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│ Programming the Keyboard │
└──────────────────────────┘
Written for the PC-GPE by Mark Feldman
e-mail address : u914097@student.canberra.edu.au
myndale@cairo.anu.edu.au
┌───────────────────────────────────────────┐
│ THIS FILE MAY NOT BE DISTRIBUTED │
│ SEPARATE TO THE ENTIRE PC-GPE COLLECTION. │
└───────────────────────────────────────────┘
┌────────────┬───────────────────────────────────────────────────────────────
│ Disclaimer │
└────────────┘
I assume no responsibility whatsoever for any effect that this file, the
information contained therein or the use thereof has on you, your sanity,
computer, spouse, children, pets or anything else related to you or your
existance. No warranty is provided nor implied with this information.
┌──────────┬─────────────────────────────────────────────────────────────────
│ Overview │
└──────────┘
The operation of the keyboard is really quite simple. Every time a key
is pressed or released an interrupt 9 is generated, and reading the value
from port 60h tells you what happened.
┌────────────────────────────┬───────────────────────────────────────────────
│ Decoding the Keyboard Byte │
└────────────────────────────┘
So let's say you've installed an interrupt handler to handle all keyboard
events and when an interrupt is generated your handler reads the byte from
port 60h. What now?
Well..each key on the keyboard has an associated scan code which is contained
in the lower 7 bits of the byte. The most significant bit (ie bit 7) tells
you what was actually done, 0 = key was just pressed, 1 = key was just
released. If someone had just pressed the ESC key for instance, the port will
show a value of 1 (1 is the ESC key's scan code). If they hold their finger
on the button the keyboard will keep generating interrupt 9's and each
time the port will still show a value of 1. When the person releases the key
a final interrupt will be generated and the port will return 129 (1 + 128,
since the high bit will be set indicating the person has released the key).
Well...it's almost this simple. Some keys are "extended" keys. When an
extended key is pressed an interrupt is generated and the keyboard port
will return a value of 224 (E0h). This means that an extended key was pressed
and it's *extended* scan code will be available during the *next* interrupt.
Note that the left control key has a scan code of 29, while the *right*
control key has an *extended* scan code of 29. The same applies to the alt
keys and the arrow keys (keypad arrows vs the other ones).
It would be nice if all keys were created equal and we could just throw away
the 224 extended bytes and handle all the other bytes normally. Unfortunately
there are two buttons which on my machine at least (and others I have tested)
do some really weird stuff:
PrtScn
──────
Pressing this button will send *2* extended characters to the handler, 42
and 55, so the actual byte sequence will be 224, 42, 224, 55. (Also note
that the left shift key has a regular scan code of 42, so there goes our
idea of just throwing 224's away). Only the extended 55's are sent during
auto-repeat. When the key is released, the two are sent again with the high
bits set (224, 170, 224, and 183). If any of the shift or control keys are
being held down when the PrtScn button is pressed then only the (224, 55) is
sent when the key is pressed and only the (224, 183) is sent when it's
released. If the alt key is being held down (System Request) then the key
behaves like an ordinary key with scan code 84. The practical upshot of all
this is that the handlers you write to handle normal keys and extended keys
will work fine with all the different PrtScn combinations (although a program
would have to check normal key 84 *AND* extended key 55 in order to determine
if the key is currently being pressed).
Pause/Break
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Welcome to hell. If you press this key while either of the the control keys
are being held down, it will behave like extended key 70, at all other times
it will send the following bytes: (225, 29, 69, 225, 157, 197). Holding the
key down does not result in autorepeat. Taking your finger off the key does
not send any extra bytes, they appear to be sent after the "key down" bytes
when you first press the key. Notice that 225 isn't 224, so our normal
extended character handler will not take care of this. My personal theory is
that while a scan code of 224 (E0h) means there is 1 more character
following, a scan code of 225 (E1h) means there are *2* more following. I've
seen a number of keyboard handler libraries and they all seem to overlook
this key. So why not be the first kid on your block to have a keyboard
handler which properly supports the Pause/Break key? CHECK IT OUT!!
┌───────────────────┬────────────────────────────────────────────────────────
│ Writing a Handler │
└───────────────────┘
Writing a keyboard handler is fairly straightforward. This section will
show how to do it in Pascal (you C and asm programmers would probably already
know this stuff anyway).
First we'll declare a few things we'll need:
const KEYBOARDINTR = 9;
KEYBOARDPORT = $60;
var BIOSKeyboardHandler : procedure;
CallBIOSHandler : boolean;
The CallBIOSHandler variable will be initialised by the calling program. If
we also want the BIOS handler to process all keystrokes then this variable
must be set to true.
