The following code reads port 0x21 (interrupt mask register) ofPIC (Programmable
ID: 3612242 • Letter: T
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. [10 marks]
; 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 forIRQ6
III. set bits forIRQ1, IRQ3 and IRQ4
IV. clear bit forIRQ5
V. clear bits for IRQ2, IRQ5, IRQ6and IRQ7
Explanation / Answer
1. Set bit to disable IRQ0
IN AL, 21H ; Read interrupt mask register
OR AL , 1 ; set bit 1
OUT 20H, AL ; modify the interrupt mask register
2. Set bit to disable IRQ6
IN AL, 21H ; Read interrupt mask register
OR AL , 10000000B ; set bit 6
OUT 21H, AL ; modify the interrupt mask register
4. Set bit to disableIRQ5
IN AL, 21H ; Read interrupt mask register
OR AL , 5 ; set bit 1
OUT 21H, AL ; modify the interrupt mask register
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