094. Asl absolute x

Add ASL Absolute,X.

Lesson 94 of 356 · tests/chapter_01_cpu/test_094_ASL_absolute_x.py

In this step, complete the ASL addressing sequence with Absolute,X.

File and symbols

emulator/cpu/opcodes.py: asl_absolute_x, OPCODE_TABLE[0x1E]

Why this step exists

This is the final ASL addressing transition. It composes the existing absolute_x resolver with the memory asl implementation introduced earlier.

Suggested implementation for this step

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

OPCODE_TABLE = {
    # existing entries unchanged
    0x1E: asl_absolute_x,
}

Important invariants

decode the little-endian 16-bit base before adding X
the instruction remains three bytes; X is a register, not another operand
asl owns read/modify/write and flag updates at the effective address
the accumulator is unchanged

Common misconception

For 1E 00 02 with X=0x04, add X to $0200, not to either operand byte.

Out of scope

LSR introduced by Tests 095-101
modifying absolute_x or asl
page-cross and read/modify/write cycle timing

Run this lesson

uv run pytest tests/chapter_01_cpu/test_094_ASL_absolute_x.py -v

Example implementation

Suggested implementation for this step

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

OPCODE_TABLE = {
    # existing entries unchanged
    0x1E: asl_absolute_x,
}

Show test codetest_094_ASL_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_asl_absolute_x_handler_exists_and_is_in_opcode_table():
    """Objective: create asl_absolute_x(cpu) and add 0x1E to OPCODE_TABLE."""
    assert hasattr(opcodes, "asl_absolute_x")
    assert callable(opcodes.asl_absolute_x)
    assert list(inspect.signature(opcodes.asl_absolute_x).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x1E] is opcodes.asl_absolute_x


def test_opcode_1E_asl_absolute_x_shifts_indexed_memory_value():
    """Objective: 1E 00 02 with X=0x04 shifts RAM[$0204]."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x1E)
    rom.write(0x0001, 0x00)
    rom.write(0x0002, 0x02)
    bus.write(0x0204, 0b0000_0011)

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

    assert bus.read(0x0204) == 0b0000_0110
    assert cpu.pc == 0x8003

Test functions (2)

test_asl_absolute_x_handler_exists_and_is_in_opcode_table
test_opcode_1E_asl_absolute_x_shifts_indexed_memory_value