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From: mahar@weitek.UUCP (mahar)
Newsgroups: net.arch
Subject: Re: RISC and MIPS
Message-ID: <237@weitek.UUCP>
Date: Mon, 29-Jul-85 13:33:57 EDT
Article-I.D.: weitek.237
Posted: Mon Jul 29 13:33:57 1985
Date-Received: Thu, 1-Aug-85 00:14:56 EDT
References: <419@kontron.UUCP>
Distribution: net
Organization: Weitek Corp. Sunnyvale Ca.
Lines: 87
Summary: fun with numbers

In article <419@kontron.UUCP>, steve@kontron.UUCP (Steve McIntosh) writes:
> "RISC ARCHITECTURES:
> what's an instruction? Ah! That's a very good question!
> Take, for example, the following 68000 instruction:
> 
> 	MOVE.W D7,(A3)+
> 
> That instruction stores the lower word of the 32-bit register D7 at the
> address contained in the 32-bit address register A3, and then
> increments A3 by two (two bytes = one word).
> The same operation in a hypothetical RISC machine:
> 
> 	MOVE R7,(R#)
> 	LOAD 2,R4
> 	ADD  R4,R#

	On the Berkeley RISC, the equivalent istruction sequence is:

	MOVE R7,(R4)
	ADD #2,R4,R4

> 
> For simplicity, suppose that those three computers each performed that
> equivalent instruction (or instruction sequence) in exactly one
> microsecond. Then the 68000 would be operating at 1 MIP, the 32000
> series at 2 MIPS, and the hypothetical RISC machine at 3 MIPS. 
> 
> EACH COMPUTER WOULD BE PERFORMING EXACTLY THE SAME AMOUNT OF WORK!
> 
> When a RISC advocate proudly points to the MIP performance rating of
> his preferred architecture, remember that a RISC HAS to run at a lot
> more MIPS than a CISC machine just to keep up. WHAT IS A MIP, INDEED!

I agree that MIP is not a very good measure of computer performance.
A rule of thumb that I have heard is that the VAX 780 is 1 MIP. If your
computer does more work in less time then a 780 it is faster then 1 MIP.
If it does less, it's slower.
> 
> THE VON NEUMANN BOTTLENECK:
> 
> No von Neumann machine can run faster than allowed by its
> bus-bandwidth. Both RISC and CISC architectures are von Neumann
> machines. Let us examine those three equivalent instruction (sequences)
> from a bus-bandwidth standpoint:
> 
> The 68000 requires two memory cycles, the 32016 three memory cycles
> and the RISC machine either three or four memory cycles depending on
> whether it includes a LOAD QUICK instruction which contains small
> integers inside the instruction field.

Once again the Berkeley RISC is two memory cycles.

> 
> In fact, the 68000 executes that instruction (singular in the case of
> the 68000) in exactly two memory cycles (eight clocks). The 32016
> requires AT LEAST three memory cycles (12 clocks) to execute, and can
> be even slower than that if the instructions are not word aligned.
> (68000 instructions are ALWAYS word aligned) The hypothetical risk
> machine requires 3 or 4 memory cycles to perform the same operation.

Since the Berkely machine is also two memory cycles (2 clocks), this is
a tie.

> 
> Therefore, the poor little 68000, operating at a mere 1 MIPS, is
> running AT LEAST 50% faster than a 2 MIPS 32016 and 50% to 100% faster
> than a 3 MIPS RISC machine!
> 
> To further emphasize this important point, our hypothetical RISC
> machine would have to have a memory cycle time 50% to 100% faster than
> the 68000 to have equal performance, and that would give it a 4.5 (50%
> faster) or 6.0 (100% faster) MIPS rating. If the 68000 is running at
> 12.5Mhz with zero wait states using 120 nsec DRAM (say, two megabytes
> of it) then JUST WHERE DO WE FIND DRAM FOR THAT RISC WHICH IS 50% TO
> 100% FASTER? (At an affordable price - srmc)

With that same 120 nsec DRAM, the Berkely RISC could run at 8 Mhz.
Since the 68000 does it work in 8 cycles it takes 8/12.5 or about 640 nsec.
The Berkely RISC does its work in 2 cycles. So, at 8 Mhz it takes 2/8 or
250 nsec. All for the same memory band width. In fact, however, the Berkeley
RISC only runs at about 4 Mhz. The memory bandwidth is 250 nsec. The same
sequence would be 500 nsec. A 4 Mhz RISC takes 25/32 the time to do what
a 12.5 Mhz 68000 would do.

I agree, In the example given the RISC took twice as many instructions
to do the same job. The MIP designation is ambiguous. One must look at
how much work is done in a given amount of time.