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？

Question

在我的麻省理工学院操作系统课程(686)中，我发现了一些我不懂的代码。我试图理解引导/引导中的指令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？什么设备或机制，它是测试忙？我对代码中的评论感到困惑吗？这句话的意思是什么，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

Answer 1

什么是0x64端口？

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PS/2 (键盘)控制器或PS/2设备本身发送数据。

什么设备或机制，它是测试忙？我对代码中的评论感到困惑吗？这句话的意思是什么，指的是什么？

这里的代码使CPU轮询PS/2键盘控制器来查看它是否繁忙。

inb     $0x64,%al               # Get status

上述指令读取长度为8位的PS/2控制器的状态寄存器。具体来说，它正在尝试读取输入缓冲区-位1的状态。

testb   $0x2,%al                # Busy?

从前一个inb指令返回的字节在这里进行测试。在这里，将检查输入缓冲区的状态，以查看它是完整的还是空的-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上面的线程详细解释了这里提到的端口0x64和0x60。

下面的链接还详细讨论了IO端口的使用方式。How are I/O port addresses and data sent?