为什么要在进入保护模式之前在引导加载程序中测试端口0x64？

Question

在我的MIT OS课程（686）中，我发现了一些我不理解的代码。我想了解指令inb $0x64, %al的启动 / boot.S。我的理解是它正在从数据端口0x64读一个字节到AL，端口0x64是什么？正在测试哪个设备或机制忙？我对代码忙中的注释感到困惑吗？评论是什么意思，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

Answer 1

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PS / 2（键盘）控制器或PS / 2设备本身发送数据。

正在测试哪个设备或机制忙？我对代码忙中的注释感到困惑吗？评论是什么意思，它指的是什么？

此处的代码使CPU轮询PS / 2键盘控制器以查看其是否繁忙。

inb     $0x64,%al               # Get status

上面的指令读取PS / 2控制器的状态寄存器，该寄存器的长度为8位。具体来说，它正在尝试读取输入缓冲区的状态-位1。

testb   $0x2,%al                # Busy?

从前一个inb指令返回的字节在此处进行测试。在此检查输入缓冲区的状态，以查看其是否已满或为空-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)

x86汇编代码 的语法上面的线程详细说明了此处引用的端口0x64和0x60。

另外，下面的链接详细讨论了如何使用IO端口。 I / O端口地址和数据如何发送？