293. Validación: la CPU lee el mando en 4016

PRUEBA DE VALIDACIÓN: un programa de CPU lee bits del mando desde $4016.

Lección 293 de 356 · tests/chapter_07_controller_input/test_293_VALIDATION_cpu_reads_controller_4016.py

Este no es un paso de implementación para el estudiante.

No hay nada nuevo que construir en este archivo. Esta prueba valida que las piezas ya construidas funcionan juntas:

CPU instructions
    -> CpuBus.write($4016, value)
    -> Controller.write_strobe(value)
    -> CpuBus.read($4016)
    -> Controller.read_bit()
    -> CPU stores observed bits into RAM

Por qué existe esta validación

Las pruebas anteriores demostraron el objeto Controller y el enrutamiento de $4016 en CpuBus de forma aislada. Esta prueba demuestra que una secuencia real de instrucciones de la CPU puede usar esa ruta.

Referencias

https://www.nesdev.org/wiki/Standard_controller
https://www.nesdev.org/wiki/Controller_reading_code

Forma del sondeo del mando de la NES

write 1 to $4016
write 0 to $4016
read $4016 eight times

Las ocho lecturas devuelven

A, B, Select, Start, Up, Down, Left, Right

Programa mínimo usado aquí

LDA #$01
STA $4016     ; strobe high, capture live buttons

LDA #$00
STA $4016     ; strobe low, serial reads advance

LDA $4016
STA $0000     ; A bit

LDA $4016
STA $0001     ; B bit

... repeated through Right bit ...

Distinción importante

Esta validación almacena cada bit serie en un byte de RAM independiente por claridad. Muchos ejemplos de ensamblador de la NES desplazan los bits hacia un único byte resultado. Eso es política del código del juego, no comportamiento del hardware del mando.

Fuera de alcance

Mapeo de teclado de pygame
Bucle de main.py
Puerto de mando 2
Mandos de expansión de Famicom
Comportamiento de glitch de lectura de DMC/mando
Fixtures de ROM comerciales

Ejecutar esta lección

uv run pytest tests/chapter_07_controller_input/test_293_VALIDATION_cpu_reads_controller_4016.py -v

Implementación de ejemplo

CPU instructions
    -> CpuBus.write($4016, value)
    -> Controller.write_strobe(value)
    -> CpuBus.read($4016)
    -> Controller.read_bit()
    -> CPU stores observed bits into RAM

Referencias

https://www.nesdev.org/wiki/Standard_controller
https://www.nesdev.org/wiki/Controller_reading_code

Forma del sondeo del mando de la NES

write 1 to $4016
write 0 to $4016
read $4016 eight times

Programa mínimo usado aquí

LDA #$01
STA $4016     ; strobe high, capture live buttons

LDA #$00
STA $4016     ; strobe low, serial reads advance

LDA $4016
STA $0000     ; A bit

LDA $4016
STA $0001     ; B bit

... repeated through Right bit ...

Ver código del testtest_293_VALIDATION_cpu_reads_controller_4016.py

from emulator.cpu.cpu import CPU
from emulator.bus.cpu_bus import CpuBus
from emulator.memory.fake_rom import FakeROM
from tests.helpers import load_program, write_reset_vector


def build_controller_poll_program() -> list[int]:
    """
    Build a tiny CPU program that polls controller port 1 and stores the eight
    serial read bits into RAM $0000-$0007.
    """
    program = [
        0xA9, 0x01,        # LDA #$01
        0x8D, 0x16, 0x40,  # STA $4016
        0xA9, 0x00,        # LDA #$00
        0x8D, 0x16, 0x40,  # STA $4016
    ]

    for ram_addr in range(0x0000, 0x0008):
        program.extend(
            [
                0xAD, 0x16, 0x40,        # LDA $4016
                0x8D, ram_addr, 0x00,    # STA $00xx
            ]
        )

    return program


def make_cpu_for_controller_validation() -> tuple[CPU, CpuBus]:
    """Create a CPU with a FakeROM containing the controller polling program."""
    rom = FakeROM()
    load_program(rom, 0x8000, build_controller_poll_program())
    write_reset_vector(rom, 0x8000)

    bus = CpuBus(program_rom=rom)
    cpu = CPU(bus)

    return cpu, bus


def test_VALIDATION_cpu_program_reads_controller_bits_from_4016_into_ram():
    """
    Validation objective:
    Prove CPU instructions can read controller port 1 through $4016.

    Pressed buttons:
        A, Select, Down, Right

    Expected serial bits:
        A      -> 1
        B      -> 0
        Select -> 1
        Start  -> 0
        Up     -> 0
        Down   -> 1
        Left   -> 0
        Right  -> 1
    """
    cpu, bus = make_cpu_for_controller_validation()
    bus.controller_1.a = True
    bus.controller_1.select = True
    bus.controller_1.down = True
    bus.controller_1.right = True

    cpu.reset()

    # 4 setup instructions + 8 * 2 instructions for LDA $4016 / STA RAM.
    for _ in range(20):
        cpu.step()

    assert [bus.read(addr) for addr in range(0x0000, 0x0008)] == [
        1,  # A
        0,  # B
        1,  # Select
        0,  # Start
        0,  # Up
        1,  # Down
        0,  # Left
        1,  # Right
    ]


def test_VALIDATION_cpu_controller_poll_uses_captured_snapshot():
    """
    Validation objective:
    Prove the CPU polling sequence captures a stable snapshot when strobe moves
    from high to low.

    The test changes live button state after the strobe sequence but before the CPU
    reads all stored values. The already-captured serial sequence should still
    reflect the buttons that were active during strobe.
    """
    cpu, bus = make_cpu_for_controller_validation()
    bus.controller_1.a = True
    bus.controller_1.right = True

    cpu.reset()

    # Execute only the strobe sequence:
    # LDA #$01, STA $4016, LDA #$00, STA $4016
    for _ in range(4):
        cpu.step()

    # Change live state after capture. Serial reads should still use captured data.
    bus.controller_1.a = False
    bus.controller_1.right = False

    # Execute the 8 LDA/STA read/store pairs.
    for _ in range(16):
        cpu.step()

    assert [bus.read(addr) for addr in range(0x0000, 0x0008)] == [
        1,  # A was captured as pressed
        0,
        0,
        0,
        0,
        0,
        0,
        1,  # Right was captured as pressed
    ]

Funciones de test (2)

test_VALIDATION_cpu_program_reads_controller_bits_from_4016_into_ram
test_VALIDATION_cpu_controller_poll_uses_captured_snapshot