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

Question

: 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      ; terminate program

06                   int 0x21

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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