Understand how keyboards work

From Linuxintro

Keyboards typically are connected to the computer via a USB or a PS/2 cable. They send signals when they transfer data, when they acknowledge and when their buffer is full or empty. The kernel receives these signals and, depending on the loaded kernel module (provided the kernel does not do the work itself), sends the data to /dev/tty0. The application (typically X or a shell) gets these signals and puts them into a queue, like stdin.

The X layer

The X Window system takes the incoming keycodes and reacts on them. Use xev to see which keyboard events X sees. You can map their keycodes to keys using xmodmap. For example this makes the right "Windows" key to the compose key:

xmodmap -e "keycode 134 = Multi_key"

Now every key is mapped to the keyboard layout, e.g. German keyboards look different from English ones. For that X11 has /usr/X11R6/lib/X11/xkb/symbols where the layout "latin" (default one) contains mappings like this:

   key <AD01>  { [         q,          Q,           at,  Greek_OMEGA ] };
   key <AD02>  { [         w,          W,      lstroke,      Lstroke ] };
   key <AD03>  { [         e,          E,            e,            E ] };
   key <AD04>  { [         r,          R,    paragraph,   registered ] };
   key <AD05>  { [         t,          T,       tslash,       Tslash ] };
   key <AD06>  { [         y,          Y,    leftarrow,          yen ] };

The layer beneath X

To find out what signals X receives from the keyboard, open a new graphical console and type

cat /dev/tty0

You will see the messages that the kernel (and his modules) put into the tty0 device, the keyboard. When you are ready, close the console with a mouse click, as because of obvious reasons, you cannot abort with a keypress.

The USB layer

If you have a USB keyboard you can watch what happens on the USB bus.

  • use lsusb to find out what USB bus your keyboard is on
  • load the kernel module usbmon
modprobe usbmon
  • analyse the respective USB bus with WireShark

The kernel layer

If you really want to see what data comes over the wire, you have to write a kernel module, because the i386's security concept will not let anything read data but the kernel. So, create /root/keyb.c:

/*  A kernel module.

#include <linux/module.h>
#include <linux/kernel.h>
#include <asm/io.h>

int init_module(void)
  printk("This is a kernel module\n");
  int retries = 0x100100;
  int input;
  while (--retries != 0){int oldinput=input; input=inb(0x60); if (oldinput!=input)
  printk("got %i",input);};

void cleanup_module(void)
  printk(KERN_ALERT "Au revoir\n");

and Makefile:

obj-m += keyb.o

and compile this:

$ make -C /lib/modules/$(uname -r)/build M=$(pwd) modules       

Now when you load this module, the kernel will, during the initialization phase (the while loop) listen to port 0x60 and print the data on wire there. So, be prepared

  • the module will take a little time to initialize
  • during the load phase, you can type a bit and you will see it in the system log
  • this is tested for the PS/2 port, but not for USB

Load the module:

insmod keyb.ko

Check the syslog:


See also