Path: utzoo!utgpu!jarvis.csri.toronto.edu!clyde.concordia.ca!antares!dcfs018
From: dcfs018@antares.Concordia.ca ( STEPHAN BOHM )
Newsgroups: comp.sys.atari.8bit
Subject: Why does my program take so long?
Summary: Cycle counting is off
Message-ID: <1894@clyde.concordia.ca>
Date: 6 Mar 90 19:07:32 GMT
Sender: usenet@clyde.concordia.ca
Reply-To: dcfs018@antares.Concordia.CA
Distribution: na
Organization: Concordia University, Montreal Quebec
Lines: 32

Hello all.  I've written a little machine language program that reads the
trigger port and stores the results in RAM until there's no more memory.

I optimized the thing so that it ran as fast as possible.  That meant counting
cycles to find the fastest instructions to do the job, and I ended up with
a routine that takes 14863 cycles to read a page full of bits (2048 bits).
Since 1 cycle takes 0.5587 microseconds, that means I can theoretically
read at 246500 bits/sec.  The thing is, when I run the program, it takes
a little over 10% longer to execute than my calculations show.  Why?

I am shutting off screen DMA and VBI's.  I wrote another little delay loop,
counted the cycles, calculated the theoretical run time, ran the program, and
ended up with an actual run time that was again 10% longer than anticipated.

Therefore the question is, why is my 130XE slow by 10%?  Is it due to the
RAM refresh?  I looked in the hardware manual and in De Re Atari but the
only stuff I found on DMA timing referred to screen DMA, but since I shut that
off (POKE 559,0) that shouldn't come into play.  Does Antic refresh RAM even
when POKE 559,0 was issued?  If so, when does it do the refresh?

Does 10% sound right for memory refresh, or is that way too much
lost time for such a simple operation (to me it sounds like
a lot of lost time, because all along I thought screen DMA was a big
time-waster at 30%, but 10% is substantial).

Your thoughts/comments are welcome.

                                             Jean Goulet
                                             Electrical Engineering
                                             Class of '89
                                             Concordia University
                                             Montreal, Canada