(Part 4)
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.
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