111. Ror accumulator

expose the already-built ROR A as opcode ``0x6A``.

Lesson 111 of 356 · tests/chapter_01_cpu/test_111_ROR_accumulator.py

In this step, use `ror and ror_a from lessons 109-110 and add only the accumulator opcode integration in emulator/cpu/opcodes.py`.

Why this step exists

ROR A needs an opcode-table entry before programs can reach the accumulator primitive through normal CPU execution without incorrectly fetching an operand.

Suggested implementation

from emulator.cpu.instructions import (..., ror_a)

OPCODE_TABLE = {
    ...
    0x6A: ror_a,
}

The direct table entry is the complete example implementation: `CPU.step has already fetched the opcode, and ror_a(cpu)` needs no operand decoder. It rotates A right through the old Carry, moves old A bit 0 to Carry, updates Zero/Negative from the 8-bit result, and leaves memory untouched. The one-byte instruction therefore advances PC by exactly one.

Misconception: accumulator mode is not an addressing mode wrapper; adding a `ror_accumulator handler or fetching an operand would consume an extra byte. Out of scope here: the memory handlers/opcodes 0x66, 0x76, 0x6E, and 0x7E` belong to lessons 112-115; AND begins at lesson 116.

Run this lesson

uv run pytest tests/chapter_01_cpu/test_111_ROR_accumulator.py -v

Example implementation

Suggested implementation

from emulator.cpu.instructions import (..., ror_a)

OPCODE_TABLE = {
    ...
    0x6A: ror_a,
}

Show test codetest_111_ROR_accumulator.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


CARRY_FLAG = 1 << 0


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_ror_accumulator_opcode_exists_and_is_in_opcode_table():
    """Objective: import ror_a and add 0x6A to OPCODE_TABLE."""
    assert hasattr(opcodes, "ror_a")
    assert callable(opcodes.ror_a)
    assert list(inspect.signature(opcodes.ror_a).parameters) == ["cpu"]
    assert opcodes.OPCODE_TABLE[0x6A] is opcodes.ror_a


def test_opcode_6A_ror_accumulator_rotates_a_and_advances_pc_by_one():
    """Objective: 6A means ROR A, so only A is rotated and PC advances by 1."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x6A)

    cpu.reset()
    cpu.a = 0b0000_0110
    cpu.step()

    assert cpu.a == 0b0000_0011
    assert cpu.pc == 0x8001


def test_opcode_6A_ror_accumulator_uses_old_carry_as_new_bit_7():
    """Objective: old Carry=1 is inserted into A bit 7."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x6A)

    cpu.reset()
    cpu.p |= CARRY_FLAG
    cpu.a = 0b0000_0010
    cpu.step()

    assert cpu.a == 0b1000_0001
    assert (cpu.p & CARRY_FLAG) == 0


def test_opcode_6A_ror_accumulator_sets_carry_from_old_a_bit_0():
    """Objective: old A bit 0 becomes Carry."""
    cpu, bus, rom = make_cpu_with_rom()
    rom.write(0x0000, 0x6A)

    cpu.reset()
    cpu.a = 0b0000_0011
    cpu.step()

    assert cpu.a == 0b0000_0001
    assert (cpu.p & CARRY_FLAG) != 0

Test functions (4)

test_ror_accumulator_opcode_exists_and_is_in_opcode_table
test_opcode_6A_ror_accumulator_rotates_a_and_advances_pc_by_one
test_opcode_6A_ror_accumulator_uses_old_carry_as_new_bit_7
test_opcode_6A_ror_accumulator_sets_carry_from_old_a_bit_0