Path: utzoo!yunexus!geac!syntron!jtsv16!uunet!unisoft!paul
From: paul@unisoft.UUCP (n)
Newsgroups: comp.arch
Subject: Re: RISC v. CISC (was The NeXT problem)
Message-ID: <1346@unisoft.UUCP>
Date: 25 Oct 88 16:48:47 GMT
Article-I.D.: unisoft.1346
References: <156@gloom.UUCP> <310@lynx.zyx.SE> <332@pvab.UUCP> <15964@agate.BERKELEY.EDU>
Reply-To: paul@unisoft.UUCP (Paul Campbell)
Lines: 34

In article <15964@agate.BERKELEY.EDU> matloff@iris.ucdavis.edu (Norm Matloff) writes:
>In article <332@pvab.UUCP> robert@pvab.UUCP (Robert Claeson) writes:
>
>*The more registers, the more to save at every context switch in a typical
>*OS (such as UNIX). Which will slow things down if you have many processes,
>*running.
>
>Based on parameters of Berkelely RISC I or II, the register-saving
>might take on the order of 0.1 msec.  If the quantum size is set to
>be in the range claimed to be typical in the Peterson and Silberschatz
>OS book, i.e. 10 to 100 msec, then we see that the register-saving
>issue for a RISC with lots of regiters has probably been greatly
>overemphasized.

Actually some modern chips run faster than this, the 29k for example
has 192 registers which it can save with a single burst write to memory,
a 25MHz part takes:

		192 x 40nS = 7.68 uS 

Since the stack cache isn't always full, and because the OS uses some of
these registers for itself the total save time is usually actually less
(and of course the 30MHz parts can save even faster). Of course compared
with a quantum size in the milliseconds range this is virtually non
existant. In fact compared with the normal Unix process switch overhead 
it's not really a big deal.

	Paul

-- 
Paul Campbell, UniSoft Corp. 6121 Hollis, Emeryville, Ca ..ucbvax!unisoft!paul  
Nothing here represents the opinions of UniSoft or its employees (except me)

	"Where was George?" - Nudge, nudge say no more