为什么在切换到保护模式之前在引导加载程序中测试端口0x64？-Java 学习之路

在我的麻省理工学院OS课程（686）中，我发现了一些代码，我试图理解boot / boot.S中的指令 inb $0x64, %al . 我的理解是它从数据端口0x64读取一个字节到AL，什么是端口0x64？它忙于测试哪种设备或机制？我对代码忙的评论感到困惑？评论意味着什么，它指的是什么？

# Enable A20:
#   For fascinating historical reasons (related to the fact that
#   the earliest 8086-based PCs could only address 1MB of physical memory
#   and subsequent 80286-based PCs wanted to retain maximum compatibility),
#   physical address line 20 is tied to low when the machine boots.
#   Obviously this a bit of a drag for us, especially when trying to
#   address memory above 1MB.  This code undoes this.

seta20.1:       inb     $0x64,%al               # Get status
                testb   $0x2,%al                # Busy?
                jnz     seta20.1                # Yes
                movb    $0xd1,%al               # Command: Write
                outb    %al,$0x64               #  output port
seta20.2:       inb     $0x64,%al               # Get status
                testb   $0x2,%al                # Busy?
                jnz     seta20.2                # Yes
                movb    $0xdf,%al               # Enable
                outb    %al,$0x60               #  A20

1 回答

4
什么是端口0x64？

端口0x64是键盘控制器的IO端口 .

键盘控制器有两个端口0x64和0x60 .

端口0x64（命令端口）用于向键盘控制器（PS / 2）发送命令 .

端口0x60（数据端口）用于向/从PS / 2（键盘）控制器或PS / 2设备本身发送数据 .

它忙于测试哪种设备或机制？我对代码忙的评论感到困惑？评论意味着什么，它指的是什么？

这里的代码使CPU轮询PS / 2键盘控制器以查看它是否正忙 .
```
inb     $0x64,%al               # Get status
```
上述指令读取PS / 2控制器的状态寄存器，其长度为8位 . 具体来说，它试图读取输入缓冲区的状态 - 位1 .
```
testb   $0x2,%al                # Busy?
```
在此处测试从前一个inb指令返回的字节 . 这里检查输入缓冲区的状态以查看它是满还是空 - 位1（0x2） .
```
Status Register - PS/2 Controller
Bit Meaning
0   Output buffer status (0 = empty, 1 = full) (must be set before attempting to read data from IO port 0x60)

1   Input buffer status (0 = empty, 1 = full) (must be clear before attempting to write data to IO port 0x60 or IO port 0x64)

2   System Flag - Meant to be cleared on reset and set by firmware (via. PS/2 Controller Configuration Byte) if the system passes self tests (POST)

3   Command/data (0 = data written to input buffer is data for PS/2 device, 1 = data written to input buffer is data for PS/2 controller command)

4   Unknown (chipset specific) - May be "keyboard lock" (more likely unused on modern systems)

5   Unknown (chipset specific) - May be "receive time-out" or "second PS/2 port output buffer full"

6   Time-out error (0 = no error, 1 = time-out error)

7   Parity error (0 = no error, 1 = parity error)
```
Syntax of x86 assembly code以上主题详细介绍了此处提到的端口0x64和0x60 .

以下链接还详细讨论了IO端口的使用方式 . How are I/O port addresses and data sent?
回复于 2024-05-11T02:48:00+08:00

为什么在切换到保护模式之前在引导加载程序中测试端口0x64？

1 回答

相关问题