243. Ppuctrl 控制 ppudata 的递增量

PPUCTRL 控制 PPUDATA 将 vram_addr 递增多少。

第 243 / 356 · tests/chapter_03_ppu_memory_and_graphics_data/test_243_ppuctrl_controls_ppudata_increment.py

参考资料

https://www.nesdev.org/wiki/PPU_registers#PPUCTRL

要更新的文件

emulator/ppu/ppu.py

要添加的常量

CTRL_VRAM_INCREMENT_BY_32 = 1 << 2

为什么需要这一步

PPUDATA ($2007) 会访问当前内部 VRAM 地址上的 PPU 内存。每次访问之后,PPU 都会将该地址递增。

递增量由 PPUCTRL ($2000) 的第 2 位控制:

bit 2 clear -> increment by 1
bit 2 set   -> increment by 32

为什么这很重要

递增 1 适用于沿一行连续的 PPU 内存进行写入。递增 32 适用于在命名表内存中沿一列向下写入。

建议的实现伪代码

CTRL_VRAM_INCREMENT_BY_32 = 1 << 2

case 0x2007:
    self.data = value
    self.ppu_bus.write(self.vram_addr, value)

    increment = 32 if self.ctrl & CTRL_VRAM_INCREMENT_BY_32 else 1
    self.vram_addr = (self.vram_addr + increment) & 0x3FFF

重要事项

请继续用 0x3FFF 对 vram_addr 进行掩码运算,因为 PPU 地址空间是 14 位的:

$0000-$3FFF

超出范围

其他 PPUCTRL 位
NMI 启用
图案表选择
基础命名表选择
渲染

运行本课

uv run pytest tests/chapter_03_ppu_memory_and_graphics_data/test_243_ppuctrl_controls_ppudata_increment.py -v

示例实现

建议的实现伪代码

CTRL_VRAM_INCREMENT_BY_32 = 1 << 2

case 0x2007:
    self.data = value
    self.ppu_bus.write(self.vram_addr, value)

    increment = 32 if self.ctrl & CTRL_VRAM_INCREMENT_BY_32 else 1
    self.vram_addr = (self.vram_addr + increment) & 0x3FFF

查看测试代码test_243_ppuctrl_controls_ppudata_increment.py

from emulator.ppu.ppu import CTRL_VRAM_INCREMENT_BY_32, PPU


def set_ppuaddr(ppu: PPU, addr: int) -> None:
    """Set PPU.vram_addr through the public $2006 two-write interface."""
    ppu.write_register(0x2006, (addr >> 8) & 0xFF)
    ppu.write_register(0x2006, addr & 0xFF)


def test_ppuctrl_vram_increment_constant_exists():
    """
    Objective:
    Name PPUCTRL bit 2, which selects PPUDATA increment-by-32 mode.
    """
    assert CTRL_VRAM_INCREMENT_BY_32 == 1 << 2


def test_ppudata_increments_vram_addr_by_one_when_increment_bit_is_clear():
    """
    Objective:
    With PPUCTRL bit 2 clear, PPUDATA should increment vram_addr by 1.
    """
    ppu = PPU()
    ppu.write_register(0x2000, 0x00)
    set_ppuaddr(ppu, 0x2000)

    ppu.write_register(0x2007, 0xAA)

    assert ppu.vram_addr == 0x2001


def test_ppudata_increments_vram_addr_by_32_when_increment_bit_is_set():
    """
    Objective:
    With PPUCTRL bit 2 set, PPUDATA should increment vram_addr by 32.
    """
    ppu = PPU()
    ppu.write_register(0x2000, CTRL_VRAM_INCREMENT_BY_32)
    set_ppuaddr(ppu, 0x2000)

    ppu.write_register(0x2007, 0xBB)

    assert ppu.vram_addr == 0x2020


def test_ppudata_increment_by_32_still_writes_to_original_address_before_incrementing():
    """
    Objective:
    PPUDATA should write to the old vram_addr first, then increment.

    Why:
    The increment affects the next PPUDATA access, not the current one.
    """
    ppu = PPU()
    ppu.write_register(0x2000, CTRL_VRAM_INCREMENT_BY_32)
    set_ppuaddr(ppu, 0x2000)

    ppu.write_register(0x2007, 0xCC)

    assert ppu.ppu_bus.read(0x2000) == 0xCC
    assert ppu.vram_addr == 0x2020


def test_ppudata_increment_wraps_to_14_bit_ppu_address_space():
    """
    Objective:
    vram_addr should stay inside the PPU $0000-$3FFF address range after
    incrementing.
    """
    ppu = PPU()
    ppu.write_register(0x2000, CTRL_VRAM_INCREMENT_BY_32)
    set_ppuaddr(ppu, 0x3FF0)

    ppu.write_register(0x2007, 0xDD)

    assert ppu.vram_addr == ((0x3FF0 + 32) & 0x3FFF)

测试函数 (5)

test_ppuctrl_vram_increment_constant_exists
test_ppudata_increments_vram_addr_by_one_when_increment_bit_is_clear
test_ppudata_increments_vram_addr_by_32_when_increment_bit_is_set
test_ppudata_increment_by_32_still_writes_to_original_address_before_incrementing
test_ppudata_increment_wraps_to_14_bit_ppu_address_space