8086 Assembler Tutorial for Beginners (Part 4)


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.

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
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. Generally other registers are used to pass parameters and data to sub-function.

The following example uses INT 10h sub-function 0Eh to type a "Hello!" message. This functions displays a character on the screen, advancing the cursor and scrolling the screen as necessary.

#MAKE_COM#        ; instruct compiler to make COM file.
ORG    100h

; The sub-function that we are using
; does not modify the AH register on
; return, so we may set it only once.

MOV    AH, 0Eh    ; select sub-function.

; INT 10h / 0Eh sub-function
; receives an ASCII code of the
; character that will be printed
; in AL register.

MOV    AL, 'H'    ; ASCII code: 72
INT    10h        ; print it!

MOV    AL, 'e'    ; ASCII code: 101
INT    10h        ; print it!

MOV    AL, 'l'    ; ASCII code: 108
INT    10h        ; print it!

MOV    AL, 'l'    ; ASCII code: 108
INT    10h        ; print it!

MOV    AL, 'o'    ; ASCII code: 111
INT    10h        ; print it!

MOV    AL, '!'    ; ASCII code: 33
INT    10h        ; print it!

RET               ; returns to operating system.

Copy & paste the above program to source code editor, and press [Compile] button. Run it!

See list of basic interrupts for information about other interrupts.

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