCODE_TABLE."""
    assert hasattr(opcodes, "asl_zero_page")
    assert callable(opcodes.asl_zero_page)
    assert list(inspect.signature(opcodes.asl_zero_page).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x06] is opcodes.asl_zero_page


def test_opcode_06_asl_zero_page_shifts_memory_value():
    """Objective: 06 10 means ASL $10, so RAM[$0010] is shifted left."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x06)
    rom.write(0x0001, 0x10)
    bus.write(0x0010, 0b0000_0011)

    cpu.reset()
    cpu.step()

    assert bus.read(0x0010) == 0b0000_0110
    assert cpu.pc == 0x8002


def test_opcode_06_asl_zero_page_sets_carry_from_old_bit_7():
    """Objective: old memory bit 7 becomes Carry."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x06)
    rom.write(0x0001, 0x10)
    bus.write(0x0010, 0b1000_0001)

    cpu.reset()
    cpu.step()

    assert bus.read(0x0010) == 0b0000_0010
    assert (cpu.p & CARRY_FLAG) != 0


def test_opcode_06_asl_zero_page_updates_zero_flag():
    """Objective: 0x80 shifts to 0x00 and sets Zero flag."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x06)
    rom.write(0x0001, 0x10)
    bus.write(0x0010, 0x80)

    cpu.reset()
    cpu.step()

    assert bus.read(0x0010) == 0x00
    assert (cpu.p & ZERO_FLAG) != 0
    assert (cpu.p & NEGATIVE_FLAG) == 0


def test_opcode_06_asl_zero_page_updates_negative_flag():
    """Objective: 0x40 shifts to 0x80 and sets Negative flag."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x06)
    rom.write(0x0001, 0x10)
    bus.write(0x0010, 0x40)

    cpu.reset()
    cpu.step()

    assert bus.read(0x0010) == 0x80
    assert (cpu.p & NEGATIVE_FLAG) != 0
    assert (cpu.p & ZERO_FLAG) == 0

Test functions (5)

test_asl_zero_page_handler_exists_and_is_in_opcode_table
test_opcode_06_asl_zero_page_shifts_memory_value
test_opcode_06_asl_zero_page_sets_carry_from_old_bit_7
test_opcode_06_asl_zero_page_updates_zero_flag
test_opcode_06_asl_zero_page_updates_negative_flag