Why so many different interrupts?
   The 8086 family of processors has the ability to recognize 256 
   different interrupts.  They also have the ability to let a 
   program invoke any of these interrupts with a special 
   instruction, known as a software interrupt (as opposed to a 
   hardware interrupt which is signalled from outside the 
   processor).  Software interrupts are treated just like 
   hardware interrupts, except that they are never disabled and 
   do not result in an acknowledgement to other chips in the 
   computer.  The software interrupt instruction on the 8086 
   family is called INT, and is given the number of the 
   interrupt.  Thus an INT 21h instruction invokes interrupt 
   number 33 decimal.

   Other processors also have software interrupts, though they 
   often use different names, such as the Motorola 68000 family 
   TRAP instruction, the Intel 8080 RST (ReSTart) instruction, or
   many mainframes' SVC (SuperVisor Call). 

   Since a program can invoke an interrupt by number rather than 
   by its address (as it has to in calling subroutines), 
   interrupts are a convenient way of providing services without 
   having to recompile a program whenever the address of the code 
   providing the service changes.  This also allows a user 
   program to enhance the services provided by directing the 
   interrupt to itself.  These enhanced services can then be made 
   available to other programs.

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