GameBoy emulator

【GameBoy模拟器 2】CPU、总线和游戏卡带

先把游戏读进来随便执行一下再说

目标：

  • 了解GameBoy的大致硬件组成和模拟器代码结构
  • 模拟游戏卡带和基本功能与结构，加载gb游戏文件，进行初步的解析校验
  • 了解CPU的工作流程，实现基础的CPU指令、CPU寻址模式
  • 实现CPU的基本工作流程，实现指令的读取、分析与执行：取opcode并解析为指令、按指令对应的寻址方式读取所需数据、运行该指令

涉及到的概念：

  • 卡带：cartridge
  • 总线：bus
  • 内存：memory
  • 内存映射：memory map
  • 中央处理器：CPU (Central Processing Unit)
  • 指令：instruction
  • 寻址模式：addressing mode
  • 操作码: opcode
  • 寄存器：register

知识点

  • 关于CPU状态

    • halted（挂起态）
      • halted ≠ 指游戏暂停，而是通过HALT指令设置的一种状态。相反，游戏暂停的时候，CPU是在一直运行的，没有进入halt状态（经过调试得出结论）
      • 有些操作会需要用到这种指令。比如需要CPU可能需要等待某个外部事件（如硬件中断）发生。在这种情况下，HALT指令可以使CPU进入低功耗状态，直到中断发生。
      • Depending on how much CPU time is required by a game, the HALT instruction can extend battery life anywhere from 5% to 50% or possibly more. - Pan Docs
    • running和paused
      • CPU其实不需要running和paused状态，这两个状态都没有实际用上。LLD的paused状态实际上就是关机

  • 各种寻址模式（从指令集归纳而出的，是否有硬件基础呢？有的，这个基础就是译码器和操作码表）

    • 在CPU设计中，指令译码器是CPU的一部分，能将存储在指令寄存器或微程序指令中的比特转换为能控制CPU其他部分的控制信号 – wikipedia

  • 我们的实现CPU和实际CPU做的事情的区别

    • 我们的实现：一个C程序，三个函数取指、译码、执行
    • 实际硬件层面发生的事情（宏观上都是数字电路的或且非门）
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    以GameBoy的LD A, (HL)指令为例，展示CPU如何处理这条指令：

取指：

PC指向LD A, (HL)指令的内存地址。
内存接口读取指令，并将其存储在IR中。

译码：

指令译码器解析LD A, (HL)指令。
操作码确定这是一个加载指令。
译码器识别出这是间接寻址模式，操作数在HL寄存器指向的内存地址中。

操作码提取：指令译码器从IR中提取操作码。例如，LD A, (HL)的操作码可能是0x7E。
查找操作码表：指令译码器查找操作码表，确定0x7E对应的指令类型是LD，并且使用间接寻址模式。
生成控制信号：指令译码器生成控制信号，指示CPU使用HL寄存器的值作为内存地址。

执行：

AGU计算HL寄存器的值，得到内存地址。
内存接口从该地址读取数据。
数据通过数据总线传送到寄存器A。

  • 关于周期

    • 时钟周期（时钟频率的倒数，系统中最原子的时间单位）
    • 机器周期（由多个时钟周期构成，是CPU完成基础操作花费的时间，一条指令由多个基础操作构成，比如取指、译码、执行）
    • 指令周期（由多个机器周期构成，是某条CPU指令花费的时间）

  • cart write，为什么要对卡带进行写？

    • MBC1类的游戏需要进行特定位置的cart write来切换rom bank（分页机制）
    • gameboy的battery save机制是通过向卡带中的SRAM写入数据，电池给SRAM供电来实现的（掉电后仍然会丢失数据）

