Input output instructions
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Input & Output Instructions. CPU communicates with the peripherals through I/O registers called I/O ports . There are 2 instructions, IN & OUT , that access the ports directly. These instructions are used when fast I/O is essential ……. in a game program. IN & OUT.

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Input output instructions

Input & Output Instructions

  • CPU communicates with the peripherals through I/O registers called I/O ports.

  • There are 2 instructions, IN & OUT, that access the ports directly.

  • These instructions are used when fast I/O is essential ……. in a game program.

CAP241


In out

IN & OUT

  • Most application programs do not use IN and OUT instructions.

  • Why?

    1) port addresses vary among computer models

    2) easier to program I/O with service routines

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2 categories of i o service routine

2 categories of I/O service routine

  • The BIOS routines.

  • The DOS routines.

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Bios routines

BIOS routines

  • Are stored in ROM and interact directly with I/O ports.

  • Used to carry basic screen operations such as moving the cursor & scrolling the screen.

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Dos routines

DOS routines

  • Can carry out more complex tasks.

  • Printing a character string…. They use the BIOS routines to perform direct I/O operations.

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The int instruction

The INT Instruction.

  • To invoke a DOS or BIOS routine , the INT (interrupt) instruction is used.

  • FORMAT

    INTinterrupt_number

is a number that specifies a routine.

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Example

Example

  • INT16h

    invokes a BIOS routine that performs keyboard input.

  • We will use a particular DOS routine

    INT 21h

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Int 21h

INT 21h

  • Used to invoke a large number of DOS functions.

  • Put the function number in AH register and then invoke INT21h

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Functions

FUNCTIONS

Function numberRoutines

1single-key input

2single-character output

9 character string output

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Int 21h functions

INT 21h functions

  • Input values are to be in certain registers & return output values in other registers.

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Function 1

Function 1

  • Single-Key Input

    Input : AH = 1

    Output :AL = ASCII code if character key is pressed.

    = o if non-character is pressed.

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Example1

Example

MOVAH,1 ; input key function

INT21h ; ASCII code in AL

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Example2

Example

  • If character k is pressed, AL gets its ASCII code; the character is also displayed on the screen

  • IfArrow key or F1-F10, ALwill contain 0

  • The instructions following the INT21h can examine AL and take appropriate action.

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Function 2

Function 2

  • INT21h, function 1 …. doesn’t prompt the user for input, he might not know whether the computer is waiting for input or it is occupied by some computation.

  • Function 2 can be used to prompt the user

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Function 21

Function 2

  • Display a character or execute a control function

    Input : AH = 2

    DL = ASCII code for the display character or control character

    Output :AL = ASCII code of the display character

    or control character

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Example3

Example

MOVAH,2; display character function

MOVDL, ‘?’; character is ‘?’

INT21h; display character

  • After the character is displayed, the cursor advances to the next position on the line.

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Control functions

Control functions

  • Function 2 may also be used to perform control functions.

  • If DL contains the ASCII code of a control character, INT21h causes the control function to be performed.

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Control functions1

Control functions

ASCII code (hex) SymbolFunction

7 BELbeep

8BSbackspace

9HTtab

ALFline feed (new line)

DCRcarriage return

(start of current line)

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A first program

A First Program

Read a character and display it at the beginning of the next line

1-We start by displaying a question mark:

MOV AH,2; display character function

MOV DL,'?‘ ; character is ‘?’

INT 21H ; display character

Move 3Fh, the ASCII code for “?” , into DL

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Read a character

Read a character

MOV AH,1; read character function

INT 21H; character in AL

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Display the character on next line

Display the character on next line

  • First , the character must be saved in another register.

    MOVBL , AL; save it in BL

  • This because the INT 21h , function 2 , changes AL

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Display the character on next line1

Display the character on next line

-Move cursor to the beginning of the next line:

  • Execute carriage return & line feed.

  • Put their ASCII codes in DL & execute INT 21h.

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Move cursor to the beginning of the next line

Move cursor to the beginning of the next line

MOVAH , 2 ; display character function

MOVDL , 0Dh ; carriage return

INT 21h ; execute carriage return

MOVDL , 0Ah ; line feed

INT21h ; execute line feed

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Display the character

Display the character

MOVDL , BL; get character

INT 21h;and display it

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Program listing

Program Listing

TITLE PGM4_1 : Echo PROGRAM

.MODEL SMALL

.STACK 100H

.CODE

MAIN PROC

;display prompt

MOV AH,2

MOV DL,'?'

