Difference between revisions of "Background: How keyboards work"

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Keyboards typically are connected to the computer via a [[USB]] or a [http://en.wikipedia.org/wiki/PS/2_connector 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 value received by X from the kernel will be the key's ''scancode''. Which scancode it is you can find out with the command xev. To change the mapping of scancode to X events, you can use the [[xmodmap]] command.
#REDIRECT [[understand how keyboards work]]
= The X layer =
Use the command xev to see which keyboard events X sees.
= The layer beneath X =
To find out what signals X receives from the keyboard, open a new graphical [http://simple.wikipedia.org/wiki/Command_Line_Interface 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 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:

Latest revision as of 13:54, 16 July 2014