测试效果

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$ gbemu/gbemu ../../roms/dmg-acid2.gb 
Opened: ../../roms/dmg-acid2.gb
Cartridge Loaded:
         Title    : DMG-ACID2
         Type     : 00 (ROM ONLY)
         ROM Size : 32 KB
         RAM Size : 00
         LIC Code : 00 (None)
         ROM Vers : 00
         Checksum : 9F (PASSED)
Cart loaded..
SDL INIT
TTF INIT
Execute Instruction: 00   PC: 0001
Not executing yet
Execute Instruction: 00   PC: 0002
Not executing yet
Execute Instruction: 00   PC: 0003
Not executing yet
Execute Instruction: 00   PC: 0004
Not executing yet
Execute Instruction: 00   PC: 0005
Not executing yet
Execute Instruction: 00   PC: 0006
Not executing yet
Execute Instruction: 00   PC: 0007
Not executing yet
Execute Instruction: 00   PC: 0008
Not executing yet
Execute Instruction: 00   PC: 0009
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Execute Instruction: 00   PC: 000A
Not executing yet
Execute Instruction: 00   PC: 000B
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Execute Instruction: 00   PC: 000C
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Execute Instruction: 00   PC: 000D
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Execute Instruction: 00   PC: 000E
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Execute Instruction: 00   PC: 000F
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Execute Instruction: 00   PC: 0010
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Execute Instruction: 00   PC: 0011
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Execute Instruction: 00   PC: 0012
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Execute Instruction: 00   PC: 0013
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Execute Instruction: 00   PC: 0014
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Execute Instruction: 00   PC: 0015
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Execute Instruction: 00   PC: 0016
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Execute Instruction: 00   PC: 0017
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Execute Instruction: 00   PC: 0018
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Execute Instruction: 00   PC: 0019
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Execute Instruction: 00   PC: 001A
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Execute Instruction: 00   PC: 001B
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Execute Instruction: 00   PC: 001C
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Execute Instruction: 00   PC: 001D
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Execute Instruction: 00   PC: 001E
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Execute Instruction: 00   PC: 001F
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Execute Instruction: 00   PC: 0020
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Execute Instruction: 00   PC: 0021
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Execute Instruction: 00   PC: 0022
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Execute Instruction: 00   PC: 0023
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Execute Instruction: 00   PC: 0024
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Execute Instruction: 00   PC: 0025
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Execute Instruction: 00   PC: 0026
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Execute Instruction: 00   PC: 0027
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Execute Instruction: 00   PC: 0028
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Execute Instruction: 00   PC: 0029
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Execute Instruction: 00   PC: 002A
Not executing yet
Execute Instruction: 00   PC: 002B
Not executing yet
Execute Instruction: 00   PC: 002C
Not executing yet
Execute Instruction: 00   PC: 002D
Not executing yet
Execute Instruction: 00   PC: 002E
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Execute Instruction: 00   PC: 002F
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Execute Instruction: 00   PC: 0030
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Execute Instruction: 00   PC: 0031
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Execute Instruction: 00   PC: 0032
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Execute Instruction: 00   PC: 0038
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Execute Instruction: 00   PC: 0039
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Execute Instruction: 00   PC: 003A
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Execute Instruction: 00   PC: 003B
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Execute Instruction: 00   PC: 003C
Not executing yet
Execute Instruction: 00   PC: 003D
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Execute Instruction: 00   PC: 003E
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Execute Instruction: 00   PC: 003F
Not executing yet
Execute Instruction: 00   PC: 0040
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Execute Instruction: 00   PC: 0041
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Execute Instruction: 00   PC: 0042
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Execute Instruction: 00   PC: 0043
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Execute Instruction: 00   PC: 0044
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Execute Instruction: 00   PC: 0045
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Execute Instruction: 00   PC: 0046
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Execute Instruction: 00   PC: 0047
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Execute Instruction: 00   PC: 0048
Not executing yet
Unkown Instruction E9

可以看到：成功加载了游戏的镜像文件，解析出了游戏相关信息，CPU的程序计数器PC在按预期自增。

这证明卡带读取、校验和数据传输功能正常，CPU结构和功能正常。如果手动调整PC从0x100开始，则可以读取到更多的正常指令，比如C3:

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$ gbemu/gbemu ../../roms/dmg-acid2.gb 

Opened: ../../roms/dmg-acid2.gb
Cartridge Loaded:
         Title    : DMG-ACID2
         Type     : 00 (ROM ONLY)
         ROM Size : 32 KB
         RAM Size : 00
         LIC Code : 00 (None)
         ROM Vers : 00
         Checksum : 9F (PASSED)
Cart loaded..
SDL INIT
TTF INIT
Execute Instruction: 00   PC: 0101
Not executing yet
Execute Instruction: C3   PC: 0104
Not executing yet
Unkown Instruction CE
C GameBoy emulator
Licensed under CC BY-NC-SA 4.0
最后更新于 Jun 25, 2025 15:02 UTC
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