Chapter Twenty (Part 2)
|Table of Content||
Chapter Twenty (Part 4)
THE PC KEYBOARD (Part 3)
|20.3 - The
Keyboard DOS Interface
20.4 - The Keyboard BIOS Interface
|20.3 The Keyboard DOS Interface|
MS-DOS provides several calls to read characters from the keyboard. The primary thing to note about the DOS calls is that they only return a single byte. This means that you lose the scan code information the keyboard interrupt service routine saves in the type ahead buffer.
If you press a key that has an extended code rather than an ASCII code MS-DOS returns two keycodes. On the first call MS-DOS returns a zero value. This tells you that you must call the get character routine again. The code MS-DOS returns on the second call is the extended key code.
Note that the Standard Library routines call MS-DOS to read
characters from the keyboard. Therefore
the Standard Library
also returns extended keycodes in this manner. The
routines throw away any non-ASCII keystrokes since it would not be a good thing to insert
zero bytes into the middle of a zero terminated string.
Although MS-DOS provides a reasonable set of routines to read ASCII and extended character codes from the keyboard the PC's BIOS provides much better keyboard input facilities. Furthermore there are lots of interesting keyboard related variables in the BIOS data area you can poke around at. In general if you do not need the I/O redirection facilities provided by MS-DOS reading your keyboard input using BIOS functions provides much more flexibility.
To call the MS-DOS BIOS keyboard services you use the int 16h instruction. The BIOS provides the following keyboard functions:
||Read character. Reads next available character from the system's type ahead buffer. Wait for a keystroke if the buffer is empty.|
|1||-||ZF- Set if no key.
ZF- Clear if key available.
|Checks to see if a character is
available in the type ahead buffer. Sets the zero flag if not key is available
zero flag if a key is available. If there is an available key
this function returns the
ASCII and scan code value in
|2||-||al- shift flags||Returns the current status of the shift flags in al. The shift flags are defined as follows: ? bit 7: Insert toggle bit 6: Capslock toggle bit 5: Numlock toggle bit 4: Scroll lock toggle bit 3: Alt key is down bit 2: Ctrl key is down bit 1: Left shift key is down bit 0: Right shift key is down|
auto repeat rate. The
keycode in buffer. This function stores the value in the
||Read extended character. Like
|11h||-||ZF- Set if no key. ZF- Clear if key available.
||Like the ah=01h call except this one does not throw away keycodes that are not PC/XT compatible (i.e. the extra keys found on the 101 key keyboard).|
|-||al- shift flags ah- extended shift flags||Returns the current status of the shift flags in ax. The shift flags are defined as follows: ? bit 15: SysReq key pressed bit 14: Capslock key currently down bit 13: Numlock key currently down bit 12: Scroll lock key currently down bit 11: Right alt key is down bit 10:Right ctrl key is down bit 9: Left alt key is down bit 8: Left ctrl key is down bit 7: Insert toggle bit 6: Capslock toggle bit 5: Numlock toggle bit 4: Scroll lock toggle bit 3: Either alt key is down (some machines left only) bit 2: Either ctrl key is down bit 1: Left shift key is down bit 0: Right shift key is down|
Note that many of these functions are not supported in every BIOS that was ever written. In fact only the first three functions were available in the original PC. However since the AT came along most BIOSes have supported at least the functions above. Many BIOS provide extra functions and there are many TSR applications you can buy that extend this list even farther. The following assembly code demonstrates how to write an int 16h TSR that provides all the functions above. You can easily extend this if you desire.
