Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!pasteur!ames!killer!elg
From: elg@killer.Dallas.TX.US (Eric Green)
Newsgroups: comp.sys.cbm
Subject: Re: C128,C128D, and C128C??? Questions
Message-ID: <7733@killer.Dallas.TX.US>
Date: 2 Apr 89 04:31:47 GMT
References: <1935.2432C063@isishq.FIDONET.ORG>
Organization: The Unix(R) Connection, Dallas, Texas
Lines: 36

in article <1935.2432C063@isishq.FIDONET.ORG>, izot@f171.n221.z1.FIDONET.ORG (Geoffrey Welsh) says:
>  > I suspect the VDC would have no
>  > trouble keeping up with 19,200 baud or even 32kbaud -- although the
>  > rest of the computer probably would get pretty stressed out at 32kbaud
>  >. If I was going to build a terminal... the VDC is such a
>  > natural for such an application, it's a shame that Commodore will
>  > never release it onto the consumer marketplace.
>  
>    Based on that, I can't help but wonder why 19,200 (twice the speed) would 
> cause problems on a C64 with an ACIA (which reduces overhead by as much as a 
> factor of ten)...

19,200 is only 1920 interrupts per second. That's not the problem,
especially since the NMI's for the ACIA are only about 80-100 cycles.
I don't know why my brother said it used so much, except that I assume
he was talking about the Kernal and screen display overhead. Also note
that his driver software had to do lots of compatiblity sniffing to see if
the baud rate had been changed under his nose -- he patched all the
RS232 routines in the Kernal, and jumpered over all the modem control
lines from the user port, but most software still insists upon poking
its RS232 baud rate into low RAM (and elsewhere -- but the elsewhere
doesn't much matter).

The VDC helps one problem (slow scrolling) by having its own hardware
block move command.

I'd give the card a twirl, except that he already tore down his
prototype. He's not such a great wire-wrapper. So of course when it
started glitching he recruited me to re-wrap it (and clean up the
drivers, and ... ;-). (maybe I'll get to it before summer, but I doubt
it... my priorities are clear -- classwork comes first).

--
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