a) : The following code reads port0x21 (interrupt mask register) of PIC (Program

Question

a)

: The following code reads port0x21 (interrupt mask register) of PIC (Programmable InterruptController), sets the 2nd bit that is mapped to IRQ1 and writes itback to port 0x21. As a result keyboard interrupt will be disabledas 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          ; terminateprogram

06                   int 0x21

Modify instruction_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

b)-

We should disable interrupts while hooking interrupt 8h, i.e.while placing its segment and offset in the interrupt vectortable.

b)-ii

We need not do this for interrupt 80h.

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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