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Interrupt Mask Register

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i am trying to mask the interrupt for the system timer in my pc,
using the 21H adress (Interrupt Mask Register) as adress, and sending the OCW1.

However, on none of the computers i ve tried it did work (AMD K6, Pentium 1, AMD Athlon 1400), although they are all IBM compatible.

The code is:
      .Stack 100h
      .Model Small

      mov ax,@data
      mov ds, ax   

      in al, 21h   
      or al, 01h ; masking timer interrupt  
      out 21h, al           

      mov ah, 4ch  
      int 21h
I have no idea why it doesnt work, as the code depicted is the same as in manuals i read,
yet the system timer interrupt is not maskable.

Help would be very much appreciated.
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