050. Transfer instructions

Add the core TAX, TXA, TAY, and TYA instructions.

Lesson 50 of 356 · tests/chapter_01_cpu/test_050_transfer_instructions.py

File to update

emulator/cpu/instructions.py

Locations

instructions.tax
instructions.txa
instructions.tay
instructions.tya

Why this step exists

These implied-addressing operations copy between A, X, and Y without fetching an operand. Each destination value then uses the same Zero/Negative update already used by load instructions.

Complete example implementation

# emulator/cpu/instructions.py
def tax(cpu: CPU):
    cpu.x = cpu.a
    cpu._update_zero_and_negative_flags(cpu.x)


def txa(cpu: CPU):
    cpu.a = cpu.x
    cpu._update_zero_and_negative_flags(cpu.a)


def tay(cpu: CPU):
    cpu.y = cpu.a
    cpu._update_zero_and_negative_flags(cpu.y)


def tya(cpu: CPU):
    cpu.a = cpu.y
    cpu._update_zero_and_negative_flags(cpu.a)

Important invariants

TAX copies A to X, TXA copies X to A, TAY copies A to Y, and TYA copies Y to A
the source register remains unchanged
Zero is set exactly for $00 and Negative mirrors destination bit 7
both flags are also cleared when the copied value no longer satisfies them
these functions fetch no bytes and perform no bus access

Common misconception

Updating only flags that become set leaves stale state behind. Reuse cpu._update_zero_and_negative_flags with the destination register after every copy.

Out of scope

transfer opcode imports and OPCODE_TABLE entries, introduced in Test 051
stack-pointer transfers
cycle timing

Run this lesson

uv run pytest tests/chapter_01_cpu/test_050_transfer_instructions.py -v

Example implementation

Complete example implementation

# emulator/cpu/instructions.py
def tax(cpu: CPU):
    cpu.x = cpu.a
    cpu._update_zero_and_negative_flags(cpu.x)


def txa(cpu: CPU):
    cpu.a = cpu.x
    cpu._update_zero_and_negative_flags(cpu.a)


def tay(cpu: CPU):
    cpu.y = cpu.a
    cpu._update_zero_and_negative_flags(cpu.y)


def tya(cpu: CPU):
    cpu.a = cpu.y
    cpu._update_zero_and_negative_flags(cpu.a)

Show test codetest_050_transfer_instructions.py

import inspect

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


def test_tax_instruction_exists():
    """
    Objective:
    Create in instructions.py:
        def tax(cpu):
            ...

    TAX means:
        X = A
    """
    assert hasattr(instructions, "tax")
    assert callable(instructions.tax)
    assert list(inspect.signature(instructions.tax).parameters) == ["cpu"]


def test_txa_instruction_exists():
    """
    Objective:
    Create in instructions.py:
        def txa(cpu):
            ...

    TXA means:
        A = X
    """
    assert hasattr(instructions, "txa")
    assert callable(instructions.txa)
    assert list(inspect.signature(instructions.txa).parameters) == ["cpu"]


def test_tay_instruction_exists():
    """
    Objective:
    Create in instructions.py:
        def tay(cpu):
            ...

    TAY means:
        Y = A
    """
    assert hasattr(instructions, "tay")
    assert callable(instructions.tay)
    assert list(inspect.signature(instructions.tay).parameters) == ["cpu"]


def test_tya_instruction_exists():
    """
    Objective:
    Create in instructions.py:
        def tya(cpu):
            ...

    TYA means:
        A = Y
    """
    assert hasattr(instructions, "tya")
    assert callable(instructions.tya)
    assert list(inspect.signature(instructions.tya).parameters) == ["cpu"]


def test_tax_transfers_a_to_x_and_updates_flags():
    """Objective: TAX copies A into X and updates Zero/Negative flags."""
    cpu = make_cpu()
    cpu.a = 0x80

    instructions.tax(cpu)

    assert cpu.x == 0x80
    assert (cpu.p & ZERO_FLAG) == 0
    assert (cpu.p & NEGATIVE_FLAG) != 0


def test_txa_transfers_x_to_a_and_updates_flags():
    """Objective: TXA copies X into A and updates Zero/Negative flags."""
    cpu = make_cpu()
    cpu.x = 0x00

    instructions.txa(cpu)

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


def test_tay_transfers_a_to_y_and_updates_flags():
    """Objective: TAY copies A into Y and updates Zero/Negative flags."""
    cpu = make_cpu()
    cpu.a = 0x42
    cpu.p |= ZERO_FLAG | NEGATIVE_FLAG

    instructions.tay(cpu)

    assert cpu.y == 0x42
    assert (cpu.p & ZERO_FLAG) == 0
    assert (cpu.p & NEGATIVE_FLAG) == 0


def test_tya_transfers_y_to_a_and_updates_flags():
    """Objective: TYA copies Y into A and updates Zero/Negative flags."""
    cpu = make_cpu()
    cpu.y = 0x80

    instructions.tya(cpu)

    assert cpu.a == 0x80
    assert (cpu.p & ZERO_FLAG) == 0
    assert (cpu.p & NEGATIVE_FLAG) != 0

Test functions (8)

test_tax_instruction_exists
test_txa_instruction_exists
test_tay_instruction_exists
test_tya_instruction_exists
test_tax_transfers_a_to_x_and_updates_flags
test_txa_transfers_x_to_a_and_updates_flags
test_tay_transfers_a_to_y_and_updates_flags
test_tya_transfers_y_to_a_and_updates_flags