RISC MIPS -- Sun vs. VA

[convex!datri at uunet.uu.net]

>In the Unix marketplace, MIPS normally means performance that many times
>that of a VAX 11/780, which is essentially the same as a MicroVAX 1. This

I'm pretty sure you're thinking of the MVII, which is usually rated at
about .9 of a 780.  As I remember, the MVI was significantly slower.

>Thus MIPS is not an actual count of instructions per second.

Million Instructions Per Second.  The problem is, as you noted, that many
people compare those figures across architectures.  It may also be
significant how they come up with that number -- do they just average out
all the instruction times and divide by n, or do they weight it according
to the instructions that people actually use?  On the MVII chipset, DEC
implemented some of the "lesser-used" instructions in software, which
could skew the meaning of an MVII "MIP" according to what you do on it.
An example is the CRC instruction, which it seems DEC did in software, but
of which the VMS BACKUP program makes heavy use.

I prefer the habit of saying "10 times a 780" or "10 VAX MIPS", since
that's a more useful number.

Another thing to be careful about is that there is a manufacturer named
MIPS who make RISCish processors (and machines), and that the DECstation
3100 uses them.