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From: norm@rocksvax.UUCP (Norm Zeck)
Newsgroups: net.micro.68k,net.micro.16k
Subject: 68020 vs 16k
Message-ID: <1646@rocksvax.UUCP>
Date: Wed, 8-Feb-84 22:27:17 EST
Article-I.D.: rocksvax.1646
Posted: Wed Feb  8 22:27:17 1984
Date-Received: Fri, 10-Feb-84 04:28:01 EST
Organization: Xerox, Rochester, N.Y.
Lines: 34

About the cache, rumors indicate that the size will be about 256 bytes, ie it
will not be a 68010.  Sounds like Motorola really put that one word instruction
hold in the 68010 to improve their benchmarks against CPU's that have string
instructions (Intel 80x86 for example).

About part speeds, I think Rob's comments about memory speeds is well
said.  We have been running with 12.5 Mhz 68000's for about a year now.
The processor board design is versabus based with a 4k byte cache, 32 bit
wide data r/w to memory, and segmented mmu (Charles River Data systems).
>From a memory speed point of view, the 12.5 Mhz 68k likes to see memory
respond in ~80 ns (DTACK') + 50 ns to data valid (give or take some time for
margin) without adding wait states.  To do this on a bus that is not local
to the processor such as multibus or versabus is difficult to say the least.
Hence the cache is VERY important in SYSTEM performance.  For example, we
also have a 8 Mhz 68k CPU board from Motorola that we have run some 
benchmarks on, and on the 12.5 Mhz system these same benchmarks run
about 2.5x faster using the same memory.  Obviously 12.5/8 != 2.5.
The cache + 32 bit r/w to keep the cache full make a significant
performance difference above and beyond the CPU clock speeds.  This will
Memory bandwidth will become more important in future CPU's as clock
speeds increase (I believe the 386 may have some type of cache ???).

Nothing against the 16k, nice chip, nice design.  Will definitely give
Motorola a run for their money.  But, increased CPU clock speeds
without consideration for memory speed requirements can be misleading
in terms of REAL acheivable system performance.  The cache can help here.
Beware of the benchmarks that sound real impressive, but require you
to use VERY fast memory to acheive as I have heard about some of the
faster versions of the 286 (not a put down on Intel, but their sales
lit which seems to always compare benchmarks is on CPU's often has
looked real good on paper, but has not turned out the same in a system).

					Norm Zeck