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ASYSTAT
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1991-09-18
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63 lines
======= NOSview [137]
asystat
=======
________________________________________________________________
asysyat
________________________________________________________________
Display statistics on attached asynchronous communications
interfaces (8250 or 16550A). 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, and the speed in
bits per second. (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, PPP and 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 (HWM).
The HWM 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 HWM 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 HWM 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 on the second line, the software FIFO overruns and HWM
are displayed. These indicate whether the <bufsize> parameter on
the 'attach' command needs to be adjusted.
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).