175. Bne relative

connect BNE to its relative opcode.

Lesson 175 of 356 · tests/chapter_01_cpu/test_175_BNE_relative.py

Prerequisite: step 174 wired BEQ. In this step, add these pieces to `emulator/cpu/opcodes.py`:

from emulator.cpu.instructions import bne

def bne_relative(cpu: CPU):
    offset = relative(cpu)
    bne(cpu, offset)

OPCODE_TABLE[0xD0] = bne_relative

Merge the import and table item into their existing grouped forms.

Why this step exists

The opcode layer consumes and decodes the displacement, then delegates the Zero-clear decision to step 167's `instructions.bne`.

Invariants: `0xD0` always consumes one operand byte; Zero clear applies the signed offset to post-operand PC and Zero set leaves that PC unchanged. The handler changes no flags or memory. Misconception: BNE does not perform a comparison, and an untaken branch must not leave PC on its operand.

Out of scope: BPL/BMI/BVC/BVS opcode wiring follows in steps 176-179. The next addressing-mode feature, indirect JMP, is step 180.

Run this lesson

uv run pytest tests/chapter_01_cpu/test_175_BNE_relative.py -v

Example implementation

from emulator.cpu.instructions import bne

def bne_relative(cpu: CPU):
    offset = relative(cpu)
    bne(cpu, offset)

OPCODE_TABLE[0xD0] = bne_relative

Show test codetest_175_BNE_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_bne_relative_handler_exists_and_is_in_opcode_table():
    """Objective: create bne_relative(cpu) and add 0xD0 to OPCODE_TABLE."""
    assert hasattr(opcodes, "bne_relative")
    assert callable(opcodes.bne_relative)
    assert list(inspect.signature(opcodes.bne_relative).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0xD0] is opcodes.bne_relative


def test_opcode_D0_bne_relative_branches_when_zero_clear():
    """Objective: BNE branches when Zero is clear."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xD0)
    rom.write(0x0001, 0x05)

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

    assert cpu.pc == 0x8007


def test_opcode_D0_bne_relative_does_not_branch_when_zero_set():
    """Objective: BNE does not branch when Zero is set."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0xD0)
    rom.write(0x0001, 0x05)

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

    assert cpu.pc == 0x8002

Test functions (3)

test_bne_relative_handler_exists_and_is_in_opcode_table
test_opcode_D0_bne_relative_branches_when_zero_clear
test_opcode_D0_bne_relative_does_not_branch_when_zero_set