066. Sbc absolute

Add SBC Absolute.

Lesson 66 of 356 · tests/chapter_01_cpu/test_066_SBC_absolute.py

File to update

emulator/cpu/opcodes.py

Symbols to add/update

opcodes.sbc_absolute and OPCODE_TABLE[0xED]

Why this step exists

This lesson connects SBC to an existing little-endian 16-bit absolute address, following the same resolve-read-delegate boundary as other memory opcodes.

Complete example implementation

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

OPCODE_TABLE = {
    # ... existing entries ...
    0xED: sbc_absolute,
}

Important invariants

absolute consumes two operand bytes and returns the 16-bit address
the handler reads one byte from that address
only sbc changes A and arithmetic flags
executing the three-byte instruction advances PC by three bytes

Common misconception

The two operand bytes form an address in little-endian order; they are not the value to subtract and should not be assembled again in this wrapper.

Out of scope

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

Run this lesson

uv run pytest tests/chapter_01_cpu/test_066_SBC_absolute.py -v

Example implementation

Complete example implementation

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

OPCODE_TABLE = {
    # ... existing entries ...
    0xED: sbc_absolute,
}

Show test codetest_066_SBC_absolute.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_absolute_handler_exists_and_is_in_opcode_table():
    """Objective: create sbc_absolute(cpu) and add 0xED to OPCODE_TABLE."""
    assert hasattr(opcodes, "sbc_absolute")
    assert callable(opcodes.sbc_absolute)
    assert list(inspect.signature(opcodes.sbc_absolute).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0xED] is opcodes.sbc_absolute


def test_opcode_ED_sbc_absolute_subtracts_value_from_memory():
    """Objective: ED 00 02 means SBC $0200, so subtract RAM[$0200] from A."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xED)
    rom.write(0x0001, 0x00)
    rom.write(0x0002, 0x02)
    bus.write(0x0200, 0x01)

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

    assert cpu.a == 0x0F
    assert cpu.pc == 0x8003

Test functions (2)

test_sbc_absolute_handler_exists_and_is_in_opcode_table
test_opcode_ED_sbc_absolute_subtracts_value_from_memory