Path: utzoo!mnetor!uunet!husc6!think!ames!ucbcad!ucbvax!RAND-UNIX.ARPA!bridger%rcc
From: bridger%rcc@RAND-UNIX.ARPA (Bridger Mitchell)
Newsgroups: comp.os.cpm
Subject: Re: MEX1.65 Character Loss
Message-ID: <8712301812.AA01201@newton.arpa>
Date: 30 Dec 87 18:11:57 GMT
References: <8712291428.AA09608@dmc-crc.arpa>
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The ARPA Internet
Lines: 25


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