The document discusses various keyboard and video interrupts used in 8086 processors. It explains that interrupts allow efficient processing of I/O without wasting CPU time waiting. INT 21h is used for keyboard and file operations depending on the value in register AH. INT 16h provides keyboard services like reading keys. INT 10h controls video modes and can set pixels, write text, and get display information. Example programs demonstrate using interrupts to get input, output text, and retrieve the date.

1. KEYBOARD INTERRUPTs
8086
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2. The Purpose of Interrupts
 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 BIOS Interrupts
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 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.

4. Introduction
SOFTWARE INTERRUPT:
 The 8086 INT instruction can be used to cause the 8086 to do any one of 256 possible
interrupt types.
 The desired type is specified as part of the instruction.
 Software interrupts produced by the INT instructions have many uses.
 For example: INT 3 instruction to insert breakpoints in programs for debugging.
 Another use of Software interrupt INT is to test various interrupts-service procedures.
 For example: INT 0 instruction to send execution to a divide-by-zero interrupt –
service procedure without having to run the actual division program.
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5. INT Instruction
The term type in the instruction format refers to a number between 0 and
255 which identifies the interrupt. When an 8086 executes an INT instruction.
It will:
1. Decrement the stack pointer by 2 and push the flags onto the stack.
2. Decrement the stack pointer by 2 and push the contents of CS onto the stack.
3. Decrement the stack pointer by 2 and push the offset of the next instruction after the INT
number instruction on the stack.
4. Get a new value for IP from an absolute memory address of 4 times the type specified in
the instruction. For an INT 8 instruction. For example, the new IP will be read from
address 00020H.
5. Get a new value for CS from an absolute memory address of 4 times the type specified in
the instruction plus 2. For an INT 8 instruction, for example, the new value of CS will be
read from address 00022H.
6. Reset both IF and TF. Other flags are not affected.
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6. INT 21h Instruction
 Hence in this section we are going to see about 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.
 The following slides will explain the different operations
performed.
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7. INT 21h Instruction
 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=2 - write character to standard output.
entry: DL = character to write, after execution AL = DL.
example:
mov ah, 2
mov dl, 'a'
int 21h
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8. INT 21h Instruction
 INT 21h / AH=5 - output character to printer.
entry: DL = character to print, after execution AL = DL.
example:
mov ah, 5
mov dl, 'a'
int 21h
 INT 21h / AH=7 - character input without echo to AL.
example:
mov ah, 7
int 21h
 INT 21h / AH=9 - output of a string at DS:DX. String must be terminated by '$'.
example:
msg db "hello world $“
.code
mov ax,@data
mov ds,ax
mov dx,offset msg
mov ah,9
int 21h
ret
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9. INT 21h Instruction
 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=25h - set interrupt vector;
input: AL = interrupt number. DS:DX -> new interrupt handler.
 INT 21h / AH=2Ah - get system date;
return: CX = year (1980-2099). DH = month. DL = day. AL = day of week (00h=Sunday)
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10. Other INT 21h Instruction
 INT 21h / 6h;direct console input or output.
INT 21h/0Bh; Get input Status
INT 21h/0Ch; Flush keyboard buffer and read standard input
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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11. Other INT 21h instruction
 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 file
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12. Example Programs
 INPUT A CHARACTER AND PRINT THE
CHARACTER IN THE OUTPUT
.model small
.stack 100h
.code
mov ax, @data
mov ds, ax
mov ah, 1h ; keyboard input subprogram
int 21h ; read character into al
mov dl, al ; copy character to dl
mov ah, 2h ; character output subprogram
int 21h ; display character in dl
end
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13. Example Programs
 INPUT THE STRING:
.model small
.stack 100h
.code
mov ax, @data
mov ds , ax
mov ah,0Ah
int 21h
end
 PRINT THE STRING IN
THE OUTPUT
.model small
.data
msg db "hello world $"
.stack 100h
.code
mov ax, @data
mov ds, ax
mov dx, offset msg
mov ah,9
int 21h
ret
end
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14. Example Programs
 GET SYSTEM DATE AND
TIME:
 // DATE:
.model small
.stack 100h
.code
mov ax, @data
mov ds, ax
mov ah, 2Ah;
int 21h ;
end
 //TIME:
.model small
.stack 100h
.code
mov ax,@data
mov ds,ax
mov ah, 2Ch;
int 21h ;
end
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15. INT 16h INSTRUCTION
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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
 INT 16h/12h: Return Keyboard Status.

16. INT 10h Instructions
 INT 10h instruction is used for the video mode.
 Even this INT instruction has many operations that
works according to the value moved onto the ‘ah’.
 Option 0H – Sets video mode.
 Registers used:
 AH = 0H
 AL = Video Mode.
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17. INT 10h Instructions
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The following are some of the services offered by INT 10 H.
 INT 10H/00H; set video mode
 INT 10H/01H; set cursor size
 INT 10H/02H; set cursor position
 INT 10H/03H; return cursor status
 INT 10H/05H; select active page
 INT 10H/06H; scroll up screen
 INT 10H/07H; scroll down screen
 INT 10H/0Ah; display character
 INT 10H/0CH; Write pixel dot
 INT 10H/13H; Return video information

18. INT 10h Instructions
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 INT 10h / AH = 09h - write character and attribute at cursor
position.
input:
AL = character to display.
BH = page number.
BL = attribute.
CX = number of times to write character.
 INT 10h / AH = 0Ah - write character only at cursor position.
input:
AL = character to display.
BH = page number.
CX = number of times to write character.

19. INT 10h Instructions
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 INT 10h / AH = 0Ch - change color for a single pixel.
input:
AL = pixel color
CX = column.
DX = row.
example: mov al, 13h mov ah, 0 int 10h ; set graphics video mode.
mov al, 1100b mov cx, 10 mov dx, 20 mov ah, 0ch int 10h ; set
pixel.
 INT 10h / AH = 0Dh - get color of a single pixel.
input:
CX = column.
DX = row.
output:
AL = pixel color

20. INT 10h Instructions
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 INT 10h / AH = 13h - write string.
input:
AL = write mode:
bit 0: update cursor after writing;
bit 1: string contains attributes.
BH = page number.
BL = attribute if string contains only characters (bit 1 of AL is
zero).
CX = number of characters in string (attributes are not
counted).
DL,DH = column, row at which to start writing.
ES:BP points to string to be printed.

21. INT 10h Instructions
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example:
mov al, 1
mov bh, 0
mov bl, 0011_1011b
mov cx, msg1end - offset msg1 ; calculate message size.
mov dl, 10
Mov dh, 7
push cs
pop es
mov bp, offset msg1
mov ah, 13h
int 10h
jmp msg1end
msg1 db " hello, world! “
msg1end:

22. THANK YOU
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