Port 3FA - Interrupt identification register (prioritized)
1111 1xxx Forced to 0
xxxx x11x Interrupt ID (see below)
xxxx xxx1 0=Interrupt pending - 1=No interrupts pending
ID Priority Type Source Reset Control
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00 Lowest Modem Status Clear to Send Read Modem
Data Set Ready Status
Ring indicator Register
Carrier Detect
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01 Low Transmitter Holding Ready to send next -
Register Empty character
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10 High Received Data Something was Read Receive
Available received Buffer Register
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11 Highest Receiver Line status Overrun Error Read Line
Parity Error Status Register
Framing Error
Break Interrupt