065. Sbc zero page x

Add SBC Zero Page,X.

Lesson 65 of 356 · tests/chapter_01_cpu/test_065_SBC_zero_page_x.py

File to update

emulator/cpu/opcodes.py

Symbols to add/update

opcodes.sbc_zero_page_x and OPCODE_TABLE[0xF5]

Why this step exists

SBC now gains indexed zero-page access by composing the existing zero_page_x resolver with a memory read and the existing sbc instruction.

Complete example implementation

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

OPCODE_TABLE = {
    # ... existing entries ...
    0xF5: sbc_zero_page_x,
}

Important invariants

zero_page_x fetches one operand, adds X, and wraps within page $00
the handler reads the resolved address exactly once
the read value, not its address, is passed to sbc
executing the two-byte instruction advances PC by two bytes

Common misconception

Do not reproduce indexing in the opcode wrapper or allow $FF + X to enter page $01; wrapping is already the responsibility of zero_page_x.

Out of scope

absolute and indirect SBC wrappers
changes to addressing helpers or SBC arithmetic
cycle timing

Run this lesson

uv run pytest tests/chapter_01_cpu/test_065_SBC_zero_page_x.py -v

Example implementation

Complete example implementation

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

OPCODE_TABLE = {
    # ... existing entries ...
    0xF5: sbc_zero_page_x,
}

Show test codetest_065_SBC_zero_page_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_zero_page_x_handler_exists_and_is_in_opcode_table():
    """Objective: create sbc_zero_page_x(cpu) and add 0xF5 to OPCODE_TABLE."""
    assert hasattr(opcodes, "sbc_zero_page_x")
    assert callable(opcodes.sbc_zero_page_x)
    assert list(inspect.signature(opcodes.sbc_zero_page_x).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0xF5] is opcodes.sbc_zero_page_x


def test_opcode_F5_sbc_zero_page_x_subtracts_indexed_value():
    """Objective: F5 10 with X=0x03 subtracts RAM[$0013] from A."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xF5)
    rom.write(0x0001, 0x10)
    bus.write(0x0013, 0x01)

    cpu.reset()
    cpu.a = 0x10
    cpu.x = 0x03
    cpu.flags.set_carry_flag(True)
    cpu.step()

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


def test_opcode_F5_sbc_zero_page_x_wraps_inside_page_zero():
    """Objective: Zero Page,X wraps before reading the SBC value."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xF5)
    rom.write(0x0001, 0xFF)
    bus.write(0x0000, 0x01)

    cpu.reset()
    cpu.a = 0x10
    cpu.x = 0x01
    cpu.flags.set_carry_flag(True)
    cpu.step()

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

Test functions (3)

test_sbc_zero_page_x_handler_exists_and_is_in_opcode_table
test_opcode_F5_sbc_zero_page_x_subtracts_indexed_value
test_opcode_F5_sbc_zero_page_x_wraps_inside_page_zero