Specifications

Serial Communications (16C452)
Interrupt ID Register
(2FAh, 3FAh; R)
The Interrupt Identification register (IIR) stores an identification code
or "ID" of pending interrupts. In order to provide minimum software
overhead during data character transfers, the serial hardware
prioritizes interrupts into four levels: Receiver Line Status (priority
1), Received Data Ready (priority 2), Transmitter Holding Register
Empty (priority 3), and Modem Status (priority 4). Refer to Table 8-5
below.
Table 8-5
16C452 Interrupt Control Functions.
Interrupt Identification
Register
Interrupt Set and Reset Functions
Bit 2 Bit 1 Bit 0
Priority
Level
Interrupt
Flag
None
Interrupt
Reset Control
1
1
0 Highest
Receiver
Line Status
Overrun Error or
Parity Error or
Framing Error or
Break Interrupt
Reading the
Line Status Register
1
0
0 Second
Received
Data Available
Receiver
Data Available
Reading The
Receiver Buffer
Register
0
1
0
Third
Transmitter
Holding Register
Empty
Transmitter
Holding Register
Empty
Reading the IIR
Register (if source
of interrupt) or
Writing into the
Transmitter
Holding Register
0
0
0 Fourth
Modem
Status
Clear to Send or
Data Set Ready or
Ring Indicator or
Received Line
Signal Detect
Reading the
Modem Status
Register
Interrupt
Source
None
001
8-28