Path: utzoo!utgpu!watmath!isishq!f171.n221.z1.FIDONET.ORG!izot
From: izot@f171.n221.z1.FIDONET.ORG (Geoffrey Welsh)
Newsgroups: comp.sys.cbm
Subject: Re: Simultaneous disk & RS-232 access
Message-ID: <1496.23E905F1@isishq.FIDONET.ORG>
Date: 2 Feb 89 20:14:16 GMT
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 > From: leblanc@eecg.toronto.edu (Marcel LeBlanc)
 > Message-ID: <89Feb1.163849est.2383@godzilla.eecg.toronto.edu>
 
 > ... let's look at the other
 > obvious solution:  read from/save to a RAM buffer, and use fast disk I/O
 > routines to read/write the file.
 
   Yeah, we spent a bit of time thinking about that one. We were going to use
the 1750 RAM expander such that the first 512K of the download went straight 
into RAM; any data received after the 512K was full would result in the OLDEST 
data block in RAM being written out. This overcame the problem of not being 
able to download files larger than your available RAM (i.e. we thought of your 
solution, and even refined it slightly).
 
   However, the #1 driving force behind our software was that it work with no 
extra hardware (we could just as easily have resuired that you install the 
UART board I designed). Thus depending on extra RAM isn't practical.
 
 > So the usage efficiency of the RS-232 connection is:
 > (M is modem speed, D is disk speed).
 >
 >         Efficiency =     size/M                   D
 >                         -----------------  =    -----
 >                          size/M + size/D        D + M
 
   That set of equations assumes that disk transmission must be stopped to 
handle the RS-232 port and vice-versa.
 
   But that is exactly the barrier that I am suggesting to break. You are 
saying that, asuming that the status quo is unchangeable, the status quo 
cannot be changed. You miss the point of the suggestion.
 
 > Modem speed     Efficiency              Efficiency
 >                 (slow disk)             (fast disk)
 > 2400               290                     6400
 >                 --------- = 0.55        ---------- = 0.96
 >                 290 + 240               6400 + 240
 
   In these calculations, you assume that the modem achieves its ideal 
throughput. If that is the case, them simultaneous disk & RS-232 access would 
mean that the net throughput was merely the slower of the two, such that 2400 
bps transfer would take place at 240 bytes/second (100%), 1200 at 120 b/s, 300 
at 30 b/s.
 
 > If you want to transfer files at 4800bps, you would have no choice
 > but to speed up disk access to get any real benefit.
 
   That is inevitable at baud rates above 2400.
 
 > All right, flame away! :-)
 
   I think I did enough, a couple of paragraphs back.
 
   Geoff ( watmath!isishq!izot )
 


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