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Computer Organization & Assembly Language

Computer Organization & Assembly Language. University of Sargodha, Lahore Campus Prepared by Ali Saeed. Interrupts. Interrupts can be seen as a number of functions. These functions make the programming much easier

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Computer Organization & Assembly Language

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  1. Computer Organization & Assembly Language University of Sargodha, Lahore Campus Prepared by Ali Saeed

  2. Interrupts • Interrupts can be seen as a number of functions. • These functions make the programming much easier • Instead of writing a code to print a character you can simply call the interrupt and it will do everything for you. • There are also interrupt functions that work with disk drive and other hardware. • We call such functions software interrupts.

  3. Interrupts • where value can be a number between 0 to 255 (or 0 to 0FFh), generally we will use hexadecimal numbers. • You may think that there are only 256 functions, but that is not correct. Each interrupt may have sub-functions. • To specify a sub-function AH register should be set before calling interrupt. • Each interrupt may have up to 256 sub-functions (so we get 256 * 256 = 65536 functions). • In general AH register is used, but sometimes other registers maybe in use.

  4. Interrupts • Interrupts are also triggered by different hardware, these are called hardware interrupts. • Currently we are interested in software interrupts only. • To make a software interrupt there is an INT instruction • it has very simple syntax: • INT value

  5. INT 21H • INT 21H may be used to invoke a large number of DOS functions • A particular number of interrupt in AH register

  6. INT 21H • INT 21H function expect input value in a certain register and return output in a certain register e.g. • MOV Ah,1 • INT 21H • Take a character as input and put the ASCII in AL register • It will put 0 in AL if non character key is pressed

  7. INT 21H • display the character ’a’ on the screen: • mov dl, ‘a‘ ; dl = ‘a‘ • mov ah, 2h ; character output subprogram • int 21h ; call ms-dos output character • Student Exercise: Write a code that takes one character input from user, and display 2 times on screen.

  8. INT 21H • String Function • AH contain 9 • Inputs: DX= offset address of string • The String must end with $ character • E.g • Msg DB ‘Hello!$’ • LEA DX, Msg • Mov AH,9 • INT 21H • Output: Hello!

  9. LEA Instruction • Load Effective Address • LEA Destination, Source • It get offset address of source and put it into destination register • E.g • LEA DX, Msg • Put offset of Msg in DX register

  10. Control Function • DL contain ASCII code of control character • INT 21H cause control function to be performed

  11. Student Assignment • Multiplication of 3x3 Matrics • Dead Line Next Week 1st Class

  12. Programming Steps

  13. Thanks

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