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From: bjj@psueclb.BITNET
Newsgroups: comp.arch
Subject: Re: 360 vs VAX stuff
Message-ID: <818@PSUECLB>
Date: Sat, 12-Sep-87 13:17:02 EDT
Article-I.D.: PSUECLB.818
Posted: Sat Sep 12 13:17:02 1987
Date-Received: Sun, 13-Sep-87 10:03:10 EDT
Organization: Engineering Computer Lab, Pennsylvania State University
Lines: 43

This is in response to the claim that a VAX is not readily pipelined,
or at least that there are limits to the pipelining.  The same problem
didn't seem to stop the designers of the Harris HCX7.

From: johnw@astroatc.UUCP (John F. Wardale)
>>> The 1st byte of each 370 instruction tells the length of the instruction!

>> You have pin-pointed one of the VAX's problems.  This does not prevent,
>> absolutely, pipelining.

> Ok, but it ties the designers hands and one foot behind his back!!!

The Harris HCX7 is roughly similar to the VAX 8700.  You have to look real
close at the instruction set to see the differences between the VAX and
HCX instruction sets.  Like the VAX, the HCX opcode does not indicate
the instruction length, rather each operand encodes its own length.

The HCX7 seems to have solved the instruction decode problem with
a separate instruction cache.  The cache contains DECODED instructions
stored as fixed length 73 bit words.

Assuming a reasonable hit ratio in the 4K word instruction cache,
the extra complexity of instruction decode won't affect speed.

The HCX7 has a 100nS clock cycle.  Most instructions execute in 1 cycle.
Harris describes a 3 level pipeline which simultaneously processes:
        1)  Instruction Fetch
        2)  Address Calculation
        3)  Instruction Execution

> I believe the encoding of VAX instructions prevents one from
> making it go fast, while still being affordable.  (Its a
> point-of-diminishing returns question.)  Comments...Anyone think it'd be
> (economically) worth building a VAX 3X or 10X the current top-vax?
> Or has it [as I feel it has] reached the limit for current technology.

I shouldn't think a cache of decoded instructions would be unaffordable,
you have to cache them somewhere anyway.

Some of us have long been amazed at what others will pay to buy IBM.
When DEC starts charging for a VAX CPU what IBM charges for a 3090,
then it will be easier compare affordability.