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From: yerazuws@CSV.RPI.EDU (Crah)
Newsgroups: mod.computers.vax
Subject: Re: VAX Instruction Timing
Message-ID: <8701091457.AA27493@csv.rpi.edu>
Date: Fri, 9-Jan-87 09:57:27 EST
Article-I.D.: csv.8701091457.AA27493
Posted: Fri Jan  9 09:57:27 1987
Date-Received: Sat, 10-Jan-87 02:27:55 EST
References: <8701090448.AA22342@ucbvax.Berkeley.EDU>
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Summary: You ask for the impossible ..... or at least the Really Hard To Do!

In article <8701090448.AA22342@ucbvax.Berkeley.EDU>, ins_ajbh@jhunix.BITNET (James B Houser) writes:
> 
> How do you go about getting instruction time numbers for VAXEN. Things
> like how long a register to register move takes etc. 

The problem is that such timing numbers depend on what instructions 
have recently gone through the machine.  As far as I know, ALL
vaxes (excepting /730 and /725) have a pipeline of some sort.  
On the big fast ones (like 8800's) this pipeline is three instructions
deep.  So, if you are on an 8800, and run a loop consisting of 
something like
    
	TOP:	Load R1 with R2
		Increment R3
		Branch if R3 .ne. 0 to TOP

the pipeline will flush at the end of each loop, artificially
making the machine appear slower than it is.  Even the trick of 
adding another load of R1 with R2 and measuring the difference 
in times between the two loops may not help, because some big 
VAXes have "bypasses" built into the microcode.  The ucode "sees"
that R2 is already on an internal bus, and grabs it without refetching
it from R2- artificially DEcreasing the time needed.
	
	Then we have to worry about cache contention, flushing, etc.
And what the cache algorithms are on the particular CPU you are on.
And what kind of memory the CPU has (different memories may have 
different behavior under different fetching parameters - and whether
those fetches were aligned - or at least fell into the same memory
block  (some VAXes fetch 128 bits at a time))
	
	Conclusion: you simply have to TRY your application
and tweak where you think it's slow.  Consider: A Sun 3 with 
the standard CPU (not the $30,000 option one) should be twice
as fast as my uVAXstation II - but I find that for what I
use them for, the uVAX II is faster.  Why?  I dunno.
	
	Hope this eases your brain.
	
		-Bill Yerazunis