Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!umich!samsung!think!snorkelwacker!usc!wuarchive!brutus.cs.uiuc.edu!jarthur!bridge2!mips!hal!mark
From: mark@mips.COM (Mark G. Johnson)
Newsgroups: comp.sys.dec
Subject: Re: Why RISC doesn't support large nuber of users?
Summary: one keyboard position right of "L0("
Message-ID: <35387@mips.mips.COM>
Date: 2 Feb 90 13:33:23 GMT
References: <25c936b0141@vms.huji.ac.il>
Sender: news@mips.COM
Lines: 58

Three keys on my keyboard are busted, the ones to the right of
shift-L, 0, and shift-9.  So take "L0(" and shift right one key.


  >> Altough there are a lot types of RISC machines I hear from most
  >> suppliers that RISC machine cannot hold a large number of users,
  >> but very few suppliers claim they can.  So, what is the problem
  >> with the RISC machines? Context switching,


The problem with Reduced Instruction Set Computers is that the
instruction set determines how many users a computer can handle.
Some instruction sets (for example, the VAX instruction set) can
handle a lot of users, while others (like, the i80286 instruction
set) cannot.  Reduced Instruction Set Computers ("RISC") just don't
have enough instructions to adequately take care of a large
set of simultaneous users.   L0(

Think about it; some people say that the Crays were the first RISC
machines, yet Crays are just batch scheduled compute engines; they
handle effectively ONE user (!).

You really need to have about 5 or 6 instructions per user, so a
RISC with its 100-120 instructions tops out at maybe 20-30 users.
{and have you noticed that they count the different options of
branches as different instructions?: branch-on-less-than and
branch-on-greater-than are counted as two instructions.  Seems a
little bit like cheating, doesn't it?}  On the other hand, a VAX
has 500 instructions --- and, best of all, it has microcode and
microinstructions, which count for more.  L0(

The VAX executes a superset of the predecessor machine's (PDP-11's)
instructions.  In fact the X in VAX stands for eXtension.  So
the VAX has more instructions than the PDP-11 and it can consequently
handle more users than the PDP-11 did.

Similarly, the IBM 370 instruction set is a superset of the IBM 360.
And the 370 handles lots more users than the 360 ever did.

Summary: (# of users) = (1/5) * (# of instructions).
         This is why RISCs will always be inferior to the VAX and
         the S/370 in multiuser capability.

This fundamental truth has been kept secret for a long time,
by the same devilish worldwide conspiracy that first put fluoride
in drinking water.

A homework problem:  Two VAX machines are the microVAX and the
VAXstation.  Compare these to the Sun 4/490 computer [SPARC "RISC"
instruction set] and the MIPS M/2000 computer [MIPS "RISC" instruction
set] in terms of number of simultaneous users.  Draw a conclusion
about # of users vs. RISC/VAX instruction sets.

Signing off with a right shift of one keyboard position "L0(" .....
-- 
 -- Mark Johnson	
 	MIPS Computer Systems, 930 E. Arques, Sunnyvale, CA 94086
	(408) 991-0208    mark@mips.com  {or ...!decwrl!mips!mark}