172. Bcc relative

connect BCC to its relative opcode.

Lesson 172 of 356 · tests/chapter_01_cpu/test_172_BCC_relative.py

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

from emulator.cpu.instructions import bcc
from emulator.cpu.addressing_modes import relative

def bcc_relative(cpu: CPU):
    offset = relative(cpu)
    bcc(cpu, offset)

OPCODE_TABLE[0x90] = bcc_relative

The imports may be folded into the file's existing grouped imports, and the table entry belongs in its literal.

Why this step exists

`CPU.step has consumed the opcode, relative consumes and signs the one-byte operand, and step 164's bcc` applies it only when Carry is clear.

Invariants: opcode `0x90 resolves to the one-argument handler; every path consumes the operand, so an untaken branch finishes at 0x8002 and a +5 branch targets 0x8007`. The handler itself changes no flags or memory. Misconception: the displacement is relative to PC after the operand, not to the opcode address, and it must be fetched even when Carry prevents the branch.

Out of scope: BCS, BEQ, BNE, BPL, BMI, BVC, and BVS handlers/table entries are steps 173-179. Indirect JMP addressing begins at step 180.

Run this lesson

uv run pytest tests/chapter_01_cpu/test_172_BCC_relative.py -v

Example implementation

from emulator.cpu.instructions import bcc
from emulator.cpu.addressing_modes import relative

def bcc_relative(cpu: CPU):
    offset = relative(cpu)
    bcc(cpu, offset)

OPCODE_TABLE[0x90] = bcc_relative

Show test codetest_172_BCC_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_bcc_relative_handler_exists_and_is_in_opcode_table():
    """Objective: create bcc_relative(cpu) and add 0x90 to OPCODE_TABLE."""
    assert hasattr(opcodes, "bcc_relative")
    assert callable(opcodes.bcc_relative)
    assert list(inspect.signature(opcodes.bcc_relative).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x90] is opcodes.bcc_relative


def test_opcode_90_bcc_relative_branches_when_carry_clear():
    """Objective: 90 05 branches from next instruction address 0x8002 to 0x8007."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x90)
    rom.write(0x0001, 0x05)

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

    assert cpu.pc == 0x8007


def test_opcode_90_bcc_relative_does_not_branch_when_carry_set():
    """Objective: branch not taken still consumes opcode and offset bytes."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x90)
    rom.write(0x0001, 0x05)

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

    assert cpu.pc == 0x8002

Test functions (3)

test_bcc_relative_handler_exists_and_is_in_opcode_table
test_opcode_90_bcc_relative_branches_when_carry_clear
test_opcode_90_bcc_relative_does_not_branch_when_carry_set