069. Sbc indirect x

Add SBC (Indirect,X).

Lesson 69 of 356 · tests/chapter_01_cpu/test_069_SBC_indirect_x.py

File to update

emulator/cpu/opcodes.py

Symbols to add/update

opcodes.sbc_indirect_x and OPCODE_TABLE[0xE1]

Why this step exists

SBC gains pre-indexed indirect access by composing the existing indirect_x resolver with the standard memory-read and instruction-delegation wrapper.

Complete example implementation

# emulator/cpu/opcodes.py
def sbc_indirect_x(cpu: CPU):
    addr = indirect_x(cpu)
    value = cpu.bus.read(addr)
    sbc(cpu, value)

OPCODE_TABLE = {
    # ... existing entries ...
    0xE1: sbc_indirect_x,
}

Important invariants

indirect_x adds X to the zero-page operand before reading the pointer
the pointer bytes and their zero-page wrap are handled by the resolver
the wrapper reads the final address and passes that byte to sbc
executing the two-byte instruction advances PC by two bytes

Common misconception

Do not read the zero-page pointer location as the SBC operand; indirect_x returns the final 16-bit target address whose contents must be read.

Out of scope

the (Indirect),Y SBC wrapper
changes to indirect addressing or SBC arithmetic
cycle timing

Run this lesson

uv run pytest tests/chapter_01_cpu/test_069_SBC_indirect_x.py -v

Example implementation

Complete example implementation

# emulator/cpu/opcodes.py
def sbc_indirect_x(cpu: CPU):
    addr = indirect_x(cpu)
    value = cpu.bus.read(addr)
    sbc(cpu, value)

OPCODE_TABLE = {
    # ... existing entries ...
    0xE1: sbc_indirect_x,
}

Show test codetest_069_SBC_indirect_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_sbc_indirect_x_handler_exists_and_is_in_opcode_table():
    """Objective: create sbc_indirect_x(cpu) and add 0xE1 to OPCODE_TABLE."""
    assert hasattr(opcodes, "sbc_indirect_x")
    assert callable(opcodes.sbc_indirect_x)
    assert list(inspect.signature(opcodes.sbc_indirect_x).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0xE1] is opcodes.sbc_indirect_x


def test_opcode_E1_sbc_indirect_x_subtracts_value_from_final_address():
    """Objective: E1 20 with X=0x04 uses pointer at $0024 and subtracts final memory value."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xE1)
    rom.write(0x0001, 0x20)
    bus.write(0x0024, 0x00)
    bus.write(0x0025, 0x02)
    bus.write(0x0200, 0x01)

    cpu.reset()
    cpu.a = 0x10
    cpu.x = 0x04
    cpu.flags.set_carry_flag(True)
    cpu.step()

    assert cpu.a == 0x0F
    assert cpu.pc == 0x8002

Test functions (2)

test_sbc_indirect_x_handler_exists_and_is_in_opcode_table
test_opcode_E1_sbc_indirect_x_subtracts_value_from_final_address