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Port 020 - 8259A Command Port
Send 20h here to signal end-of-interrupt (EOI).
1xxx xxxx Rotate Mode
x1xx xxxx Set Interrupt Level
xx1x xxxx End of Interrupt Mode
xxx1 1xxx Reserved - must be set to 0
xxxx x111 Interrupt level (when bit 6=1)
Bits 7-5: These bits define the rotate mode, end of interrupt
mode, or a combination of the two.
001 Nonspecific End of Interrupt Command
010 No operation
011 Specific End of Interrupt Command
(bits 2-0 select interrupt to end).
101 Rotate on nonspecific End of Interrupt Command
110 Set prioriry Command
(bits 2-0 select interrupt to act on).
111 Rotate on specific end of interrupt command
See Also: 8259A 021
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