Path: utzoo!attcan!uunet!cs.utexas.edu!tut.cis.ohio-state.edu!purdue!bu-cs!mirror!prism!rob
From: rob@prism.TMC.COM
Newsgroups: comp.sys.ibm.pc
Subject: Re: Ami Bios and the V20
Message-ID: <206900137@prism>
Date: 17 Nov 89 13:40:00 GMT
References: <6129@merlin.usc.edu>
Lines: 41
Nf-ID: #R:merlin.usc.edu:-612900:prism:206900137:000:2108
Nf-From: prism.TMC.COM!rob    Nov 17 08:40:00 1989



>>  It's been said before, but it's worth repeating - The 1.8 number from
>>SI is unrealistic (in general, any number from SI comparing different
>>
>>   In general, a V20 should give you about a 5 - 10% speedup, possibly
                      ^^^^^^
>>as high as 20 - 30% when running floating point code, which tends to be 

>Have you ever actually _tried_ it?  I replaced the CPU in my XT with a V20
>and realized very noticable increase in speed, running a variety of apps.
>If the increase was only 5 - 10%, I really doubt that I would notice it.
>I'd say, on the average, I got a 40 - 50% boost.  (Just for GP, it went
>from 1.0 to 1.8, just like the original poster.)

   Actually, I did try it a few years ago. I should have been more
specific about what I meant by 'in general'. Running non-floating
point code (program compiles, spreadsheets, and databases), the speedup
was from 5 to 10%. I wouldn't have noticed it if I hadn't been timing
it. As mentioned, floating point code, which makes heavy use of the
integer multiplies and divides at which the V20 excels, shows a greater
increase (in my experience, around 25%).

   It's sort of surprising how little difference a V20 makes. As another
note mentioned, it also claims to drastically speed up (by a factor of 3 
to 6) effective address calculation, which, unlike integer multiplies and 
divides, is a real factor in most code. Yet a test program I wrote that 
simply looped around a bunch of statements like

		   MOV   AX, [BX+SI+2]

showed a speedup of only about 20%, as I recall. Looping overhead was
clearly a consideration (the V20 doesn't claim to speed up loops
significantly), but I still expected a larger gain.

   Still, whether it's worthwhile to you depends on what you're running,
and what you consider a significant speedup. You could probably also realize 
a larger gain if you optimized code for the V20. Given how inexpensive it
is, getting a V20 or V30 is probably worth it. The point remains, though: 
someone expecting the 80% speedup that SI promises will be disappointed
(i.e. my complaint is with SI, not the V20).