; INT16.ASM ; ; A short passive TSR that replaces the BIOS' int 16h handler. ; This routine demonstrates the function of each of the int 16h ; functions that a standard BIOS would provide. ; ; Note that this code does not patch into int 2Fh (multiplex interrupt) ; nor can you remove this code from memory except by rebooting. ; If you want to be able to do these two things (as well as check for ; a previous installation) see the chapter on resident programs. Such ; code was omitted from this program because of length constraints. ; ; ; cseg and EndResident must occur before the standard library segments! cseg segment para public 'code' cseg ends ; Marker segment to find the end of the resident section. EndResident segment para public 'Resident' EndResident ends .xlist include stdlib.a includelib stdlib.lib .list byp equ <byte ptr> cseg segment para public 'code' assume cs:cseg ds:cseg OldInt16 dword ? ; BIOS variables: KbdFlags1 equ <ds:[17h]> KbdFlags2 equ <ds:[18h]> AltKpd equ <ds:[19h]> HeadPtr equ <ds:[1ah]> TailPtr equ <ds:[1ch]> Buffer equ 1eh EndBuf equ 3eh KbdFlags3 equ <ds:[96h]> KbdFlags4 equ <ds:[97h]> incptr macro which local NoWrap add bx 2 cmp bx EndBuf jb NoWrap mov bx Buffer NoWrap: mov which bx endm ; MyInt16- This routine processes the int 16h function requests. ; ; AH Description ; -- ------------------------------------------------ ; 00h Get a key from the keyboard return code in AX. ; 01h Test for available key ZF=1 if none ZF=0 and ; AX contains next key code if key available. ; 02h Get shift status. Returns shift key status in AL. ; 03h Set Autorepeat rate. BH=0 1 2 3 (delay time in ; quarter seconds) BL=0..1Fh for 30 char/sec to ; 2 char/sec repeat rate. ; 05h Store scan code (in CX) in the type ahead buffer. ; 10h Get a key (same as 00h in this implementation). ; 11h Test for key (same as 01h). ; 12h Get extended key status. Returns status in AX. MyInt16 proc far test ah 0EFh ;Check for 0h and 10h je GetKey cmp ah 2 ;Check for 01h and 02h jb TestKey je GetStatus cmp ah 3 ;Check for AutoRpt function. je SetAutoRpt cmp ah 5 ;Check for StoreKey function. je StoreKey cmp ah 11h ;Extended test key opcode. je TestKey cmp ah 12h ;Extended status call je ExtStatus ; Well it's a function we don't know about so just return to the caller. iret ; If the user specified ah=0 or ah=10h come down here (we will not ; differentiate between extended and original PC getc calls). GetKey: mov ah 11h int 16h ;See if key is available. je GetKey ;Wait for keystroke. push ds push bx mov ax 40h mov ds ax cli ;Critical region! Ints off. mov bx HeadPtr ;Ptr to next character. mov ax [bx] ;Get the character. incptr HeadPtr ;Bump up HeadPtr pop bx pop ds iret ;Restores interrupt flag. ; TestKey- Checks to see if a key is available in the keyboard buffer. ; We need to turn interrupts on here (so the kbd ISR can ; place a character in the buffer if one is pending). ; Generally you would want to save the interrupt flag here. ; But BIOS always forces interrupts on so there may be some ; programs out there that depend on this so we won't "fix" ; this problem. ; ; Returns key status in ZF and AX. If ZF=1 then no key is ; available and the value in AX is indeterminate. If ZF=0 ; then a key is available and AX contains the scan/ASCII ; code of the next available key. This call does not remove ; the next character from the input buffer. TestKey: sti ;Turn on the interrupts. push ds push bx mov ax 40h mov ds ax cli ;Critical region ints off! mov bx HeadPtr mov ax [bx] ;BIOS returns avail keycode. cmp bx TailPtr ;ZF=1 if empty buffer pop bx pop ds sti ;Inst back on. retf 2 ;Pop flags (ZF is important!) ; The GetStatus call simply returns the KbdFlags1 variable in AL. GetStatus: push ds mov ax 40h mov ds ax mov al KbdFlags1 ;Just return Std Status. pop ds iret ; StoreKey- Inserts the value in CX into the type ahead buffer. StoreKey: push ds push bx mov ax 40h mov ds ax cli ;Ints off critical region. mov bx TailPtr ;Address where we can put push bx ; next key code. mov [bx] cx ;Store the key code away. incptr TailPtr ;Move on to next entry in buf. cmp bx HeadPtr ;Data overrun? jne StoreOkay ;If not jump if so pop TailPtr ; ignore key entry. sub sp 2 ;So stack matches alt path. StoreOkay: add sp 2 ;Remove junk data from stk. pop bx pop ds iret ;Restores interrupts. ; ExtStatus- Retrieve the extended keyboard status and return it in ; AH also returns the standard keyboard status in AL. ExtStatus: push ds mov ax 40h mov ds ax mov ah KbdFlags2 and ah 7Fh ;Clear final sysreq field. test ah 100b ;Test cur sysreq bit. je NoSysReq ;Skip if it's zero. or ah 80h ;Set final sysreq bit. NoSysReq: and ah 0F0h ;Clear alt/ctrl bits. mov al KbdFlags3 and al 1100b ;Grab rt alt/ctrl bits. or ah al ;Merge into AH. mov al KbdFlags2 and al 11b ;Grab left alt/ctrl bits. or ah al ;Merge into AH. mov al KbdFlags1 ;AL contains normal flags. pop ds iret ; SetAutoRpt- Sets the autorepeat rate. On entry bh=0 1 2 or 3 (delay ; in 1/4 sec before autorepeat starts) and bl=0..1Fh (repeat ; rate about 2:1 to 30:1 (chars:sec). SetAutoRpt: push cx push bx mov al 0ADh ;Disable kbd for now. call SetCmd and bh 11b ;Force into proper range. mov cl 5 shl bh cl ;Move to final position. and bl 1Fh ;Force into proper range. or bh bl ;8042 command data byte. mov al 0F3h ;8042 set repeat rate cmd. call SendCmd ;Send the command to 8042. mov al bh ;Get parameter byte call SendCmd ;Send parameter to the 8042. mov al 0AEh ;Reenable keyboard. call SetCmd mov al 0F4h ;Restart kbd scanning. call SendCmd pop bx pop cx iret MyInt16 endp ; SetCmd- Sends the command byte in the AL register to the 8042 ; keyboard microcontroller chip (command register at ; port 64h). SetCmd proc near push cx push ax ;Save command value. cli ;Critical region no ints now. ; Wait until the 8042 is done processing the current command. xor cx cx ;Allow 65 536 times thru loop. Wait4Empty: in al 64h ;Read keyboard status register. test al 10b ;Input buffer full? loopnz Wait4Empty ;If so wait until empty. ; Okay send the command to the 8042: pop ax ;Retrieve command. out 64h al sti ;Okay ints can happen again. pop cx ret SetCmd endp ; SendCmd- The following routine sends a command or data byte to the ; keyboard data port (port 60h). SendCmd proc near push ds push bx push cx mov cx 40h mov ds cx mov bx ax ;Save data byte mov bh 3 ;Retry cnt. RetryLp: cli ;Disable ints while accessing HW. ; Clear the Error Acknowledge received and resend received flags ; in KbdFlags4 and byte ptr KbdFlags4 4fh ; Wait until the 8042 is done processing the current command. xor cx cx ;Allow 65 536 times thru loop. Wait4Empty: in al 64h ;Read keyboard status register. test al 10b ;Input buffer full? loopnz Wait4Empty ;If so wait until empty. ; Okay send the data to port 60h mov al bl out 60h al sti ;Allow interrupts now. ; Wait for the arrival of an acknowledgement from the keyboard ISR: xor cx cx ;Wait a long time if need be. Wait4Ack: test byp KbdFlags4 10 ;Acknowledge received bit. jnz GotAck loop Wait4Ack dec bh ;Do a retry on this guy. jne RetryLp ; If the operation failed after 3 retries set the error bit and quit. or byp KbdFlags4 80h ;Set error bit. GotAck: pop cx pop bx pop ds ret SendCmd endp Main proc mov ax cseg mov ds ax print byte "INT 16h Replacement" cr lf byte "Installing...." cr lf 0 ; Patch into the INT 9 and INT 16 interrupt vectors. Note that the ; statements above have made cseg the current data segment ; so we can store the old INT 9 and INT 16 values directly into ; the OldInt9 and OldInt16 variables. cli ;Turn off interrupts! mov ax 0 mov es ax mov ax es:[16h*4] mov word ptr OldInt16 ax mov ax es:[16h*4 + 2] mov word ptr OldInt16+2 ax mov es:[16h*4] offset MyInt16 mov es:[16h*4+2] cs sti ;Okay ints back on. ; We're hooked up the only thing that remains is to terminate and ; stay resident. print byte "Installed." cr lf 0 mov ah 62h ;Get this program's PSP int 21h ; value. mov dx EndResident ;Compute size of program. sub dx bx mov ax 3100h ;DOS TSR command. int 21h Main endp cseg ends sseg segment para stack 'stack' stk db 1024 dup ("stack ") sseg ends zzzzzzseg segment para public 'zzzzzz' LastBytes db 16 dup (?) zzzzzzseg ends end Main
Chapter Twenty: The PC Keyboard (Part
29 SEP 1996