096. Instruction lsr a

Add the LSR accumulator instruction behavior.

Lesson 96 of 356 · tests/chapter_01_cpu/test_096_instruction_lsr_a.py

In this step, add accumulator-targeted LSR behavior after the memory primitive from Test 095.

File and symbol

emulator/cpu/instructions.py: lsr_a

Why this step exists

Accumulator LSR shares the shift and flags of Test 095 but has a different data destination, so it must update cpu.a without treating that register as an address.

Suggested implementation for this step

# emulator/cpu/instructions.py
def lsr_a(cpu: CPU):
    value = cpu.a
    result = value >> 1
    result_8 = result & 0xFF

    # Set flags
    cpu.flags.set_carry_flag((value & 0x01) != 0)
    cpu.flags.set_negative_flag(False)
    cpu.flags.set_zero_flag(result_8 == 0)

    cpu.a = result_8

Important invariants

old A bit 0 replaces Carry
Zero follows the final A value and Negative is always cleared
no bus read or write occurs
memory at an address numerically equal to the old A remains unchanged

Common misconception

cpu.a is the value and destination, not an address to pass to memory lsr.

Out of scope

opcode 0x4A wiring in Test 097
memory addressing opcodes in Tests 098-101
refactoring shared LSR logic or adding cycle timing

Run this lesson

uv run pytest tests/chapter_01_cpu/test_096_instruction_lsr_a.py -v

Example implementation

Suggested implementation for this step

# emulator/cpu/instructions.py
def lsr_a(cpu: CPU):
    value = cpu.a
    result = value >> 1
    result_8 = result & 0xFF

    # Set flags
    cpu.flags.set_carry_flag((value & 0x01) != 0)
    cpu.flags.set_negative_flag(False)
    cpu.flags.set_zero_flag(result_8 == 0)

    cpu.a = result_8

Show test codetest_096_instruction_lsr_a.py

from emulator.cpu.instructions import lsr_a
from tests.helpers import NEGATIVE_FLAG, ZERO_FLAG, make_cpu


CARRY_FLAG = 1 << 0


def test_lsr_a_shifts_accumulator_right():
    """Objective: A value 0b0000_0110 becomes 0b0000_0011."""
    cpu = make_cpu()
    cpu.a = 0b0000_0110

    lsr_a(cpu)

    assert cpu.a == 0b0000_0011


def test_lsr_a_sets_carry_from_old_accumulator_bit_0():
    """Objective: old A bit 0 is moved into Carry before the right shift."""
    cpu = make_cpu()
    cpu.a = 0b0000_0011

    lsr_a(cpu)

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


def test_lsr_a_clears_carry_when_old_accumulator_bit_0_was_clear():
    """Objective: Carry is cleared when original A did not have bit 0 set."""
    cpu = make_cpu()
    cpu.p |= CARRY_FLAG
    cpu.a = 0b0000_0010

    lsr_a(cpu)

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


def test_lsr_a_sets_zero_flag_when_result_is_zero():
    """Objective: A=0x01 shifts to 0x00 and sets Zero flag."""
    cpu = make_cpu()
    cpu.a = 0x01

    lsr_a(cpu)

    assert cpu.a == 0x00
    assert (cpu.p & ZERO_FLAG) != 0


def test_lsr_a_always_clears_negative_flag():
    """Objective: LSR A fills bit 7 with 0, so Negative must be cleared."""
    cpu = make_cpu()
    cpu.p |= NEGATIVE_FLAG
    cpu.a = 0x80

    lsr_a(cpu)

    assert cpu.a == 0x40
    assert (cpu.p & NEGATIVE_FLAG) == 0


def test_lsr_a_does_not_write_shifted_result_to_memory():
    """Objective: LSR A writes to A only; it must not treat A as a memory address."""
    cpu = make_cpu()
    cpu.a = 0x20
    cpu.bus.write(0x0020, 0x99)

    lsr_a(cpu)

    assert cpu.a == 0x10
    assert cpu.bus.read(0x0020) == 0x99

Test functions (6)

test_lsr_a_shifts_accumulator_right
test_lsr_a_sets_carry_from_old_accumulator_bit_0
test_lsr_a_clears_carry_when_old_accumulator_bit_0_was_clear
test_lsr_a_sets_zero_flag_when_result_is_zero
test_lsr_a_always_clears_negative_flag
test_lsr_a_does_not_write_shifted_result_to_memory