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The Keyboard Interface Tutorial
Preface:
The Keyboard on the IBM PC and compatible is connected to the PPI -
Programmable Peripheral Interface, which generally takes care of Keyboard
functions, equipment information and timer functions. The PPI is addressable
via port reads and writes, to addresses 060h thru 063h.
There are several type of keyboards available. The main difference between the
various types is the key count. Existing sizez are:
83 Key (Original PC)
84 Key (XT and clones)
101 Key (Usually on AT's and up, called Enhanced Keyboard)
In addition to those standard types, there are numerous non-standard types,
which varry from manufacturer to manufacturer. There are 102 Key keyboards,
keyboards with two F-Key sets (upper and left), keyboards with weird
hardly-used keys like 'Macro' and 'Turbo' (which DO have a scan code, though)
etcetera.
The IBM AT and up provides a better way of negotiating the keyboard, by
allowing the user to directly access and command the keyboard, as it will be
demonstrated later.
Using assembly language, there are two major ways to handle the keyboard and
keystrokes:
1. Using DOS functions:
DOS provides a useful set of functions to use in context with the keyboard.
This includes one-key-input and string input.
2. Using BIOS interrupts:
The IBM PC BIOS (Basic Input Output System) provides an interrupt to handle
the keyboard - Interrupt 016h (022d). These functions, titles KEYBOARD I/O,
are an efficient medium level way of using the keyboard
In addition to the Interrupt 016h interface, there is the Keyboard
Interrupt, Interrupt 009h. As opposed to the other interrupt, Interrupt
009h is an IRQ - Interrupt Request, which means that it is never called by
software. Only hardware, when certain conditions are met, calls IRQ
interrupts. Naturally, the user is able to invoke them himself, but that is
not commonly done.
3. Direct port access:
It is possible to access the keyboard using direct port reads and writes.
As mentioned before, this is usually done by addressing the PPI. On AT's
and up, however, it is possible to directly access the keyboard itself.
The BIOS also supplies a certain amount of information about the keyboard
within it's BDA - BIOS Data Area, aka BDS - BIOS Data Segment - which is the
segment that lies at paragraph 040h (0040h:XXXXh or 0000h:04XXh):
Address Size Contents
------- ---- --------
0:0412 1 Errors in PCjr InfraRed keyboard link
0:0417 2 Keyboard status bits. See Keyboard Flags
0:0419 1 Current (accumulating) value of Alt+numpad pseudo-key input.
Normally 0. When [Alt] is released, value is stored in kbd buf.
0:041A 2 Address of keyboard buffer head (keystroke at that addr is next)
0:041C 2 Address of keyboard buffer tail
0:041E 32 Keyboard buffer. BIOS stores keystrokes here (head and tail
point to addresses from 041Eh to 043Dh inclusive).
0:0480 2 AT PS/2 Keyboard buffer offset start address (usually 01Eh)
0:0482 2 end address (usually 03Eh)
0:0496 1 AT Keyboard flag bit 4=1 (10h) if 101-key keyboard is attached
0:0497 1 AT Keyboard flag for LED 'key lock' display
bits 0-2 are ScrollLock, NumLock, CapsLock respectively
******************************************************************************
Using DOS functions:
--------------------
DOS provides a set of 7 functions to handle the keyboard:
01h Keyboard Input
06h Console I/O
07h No Echo Unfiltered Input
08h No Echo Filtered Input
0Ah Buffered Input
0Bh Input Status
0Ch Clear Keyboard Buffer & Input
In addition to these functions, it is also possible to read a selected amount
of characters from the keyboard, using DOS's File Handle functions, as the
Keyboard, aka Standard Input, has a pre-set handle of 0000h:
DOS Fn 3Fh: Read from keyboard via Handle
------------------------------------------------------------------------------
/-----------------------------------------------------------------\
| Expects | AH | 3Fh |
\---------| BX | 0000h - Handle for Standard Input (Keyboard) |
| DS:DX | Address of buffer to receive data |
| CX | Number of bytes to read |
/---------|-------|-----------------------------------------------|
| Returns | AX | Error code if CF is set to CY |
\---------| AX | Number of bytes actually read |
\-------------------------------------------------------/
Description: CX bytes of data are read from the keyboard. The data is placed
into the caller's buffer pointed to by DS:DX.
Notes: It is handy to use this function for reading default handles
such as the Standard I/O handles, instead of the buffered input
or character-by-character input functions.
When you read from a device, AX returns the length of the line
up to and including the termination CR (ASCII 13h).
******************************************************************************
Using BIOS Interrupts:
----------------------
As mentioned before, there are two ways to use interrupts to access the
keyboard.
The first is Interrupt 016h - Keyboard I/O.
INT 16h: Keyboard Services
------------------------------------------------------------------------------
This is the application-level interface to the keyboard. Keystrokes are
actually processed asynchronously in the background. As each keystroke is
received from the keyboard, it is processed by INT 09h and placed into a
circular queue.
The second is Interrupt 009h - Keyboard Interrupt, IRQ 1
INT 09h: Keyboard Interrupt
------------------------------------------------------------------------------
This hardware-generated interrupt (IRQ 1) is executed upon each press and
release of a key. The ROM-BIOS code interprets the keystroke, storing values
into the keyboard buffer at 40:1Eh. It also handles the special cases of the
PrtSc and SysReq keys, and tracks the status of the shift and case-lock keys.
See: INT 16h .......... BIOS service to access the keys stored in the buffer
and obtain status of the certain shift keys.
Scan Codes ....... a list of the values of each possible keystroke as
it is received by INT 09h.
Extended ASCII ... for a summary of the values that BIOS stores into the
the keyboard buffer after it translates a scan code.