Next we need to store the value of the current handler and set up own our
own one. We'll use a procedure called KeyboardHandler to handle the actual
interrupt.
CallBIOSHandler := false; { ...or set it to true if you want. }
GetIntVec(KEYBOARDINTR, @BIOSKeyboardHandler);
SetIntVec(KEYBOARDINTR, Addr(KeyboardHandler));
Ok, so everything is now set up and our handler will now be able to process
all keyboard events. The actual interrupt handler could look like this:
{$F+}
procedure KeyboardHandler(Flags, CS, IP, AX, BX, CX, DX,
SI, DI, DS, ES, BP: Word);
interrupt;
var key : byte;
begin
key := Port[KEYBOARDPORT];
{ PROCESS THE KEYSTROKE HERE }
if CallBIOSHandler then
{ Call the BIOS keyboard handler if the calling program wants us to }
begin
asm pushf end;
BIOSKeyboardHandler;
end
{ Otherwise just acknowledge the interrupt }
else Port[$20] := $20;
end;
{$F-}
When the program is finished we can set the old keyboard handler again:
SetIntVec(KEYBOARDINTR, @BIOSKeyboardHandler);
┌───────────────────┬────────────────────────────────────────────────────────
│ A Word of Warning │
└───────────────────┘
When I was writing a simple handler to test the info in this file I did
something REALLY stoopid which I would like to share with the world. I
thought that my program was stuffing the keyboard up because when I exited
the program my editor (Borland Pascal 7.0) would act as though the control
button was being held down (I'm sure some of you have already started
laughing by now). I had to press it after each time I ran the program
just to sort it out. After spending a few hours looking all over the place
for info on what could possibly be wrong I realised what I was doing. I was
pressing CTRL-F9 to compile the program which would also immediately make it
run and I was releasing the control key when my program was running, ie the
regular BIOS handler was not getting the control key's "key up" command and
still thought it was being held down when my program returned control to
it. Moron.....
┌────────────┬───────────────────────────────────────────────────────────────
│ Scan Codes │
└────────────┘
The following is a list of all the regular key scan codes in numerical
order:
Scan Scan
Code Key Code Key
────────────────────────── ──────────────────────────
1 ESC 44 Z
2 1 45 X
3 2 46 C
4 3 47 V
5 4 48 B
6 5 49 N
7 6 50 M
8 7 51 , <
9 8 52 . >
10 9 53 / ?
11 0 54 RIGHT SHIFT
12 - _ 55 * (KEYPAD)
13 = + 56 LEFT ALT
14 BACKSPACE 57 SPACEBAR
15 TAB 58 CAPSLOCK
16 Q 59 F1
17 W 60 F2
18 E 61 F3
19 R 62 F4
20 T 63 F5
21 Y 64 F6
22 U 65 F7
23 I 66 F8
24 O 67 F9
25 P 68 F10
26 [ { 69 NUMLOCK (KEYPAD)
27 ] } 70 SCROLL LOCK
28 ENTER (RETURN) 71 7 HOME (KEYPAD)
29 LEFT CONTROL 72 8 UP (KEYPAD)
30 A 73 9 PGUP (KEYPAD)
31 S 74 - (KEYPAD)
32 D 75 4 LEFT (KEYPAD)
33 F 76 5 (KEYPAD)
34 G 77 6 RIGHT (KEYPAD)
35 H 78 + (KEYPAD)
36 J 79 1 END (KEYPAD)
37 K 80 2 DOWN (KEYPAD)
38 L 81 3 PGDN (KEYPAD)
39 ; : 82 0 INSERT (KEYPAD)
40 ' " 83 . DEL (KEYPAD)
41 ` ~ 87 F11
42 LEFT SHIFT 88 F12
The following is a list of all the extended key scan codes in numerical
order:
Scan Scan
Code Key Code Key
─────────────────────────────── ───────────────────────────────
28 ENTER (KEYPAD) 75 LEFT (NOT KEYPAD)
29 RIGHT CONTROL 77 RIGHT (NOT KEYPAD)
42 PRINT SCREEN (SEE TEXT) 79 END (NOT KEYPAD)
53 / (KEYPAD) 80 DOWN (NOT KEYPAD)
55 PRINT SCREEN (SEE TEXT) 81 PAGE DOWN (NOT KEYPAD)
56 RIGHT ALT 82 INSERT (NOT KEYPAD)
71 HOME (NOT KEYPAD) 83 DELETE (NOT KEYPAD)
72 UP (NOT KEYPAD) 111 MACRO
73 PAGE UP (NOT KEYPAD)