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ASYSTAT
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1995-09-17
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55 lines
asystat
Display statistics on attached asynchronous communications
interfaces (8250 or 16550A), if any. The display for each port
consists of three lines. The first line gives the port label and the
configuration flags; these indicate whether the port is a 16550A chip,
the trigger character if any, whether CTS flow control is enabled,
whether RLSD (carrier detect) line control is enabled, the speed
in bits per second, and the port address and IRQ number in hexadecimal.
(Receiving the trigger character causes the driver to signal upper
layer software that data is ready; it is automatically set to the
appropriate frame end character for SLIP, PPPand NRS lines.)
The second line of the status display shows receiver (RX) event
counts: the total number of receive interrupts, received characters,
receiver overruns (lost characters) and the receiver high water
mark. The high water mark is the maximum number of characters ever
read from the device during a single interrupt. This is useful for
monitoring system interrupt latency margins as it shows how close
the port hardware has come to overflowing due to the inability of the
CPU to respond to a receiver interrupt in time. 8250 chips have no
FIFO, so the high water mark cannot go higher than 2 before
overruns occur. The 16550A chip, however, has a 16-byte receive FIFO
which the software programs to interrupt the CPU when the FIFO is
one-quarter full. The high water mark should typically be 4 or 5 when
a 16550A is used; higher values indicate that the CPU has at least
once been slow to respond to a receiver interrupt.
When the 16550A is used, a count of FIFO timeouts is also
displayed on the RX status line. These are generated automatically
by the 16550A when three character intervals go by with more than 0
but less than 4 characters in the FIFO. Since the characters that
make up a SLIP or NRS frame are normally sent at full line speed, this
count will usually be a lower bound on the number of frames received
on the port, as only the last fragment of a frame generally results
in a timeout (and then only when the frame is not a multiple of 4
bytes long.)
Finally, the software fifo overruns and high water mark are
displayed. These indicate whether the <bufsize> parameter on the
attach command needs to be adjusted (see the Attach Commands
chapter).
The third line shows transmit (TX) statistics, including a total
count of transmit interrupts, transmitted characters, the length of
the transmit queue in bytes, the number of status interrupts, and
the number of THRE timeouts. The status interrupt count will be zero
unless CTS flow control or RLSD line control has been enabled.
The THRE timeout is a stopgap measure to catch lost transmit
interrupts, which seem to happen when there is a lot of
activity (ideally, this will be zero).