Keyboard Flags ... for a summary of how to obtain, test for, and modify
the bit-settings of shift and case-lock flags.
TSRs (RAM-resident programs) that have a hot-key to trigger a popup usually
intercept INT 09h and test for a certain key with a sequence such as this:
push ax
in al,60h ; Read the key
cmp al,POP_KEY ; Is this the hot key?
je do_pop ; Yes, trigger the popup
; No, drop through to original driver
pop ax
jmp cs:[int9_vect] ; Just hop out to original int handler
do_pop: ;------ the following housekeeping is needed to satisfy the hdwr int
in al,61h ; Get value of keyboard control lines
mov ah,al ; Save it
or al,80h ; Set the "enable kbd" bit
out 61h,al ; And write it out the control port
xchg ah,al ; Fetch the original control port value
out 61h,al ; And write it back
mov al,20h ; Send End-Of-Interrupt signal
out 20h,al ; to the 8259 Interrupt Controller
;------ other code handles other tests and finally the popup code
******************************************************************************
Direct port access:
-------------------
The keyboard is addressable in two manners.
The first, available on all IBM PCs of all types, is by addressing the PPI -
Programmable Peripheral Interface.
Port Description
---- -----------
060h PC/XT PPI port A. Read keyboard scan code:
IN al,60h ; Fetche most recent scan code. See INT 09h and Scan Code
AT keyboard data register. See AT Keyboard
061h PC/XT PPI (Programmable Peripheral Interface) port B.
7 6 5 4 3 2 1 0
| | | | | | | |
| | | | | | | \-> 0: Timer 2 gate (speaker) --/--> OR 03h=speaker ON
| | | | | | \---> 1: Timer 2 data ---------/ AND 0FCh=speaker OFF
| | | | | \-----> 2: Always 0
| | | | \-------> 3: 1=Read high switches; 0=read low switches(see 62h)
| | | \---------> 4: 0=Enable RAM parity checking; 1=disable
| | \-----------> 5: 0=Enable I/O channel check
| \-------------> 6: 0=Hold keyboard clock low
\---------------> 7: 0=Enable keyboard; 1=disable keyboard
------------------------------------------------------------------------------
The second, available only on IBM PC/ATs and up, is by directly commanding the
keyboard itself. -= This is for the AT keyboard only. =-
Port Description
---- -----------
064h AT keyboard command register. This port communicates with the 8042
which runs an on-chip control program for the keyboard. It accepts
command codes and data bytes.
The keyboard of the AT (and its Intel 8042 microcomputer interface) is
programmable and a lot more interesting than the old-style PC keyboards.
Using the information below, you can set the key repeat speed and play
games with the "lock" key LED display.
Note: This is not a comprehensive coverage of the details of keyboard and
8042 operation, but it should provide some food for thought.
Port 60h is for writing data and is maintained for compatibility with
earlier models. If the examples using port 64h don't work, try using 60h.
Port 64h is for writing commands and data and for reading keyboard status.
Before sending a command to the keyboard, the BIOS tests its status
(IN al,64h) and makes sure a key isn't being pressed or the internal buffer
isn't full, etc. There's a small risk if you just go ahead and send the
command:
mov al,cmd_code
out 64h,al
For a two-part command such as setting the typeamatic rate, it's wise to
delay a little while between OUTs:
mov al,cmd_code
out 64h,al
mov cx,2000h ;arbitrary <20> 10ms+
delay: loop delay
mov al,data_value
out 64h,al
Cmd Description
--- -----------
0FFh Reset the keyboard and start internal diagnostics
0FEh Resend the last transmission
0FDh-0F7h (NOP)
0F6h Set keyboard to defaults and continue scanning
0F5h Set keyboard to defaults and disable keyboard scanning
0F4h Enable the keyboard. Kybd sends 'ACK', clears buffer, and starts scanning
0F3h Set typeamatic rate and delay. First send 0F3h, then send data byte:
-7-6-5-4-3-2-1-0-
|0|dly|rept rate|
-----------------
| | | \---------> bits 0-4 set the repeat rate (see below)
| \-------------> bits 5-6 set initial delay before first repeat:
| 00=250ms; 01=500ms; 10=750ms; 11=1000ms
\---------------> bit 7 is always 0
Value Rate Value Rate
This chart is a partial guide for the repeat 0 = 30.0 0Ah = 10.0
rate (bits 0-4). You can interpolate for 1 = 26.7 0Dh = 9.2
values not shown, but let's face it, you're 2 = 24.0 10h = 7.5
only interested in the fastest rates. 4 = 20.0 14h = 5.0
8 = 15.0 1Fh = 2.0
The keyboard is initially set to begin repeating after 1/2-second and
repeat at a rate of 10 repeats per second. This is much too slow. A
data byte of 01h sets the delay to 1/4-second with 26 repeats per second.
0F2h-0EFh (NOP)
0EEh Echo. Diagnostics aid. Simply sends 0EEh right back.
0EDh Turn LED 'lock key' lights on or off. First send 0EDh, then send byte:
-7-6-5-4-3-2-1-0-
| not used|c|n|s|
-----------|-|-|-
| | \-> ScrollLock light 01h=turn on
| \---> NumLock light 02h=turn on
\-----> CapsLock light 04h=turn on
The bit positions 0-3 correspond to bits 4-6 of the keyboard flags
variable in the BIOS Data area. You should make an effort to keep the
flags in sync with the lights. For instance, if you do a big favor for
the user and set his ten-key pad into NumLock mode (by setting bit 5 of
0:0417h) then be sure to turn on the corresponding LED (eg, bit 1).