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.