Interrupts
==========
When a hardware component (ex: a peripheral device) needs CPU attention, 
the controller associated with this component:
   1. sends Interrupt Request (INTR) signal to the CPU
   2. puts Interrupt Number (0 to FFh) on data bus
   
CPU uses interrupt number to index into interrupt vector table (IVT) 
   Each entry of this table is called an interrupt vector
   Each entry contains address of Interrupt Handler (ISR) 
   CPU transfers control to the corresponding ISR
   Interrupt returns with an IRET

   Allows the program to "call" 
      1. the operating system
      2. the BIOS
      
   The program uses functionality already present to accomplish tasks that may
   be quite complex.
   

   Example:
   INT 21h / AH=2 - write character to standard output.
   entry: DL = character to write, after execution AL = DL.

      ; print character 'a' 
         mov ah, 2
         mov dl, 'a'
         int 21h
         
      ; print carrige return and new line: 
         mov ah, 2      
         mov dl, 0Dh
         int 21h
         mov dl, 0Ah
         int 21h

      ; prints a byte in binary         
            mov cx, 8
      p1:   mov ah, 2    
            mov dl, '0'
            test bl, 10000000b ;test leftmost bit - "and" & set flag
            jz zero
            mov dl, '1'
      zero: int 21h  ;print function.
            shl bl, 1
      loop p1    
      

      ; wait 5 seconds (5 million microseconds)
            mov     cx, 4Ch    ; 004C4B40h = 5,000,000
            mov     dx, 4B40h
            mov     ah, 86h
            int     15h      


   about interrupts

   functional list of interrupts

   ONLINE emu

   emu: list of interrupts