Path: utzoo!utgpu!water!watmath!clyde!bellcore!tness7!killer!elg
From: elg@killer.UUCP (Eric Green)
Newsgroups: comp.sys.cbm
Subject: Re: Burst Mode on C-64 ?
Message-ID: <4376@killer.UUCP>
Date: 7 Jun 88 02:08:06 GMT
References: <1988Jun5.024514.28021@ziebmef.uucp>
Distribution: na
Organization: The Unix(R) Connection, Dallas, Texas
Lines: 32

in article <1988Jun5.024514.28021@ziebmef.uucp>, ross@ziebmef.uucp (Ross Ridge) says:
> In article <4241@killer.UUCP> elg@killer.UUCP (Eric Green) writes:
>>in article <1988May27.175829.5648@ziebmef.uucp>, ross@ziebmef.uucp (Ross Ridge) says:
>>Note that bit-rate for each byte during burst mode is 250,000 baud, not 4800
>>baud. Burst mode is FAST. With the 1581, which reads data off disk at a
>>reasonable rate, the 128 is almost as useful as a real computer.
>>
> 250 000 baud? That's almost 32k a sec. and as I understand it burst mode is
> only slightly faster than improved serial bus the 128/1571 uses. Hmm... 

The BIT rate is 250,000 baud (the fast-mode routines shove a "4" into the
timer which provides the shift-register clock). 

Actual throughput is much less, because between each byte, a sizable amount of
time must be spent handshaking. However, because the disk drive shoves data
out its serial port at that bit rate, you must still be capable of reading a
single byte of data at that bit rate. The 6526's serial shift register can do
that. Your software never will.

As for the speed of burst mode: It is considerably faster than "fast mode",
because it has much less handshaking overhead. For example, one test I did was
loading Nezterm (a 200-block file). Took about 20 seconds in fast-mode. Took 8
seconds in burst mode. Try it yourself. Write a tight, clean assembly-language
program which does nothing but read bytes from the file, in fast-mode. Then do
a burst-mode load. The principle is the same (all that the "load" routine
does, in the absense of burst mode, is read bytes from the file one-at-a-time
and shove'em into RAM).

--
Eric Lee Green    ..!{ames,att,decwrl,ihnp4,mit-eddie,osu-cis}!killer!elg
          Snail Mail P.O. Box 92191 Lafayette, LA 70509              
"Is a dream a lie if it don't come true, or is it something worse?"