086. Instruction dey

Add DEY instruction behavior.

Lesson 86 of 356 · tests/chapter_01_cpu/test_086_instruction_dey.py

In this step, add only dey; Test 087 wires the opcode after the behavior exists.

Production location and symbol

emulator/cpu/instructions.py: `dey(cpu: CPU)`

Why this step exists

DEY owns the Y-register decrement and status effects independently of decoding. Masking is required to model underflow in an 8-bit register.

Suggested implementation

def dey(cpu: CPU):
    result = cpu.y - 1
    result_8 = result & 0xFF

    # Set flags
    cpu.flags.set_negative_flag((result_8 & 0b1000_0000) != 0)
    cpu.flags.set_zero_flag(result_8 == 0)

    cpu.y = result_8

Important invariants

Y wraps from 0x00 to 0xFF
Zero and Negative reflect the masked 8-bit result
Carry and Overflow are preserved; memory is not accessed

Common misconception

DEY is not SBC applied to Y and must not use Carry as a borrow input.

Out of scope

importing dey and mapping opcode 0x88 (test 087)
prior INY behavior and dispatch
later ASL work and cycle timing

Run this lesson

uv run pytest tests/chapter_01_cpu/test_086_instruction_dey.py -v

Example implementation

Suggested implementation

def dey(cpu: CPU):
    result = cpu.y - 1
    result_8 = result & 0xFF

    # Set flags
    cpu.flags.set_negative_flag((result_8 & 0b1000_0000) != 0)
    cpu.flags.set_zero_flag(result_8 == 0)

    cpu.y = result_8

Show test codetest_086_instruction_dey.py

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


CARRY_FLAG = 1 << 0
OVERFLOW_FLAG = 1 << 6


def test_dey_decrements_y_register():
    """Objective: Y decreases by one and remains an 8-bit register value."""
    cpu = make_cpu()
    cpu.y = 0x10

    dey(cpu)

    assert cpu.y == 0x0F


def test_dey_wraps_from_00_to_ff_and_sets_negative_flag():
    """Objective: 0x00 - 1 wraps to 0xFF and sets Negative flag."""
    cpu = make_cpu()
    cpu.y = 0x00

    dey(cpu)

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


def test_dey_sets_zero_flag_when_result_is_zero():
    """Objective: 0x01 - 1 becomes 0x00 and sets Zero flag."""
    cpu = make_cpu()
    cpu.y = 0x01

    dey(cpu)

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


def test_dey_does_not_modify_carry_or_overflow_flags():
    """Objective: DEY updates Z/N only; Carry and Overflow are preserved."""
    cpu = make_cpu()
    cpu.y = 0x10
    cpu.p |= CARRY_FLAG
    cpu.p |= OVERFLOW_FLAG

    dey(cpu)

    assert cpu.y == 0x0F
    assert (cpu.p & CARRY_FLAG) != 0
    assert (cpu.p & OVERFLOW_FLAG) != 0

Test functions (4)

test_dey_decrements_y_register
test_dey_wraps_from_00_to_ff_and_sets_negative_flag
test_dey_sets_zero_flag_when_result_is_zero
test_dey_does_not_modify_carry_or_overflow_flags