173. Bcs relative

connect BCS to its relative opcode.

Lesson 173 of 356 · tests/chapter_01_cpu/test_173_BCS_relative.py

Prerequisite: step 172 imported `relative and wired BCC. In this step, add these pieces to emulator/cpu/opcodes.py`:

from emulator.cpu.instructions import bcs

def bcs_relative(cpu: CPU):
    offset = relative(cpu)
    bcs(cpu, offset)

OPCODE_TABLE[0xB0] = bcs_relative

Fold the import and table item into the existing grouped import and table literal.

Why this step exists

Addressing remains in the opcode layer, while step 165's `instructions.bcs` owns the complementary Carry-set decision.

Invariants: `0xB0 maps to bcs_relative(cpu)`; the signed operand is always consumed; Carry set branches from post-operand PC and Carry clear leaves that PC unchanged. Flags and memory are untouched. Misconception: BCS means Carry set, not Carry clear, and the handler must not duplicate sign conversion.

Out of scope: BEQ/BNE/BPL/BMI/BVC/BVS opcode wiring remains for steps 174-179; JMP addressing is step 180.

Run this lesson

uv run pytest tests/chapter_01_cpu/test_173_BCS_relative.py -v

Example implementation

from emulator.cpu.instructions import bcs

def bcs_relative(cpu: CPU):
    offset = relative(cpu)
    bcs(cpu, offset)

OPCODE_TABLE[0xB0] = bcs_relative

Show test codetest_173_BCS_relative.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_bcs_relative_handler_exists_and_is_in_opcode_table():
    """Objective: create bcs_relative(cpu) and add 0xB0 to OPCODE_TABLE."""
    assert hasattr(opcodes, "bcs_relative")
    assert callable(opcodes.bcs_relative)
    assert list(inspect.signature(opcodes.bcs_relative).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0xB0] is opcodes.bcs_relative


def test_opcode_B0_bcs_relative_branches_when_carry_set():
    """Objective: BCS branches when Carry is set."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xB0)
    rom.write(0x0001, 0x05)

    cpu.reset()
    cpu.flags.set_carry_flag(True)
    cpu.step()

    assert cpu.pc == 0x8007


def test_opcode_B0_bcs_relative_does_not_branch_when_carry_clear():
    """Objective: BCS does not branch when Carry is clear."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xB0)
    rom.write(0x0001, 0x05)

    cpu.reset()
    cpu.flags.set_carry_flag(False)
    cpu.step()

    assert cpu.pc == 0x8002

Test functions (3)

test_bcs_relative_handler_exists_and_is_in_opcode_table
test_opcode_B0_bcs_relative_branches_when_carry_set
test_opcode_B0_bcs_relative_does_not_branch_when_carry_clear