084. Instruction iny

Add INY instruction behavior.

Lesson 84 of 356 · tests/chapter_01_cpu/test_084_instruction_iny.py

In this step, add only iny, before its Test 085 opcode mapping and the DEY work in Tests 086-087.

Production location and symbol

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

Why this step exists

INY increments the Y register independently of opcode decoding and must emulate an 8-bit register despite Python's unbounded integers.

Suggested implementation

def iny(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 0xFF to 0x00
Zero and Negative are computed from the masked result
Carry, Overflow, memory, and other registers remain unchanged

Common misconception

The increment does not set Carry when Y wraps; INY only updates Z and N.

Out of scope

opcode 0xC8 dispatch (test 085)
DEY behavior and dispatch (tests 086-087)
later ASL work and cycle timing

Run this lesson

uv run pytest tests/chapter_01_cpu/test_084_instruction_iny.py -v

Example implementation

Suggested implementation

def iny(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_084_instruction_iny.py

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


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


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

    iny(cpu)

    assert cpu.y == 0x11


def test_iny_wraps_from_ff_to_00_and_sets_zero_flag():
    """Objective: 0xFF + 1 wraps to 0x00 and sets Zero flag."""
    cpu = make_cpu()
    cpu.y = 0xFF

    iny(cpu)

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


def test_iny_sets_negative_flag_when_result_has_bit_7_set():
    """Objective: 0x7F + 1 becomes 0x80, so Negative flag is set."""
    cpu = make_cpu()
    cpu.y = 0x7F

    iny(cpu)

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


def test_iny_does_not_modify_carry_or_overflow_flags():
    """Objective: INY updates Z/N only; Carry and Overflow are preserved."""
    cpu = make_cpu()
    cpu.y = 0x01
    cpu.p |= CARRY_FLAG
    cpu.p |= OVERFLOW_FLAG

    iny(cpu)

    assert cpu.y == 0x02
    assert (cpu.p & CARRY_FLAG) != 0
    assert (cpu.p & OVERFLOW_FLAG) != 0

Test functions (4)

test_iny_increments_y_register
test_iny_wraps_from_ff_to_00_and_sets_zero_flag
test_iny_sets_negative_flag_when_result_has_bit_7_set
test_iny_does_not_modify_carry_or_overflow_flags