keyboard interrupt
DESCRIPTION
TRANSCRIPT
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KEYBOARD INTERRUPTs8086
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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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DOS and BIOS Interrupts
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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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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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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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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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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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 $“.codemov ax,@datamov ds,axmov dx,offset msgmov ah,9int 21hret
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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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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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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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Example Programs
INPUT A CHARACTER AND PRINT THE CHARACTER IN THE OUTPUT
.model small
.stack 100h
.codemov ax, @datamov ds, axmov ah, 1h ; keyboard input subprogramint 21h ; read character into almov dl, al ; copy character to dlmov ah, 2h ; character output subprogramint 21h ; display character in dl
end
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Example Programs
INPUT THE STRING:.model small.stack 100h.code
mov ax, @datamov ds , axmov ah,0Ahint 21h
end
PRINT THE STRING IN THE OUTPUT.model small.data
msg db "hello world $".stack 100h.code
mov ax, @datamov ds, axmov dx, offset msgmov ah,9int 21hret
end
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Example Programs
GET SYSTEM DATE AND TIME:
// DATE:.model small.stack 100h.code
mov ax, @datamov ds, axmov ah, 2Ah; int 21h ; end
//TIME:.model small.stack 100h.code
mov ax,@datamov ds,axmov ah, 2Ch; int 21h ;
end
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INT 16h INSTRUCTION
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.
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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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INT 10h Instructions
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
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INT 10h Instructions
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.
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INT 10h Instructions
INT 10h / AH = 0Ch - change color for a single pixel.
input:AL = pixel colorCX = 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
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INT 10h Instructions
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.
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INT 10h Instructions
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:
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THANK YOU