The following code reads port 0x21 (interrupt mask register) ofPIC (Programmable

Question

The following code reads port 0x21 (interrupt mask register) ofPIC (Programmable Interrupt Controller), sets the 2nd bit that ismapped to IRQ1 and writes it back to port 0x21. As a resultkeyboard interrupt will be disabled as IRQ1 is used bykeyboard.                           

; disable keyboard interrupt in PIC mask register

01        [org 0x0100]

02                  in al, 0x21            ; read interrupt mask register

03                  or al, 2                    ; set bit for IRQ1

04                  out 0x21, al                    ; write back mask register

05                  mov ax, 0x4c00           ; terminate program

06                  int 0x21

Modify instruction line 03 for the following:

             I.      set bit for IRQ0

          II.      set bit for IRQ6

        III.      set bits for IRQ1, IRQ3 andIRQ4

       IV.      clear bit for IRQ5

          V.      clear bits for IRQ2, IRQ5, IRQ6and IRQ7

Explanation / Answer

Question 1: 1. Set bit to disable IRQ0                               IN AL, 21H                   ; Read interruptmaskregister
                              OR AL , 1                      ; set bit 1                               OUT 20H , AL               ; modify the interrupt mask register 2. Set bit to disable IRQ6                                                IN AL, 21H                   ; Read interruptmaskregister
                               OR AL , 10000000B      ; set bit 6                                OUT 21H , AL               ; modify the interrupt maskregister 2. Set bit to disable IRQ6                                                IN AL, 21H                   ; Read interruptmaskregister
                               OR AL , 10000000B      ; set bit 6                                OUT 21H , AL               ; modify the interrupt maskregister 2. Set bit to disable IRQ6                                                IN AL, 21H                   ; Read interruptmaskregister
                               OR AL , 10000000B      ; set bit 6                                OUT 21H , AL               ; modify the interrupt maskregister                                 4.   Set bit to disableIRQ5                                    IN AL, 21H                   ; Read interruptmaskregister
                              OR AL , 5                      ; set bit 1                               OUT 21H , AL               ; modify the interrupt maskregister Question 2: Inorder to hook any interrupt we need to disable remaininginterrupts.But comming to interrupt 80h as it is a defaultinterrupt handler has the irritating behavior of switching the RealTime Clock interrupt off after a time-out expires i.e doesn't needoffset in the interrupt vector table . Hope this will helpyou..

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