179. Bvs relative

connect BVS to its relative opcode.

Lesson 179 of 356 · tests/chapter_01_cpu/test_179_BVS_relative.py

Prerequisite: step 171 added `instructions.bvs. In this step, add these pieces to emulator/cpu/opcodes.py`:

from emulator.cpu.instructions import bvs

def bvs_relative(cpu: CPU):
    offset = relative(cpu)
    bvs(cpu, offset)

OPCODE_TABLE[0x70] = bvs_relative

Fold the import and mapping into the existing grouped structures.

Why this step exists

`relative consumes and signs the displacement, then step 171's instructions.bvs` branches from post-operand PC when Overflow is set.

Invariants: `0x70` maps to a one-argument handler and consumes its operand on both paths; Overflow set applies the offset and Overflow clear leaves PC after the operand. Flags and memory are unchanged. Misconception: BVS does not set Overflow, and an untaken branch still occupies and consumes two instruction bytes including its opcode.

Out of scope: step 180 independently adds `addressing_modes.indirect for JMP. The jmp` instruction and absolute/indirect JMP opcode APIs arrive only in later steps 181-183 and must not be added here.

Run this lesson

uv run pytest tests/chapter_01_cpu/test_179_BVS_relative.py -v

Example implementation

from emulator.cpu.instructions import bvs

def bvs_relative(cpu: CPU):
    offset = relative(cpu)
    bvs(cpu, offset)

OPCODE_TABLE[0x70] = bvs_relative

Show test codetest_179_BVS_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_bvs_relative_handler_exists_and_is_in_opcode_table():
    """Objective: create bvs_relative(cpu) and add 0x70 to OPCODE_TABLE."""
    assert hasattr(opcodes, "bvs_relative")
    assert callable(opcodes.bvs_relative)
    assert list(inspect.signature(opcodes.bvs_relative).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x70] is opcodes.bvs_relative


def test_opcode_70_bvs_relative_branches_when_overflow_set():
    """Objective: BVS branches when Overflow is set."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x70)
    rom.write(0x0001, 0x05)

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

    assert cpu.pc == 0x8007


def test_opcode_70_bvs_relative_does_not_branch_when_overflow_clear():
    """Objective: BVS does not branch when Overflow is clear."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x70)
    rom.write(0x0001, 0x05)

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

    assert cpu.pc == 0x8002

Test functions (3)

test_bvs_relative_handler_exists_and_is_in_opcode_table
test_opcode_70_bvs_relative_branches_when_overflow_set
test_opcode_70_bvs_relative_does_not_branch_when_overflow_clear