Date   : Wed, 30 Dec 1987 10:11:57 PST
From   : Bridger Mitchell <bridger%rcc@rand-unix.ARPA>
Subject: Re: MEX1.65 Character Loss 

If mex is "smart", it should be polling the modem input status between
each sector-write call in the flushing routine, and buffering any
received characters for "catch-up" processing when the buffer-flushing
is completed.  [Ron Fowler: does mex do this?]  But if not, it's
likely that characters are arriving at the modem port before the
buffer-flushing operation completes.  Unless the arriving bytes are
buffered before they reach mex, the first ones will be overwritten by
the later ones, creating a gap in the input stream received by the mex
software.

The real time required to flush a buffer is system-dependent, probably
dominated by the drive's step rate and the track number of the capture
file, and also affected by the CPU clock rate.  Another system with
faster disks, a faster cpu, or even a differently located file, could
have no lossage.  On your system, if you capture to a ram disk, for
example, there may be no lossage.

Many systems have 1-3 byte buffering in the UART chip that receives
the bytes from the modem; different systems use different UARTS.  More
buffering can be obtained by writing an interrupt-service routine for
the modem ports; only a few bioses have such support, but it may be
possible to write the mex overlay that way.

--bridger