080. Instruction inx

add the INX instruction primitive.

Lesson 80 of 356 · tests/chapter_01_cpu/test_080_instruction_inx.py

In this step, add only emulator/cpu/instructions.py:inx. Opcode 0xE8 is lesson 081, while dex and opcode 0xCA are lessons 082-083.

Why this step exists

INX is implied register behavior, so it needs no addressing helper or bus access. The primitive performs 8-bit arithmetic and derives flags directly from the new X value.

Suggested implementation in emulator/cpu/instructions.py, after dec:

def inx(cpu: CPU):
    result = cpu.x + 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.x = result_8

Invariants: X remains eight-bit ($FF + 1 == $00); Zero and Negative reflect the masked result; A, Y, memory, Carry, and Overflow remain unchanged; this primitive itself does not fetch operands or advance PC.

Misconception: INX is not memory INC with X as an address. It mutates the X register directly and takes only cpu.

Out of scope: importing and mapping inx, adding dex, and mapping 0xCA belong to lessons 081-083.

Run this lesson

uv run pytest tests/chapter_01_cpu/test_080_instruction_inx.py -v

Example implementation

Why this step exists

def inx(cpu: CPU):
    result = cpu.x + 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.x = result_8

Show test codetest_080_instruction_inx.py

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


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


def test_inx_increments_x_register():
    """Objective: X increases by one and remains an 8-bit register value."""
    cpu = make_cpu()
    cpu.x = 0x10

    inx(cpu)

    assert cpu.x == 0x11


def test_inx_wraps_from_ff_to_00_and_sets_zero_flag():
    """Objective: 0xFF + 1 wraps to 0x00 and sets Zero flag."""
    cpu = make_cpu()
    cpu.x = 0xFF

    inx(cpu)

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


def test_inx_sets_negative_flag_when_result_has_bit_7_set():
    """Objective: 0x7F + 1 becomes 0x80, so Negative flag is set."""
    cpu = make_cpu()
    cpu.x = 0x7F

    inx(cpu)

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


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

    inx(cpu)

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

Test functions (4)

test_inx_increments_x_register
test_inx_wraps_from_ff_to_00_and_sets_zero_flag
test_inx_sets_negative_flag_when_result_has_bit_7_set
test_inx_does_not_modify_carry_or_overflow_flags