The document discusses interrupt processing and how it allows a microprocessor to execute other software while waiting for input from an I/O device like a keyboard. It then summarizes various INT instructions used for keyboard input/output like INT 21h and INT 16h, providing details on specific functions accessed through different register values.

1. INTERRUPT OPERATIONS
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2.  Interrupts are particularly useful when interfacing I/O devices that provide or
require data at relatively low data transfer rates .
 If the person using the keyboard typed one character per second. The software of
the Processors waited an entire second between each key stroke for the person to
type another key .
 This process was such a tremendous waste of time that designers developed another
process, Interrupt processing to handle this situation.
 Interrupt processing allows the microprocessor to execute other software
while the keyboard operator is thinking about what key to type next.
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3. DOS and BI OS
I nt errupt s
DOS and BIOS interrupts are used to perform some very
useful functions, such as displaying data to the monitor,
reading data from keyboard, etc.
They are used by identifying the interrupt option type,
which is the value stored in register AH and providing,
whatever extra information that the specific option requires.
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4. I NT 21h
I ns t ruct i on
Hence this INT instructions used for Keyboard.(INT
21 h-DOS Interrupt)
The int 21 h instructions are used to interrupt through
the Keyboard, Files, Directives.
Depending on the value that is moved to the ‘ah’ the
operations will be done.
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5. INT 21h / AH=1 - read character from standard input, with echo,
result is stored in AL.
if there is no character in the keyboard buffer, the function waits until any key is
pressed.
example:
mov ah, 1
int 21h
INT 21h / AH=7 - character input without echo to AL.
example: mov ah, 7
int 21h
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6. INT 21h / AH=0Ah - input of a string to DS:DX,
Example:
mov ax,@data
mov ds,ax
mov ah,0Ah
int 21h
INT 21h/AH=0Bh - checks keyboard status.returns FFH to AL if character is
available.Otherwise returns 00H to AL.
INT 21h/0Ch- this clear keyboard buffer and invoke function. Loads required
function in AL.
mov ah,0ch //request keyboard function
mov al,function //required function
int 21h03/17/18 6

7. INT 21h / 6h :direct console input or output.
INT 21h/0Eh :Select default drive
INT 21h/19h :Get current default drive
INT 21h/25h :Set interrupt vector
INT 21h/35h :Get interrupt Vector
INT 21h/39h :Make directory
INT 21h/3Ah :Remove directory
INT 21h/3Bh :Set current directory
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8. INT 21h/3Ch;create or truncate file
INT 21h/3Dh;Open Existing file
INT 21h/3Eh;Close file
INT 21h/3Fh;Read from File
INT 21h/40h;Write to file
INT 21h/41h;Delete File
INT 21h/42h;Set current File Position
INT 21h/47h;Get current Directory
INT 21h/4Ch;return control to the operating system(STOP
PROGRAM)
INT 21h/56h; rename file/move file03/17/18 8

9. INT 16h is the basic BIOS keyboard operation
used extensively by software developers and provides the
following services according to a function code that you
load in AH.
INT 16h/03h: set typematic Repeat rate
INT 16h/05h: Keyboard write.
 INT 16h/10h: Read keyboard Character
INT 16h/11h: Determine whether character is present or not
I NT 16h I ns t ruct i on
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10. 03/17/18 10
INT 16h/03h : set typematic Repeat rate
MOV AH,03H ; SETS TYPEMATIC REPEAT RATE
MOV AL,05H ;REQUIRED SUBFUNCTION
MOV BH,REPEAT –DELAY ;DELAY BEFOR E START(0=1/4 SEC.
,1=1/2 SEC- DEFAULT. ,2=3/4 SEC. ,3=1
SEC.)
MOV BL,REPEAT- RATE SPEED OF REPEATION (0– FASTEST
THROUGH 31-- SLOWEST)
INT 16h/05h: Keyboard write
Loading ASCII character to CH and its scan code to CL.
Allows to enter characters to the buffer until it is full.
If full, operation sets Carry flag and AL to 1

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INT 16h/10h: Read keyboard Character
If a character is pressed ,then it delivered to the AL and its scan code to AH.
If a extended key scan code is delivered to AH and the value 00H to AL
If a extended duplicate key scan code is delivered to AH and the value E0H
to AL
MOV AH,10H ;REQUEST FOR BIOS KEYBOARD INPUT.
INT 16H ;CALLING INTERRUPT SERVICE
CMP AL,00H ; EXTENDED FUNCTION KEY ??
JE ;YES.EXIT
CMP AL,E0H ; EXTENDED FUNCTION KEY ??
JE ;YES.EXIT

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INT 16h/11h: Determine whether character is present or not
If the character is present in the buffer , clears ZERO flag and delivered it to
AL and its scan code to AH and still character is remained in the buffer.
If the character is not present in the buffer , sets ZERO flag.
INT 16h/12h: Return Keyboard Status.
Delivers keyboard status byte from BIOS DATA AREA 1 at location 40 :
17H to AL & byte from 40 : 18 H to AH.
Key pressed Status Bit
Caps lock 6
Num lock 5
Scroll lock 4
Right ALT 3
Right CTRL 2
Left ALT 1
Left CTRL 0

13. How t he Keyboard
WorksKeyboard controller chip sends an 8-
bit scan code to the keyboard serial
input port
Interrupt triggered, INT 9h routine
executes
Scan code and ASCII code inserted
into keyboard typeahead buffer
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