INT 21H

; input a character

MOV AH,1

INT 21H

MOV BL,AL

; go to a new line

MOV AH,2

MOV DL,0DH

INT 21H

MOV DL,0AH

INT 21H

; display characters

MOV DL,BL

INT 21H

; return to DOS

MOV AH,4CH

INT 21H

MAIN ENDP

END MAIN

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When a program terminates it should return control to dos

When a program terminates, it should return control to DOS

MOV AH,4CH; DOS exit function

INT 21H; exit to DOS

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Displaying a string

Displaying a string

INT 21h , Function 9:

Display a string

Input :DX = offset address of string.

The string must end with a ‘$’ character.

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Input output instructions

  • If the string contains the ASCII code of a control character , the control function is performed.

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Example4

Example

  • Print HELLO! on the screen.

    MSGDB ‘HELLO!$’

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The lea instruction

The LEA instruction

  • INT 21h, function 9, expects the offset address of the character string to be in DX.

  • To get it there, we use

    LEAdestination , source

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Lea destination source

LEAdestination , source

  • LEA ….. Load Effective Address

  • Destination … is a general register.

  • Source ………… is a memory location.

  • It puts a copy of the source offset address into destination.

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Example5

Example

MSGDB ‘HELLO!$’

LEADX , MSG; puts the offset ;address of variable ; MSG in DX

  • This example contains data segments initialize DS.

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P rogram s egment p refix

Program Segment Prefix

  • When a program is loaded in memory, DOS prefaces it with PSP. The PSP contains information about program.

  • DOS places in DS & ES segment # of PSP.

  • DS must be loaded with the segment # of data segment

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Ds initialization

DS initialization

  • A program containing a data segment begins with:

    MOVAX,@DATA

    MOVDS,AX

  • @Data is the name of the data segment defined by .DATA. It is translated into a segment #.

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Print the message

Print the message

  • With DS initialized, we may print the “HELLO!” message:

    LEADX,MSG;get message

    MOVAH,9;display string function

    INT21h;display string

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Input output instructions

Sample Program

directive giving title for printed listings

program title (comment)

  • TITLE PGM4-2: PRINT STRING PROGRAM         

  • ; This program displays “Hello!”

  • .MODEL SMALL

  • .STACK 100H

comment line

memory model: small programs use at most 64K code and 64K data

set the stack size

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Sample program

Sample Program

starts the data segment where variables are stored

carriage return and line feed

reserve room for some bytes

  • .DATA

  • MSG DB “HELLO!”,0DH,0AH,’$’

  • .CODE

  • MAIN PROC

  •     MOV  AX,@DATA

  •     MOV  DS,AX;initialize DS

  • LEA DX,MSG;get message

  •     MOV  AH,9;display string function

  •     INT  21H;display message

  •     MOV  AH,4CH

  •     INT  21H;DOS exit

  • MAIN ENDP

  • END MAIN

variable name

starts the code segment

Declares the beginning of the procedure which is called main

marks the end of the current procedure

marks the end of the program. “main” specifies the program execution is to begin with the procedure “main”

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Input output instructions

  • Sample execution:

    A> PGM4_2

    HELLO!

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Case conversion program

Case Conversion Program

ENTER A LOWER CASE LETTER : a

IN UPPER CASE IT IS : A

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Case conversion program1

Case Conversion Program

  • Use EQU to define CR & LF as names for the constants 0DH & 0AH.

    CREQU0DH

    LFEQU0AH

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Input output instructions

The messages and input character can be stored in the Data Segment like this:

MSG1DB'ENTER A LOWER CASE LETTER : $‘

MSG2DBCR,LF , 'IN UPPER CASE IT IS : ‘

CHARDB?,'$'

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Input output instructions

Our program begins by displaying the first message and reading the character:

LEADX,MSG1 ; get first message

MOV AH,9 ; display string function

INT21H ;display first message

MOV AH,1 ; read character function

INT21H ; read a small letter into AL

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Convert to upper case

Convert to upper case

SUB AL,20H ; convert into uppercase

MOV CHAR,AL ; and store it

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Display second message uppercase

Display second message & uppercase

LEADX,MSG2 ; get second message

MOVAH,9; display string function

INT 21H ;display message & uppercase letter

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Input output instructions

Program Listing

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Input output instructions

.MODEL SMALL

.STACK 100H

.DATA

CREQU0DH

LFEQU0AH

MSG1DB 'ENTER A LOWER CASE LETTER : $'

MSG2DB CR,LF,'IN UPPER CASE IT IS : '

CHARDB ?,'$'

.CODE

MAIN PROC

; initialize DS

MOV AX,@DATA

MOV DS,AX

;print user prompt

LEA DX,MSG1

MOV AH,9

INT 21H

; input a character and convert to upper case

MOV AH,1

INT 21H

SUB AL,20H

MOV CHAR,AL

; display on the next line

LEA DX,MSG2

MOV AH,9

INT 21H

; return TO DOS

MOV AH,4CH

INT 21H

MAIN ENDP

END MAIN

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