🚀 go-pugleaf

In article <161@astroatc.UUCP> johnw@astroatc.UUCP (John F. Wardale) writes:
>Its clear to me from the questions, that many people don't understand RISC.
Well, I am confident in *my* understanding.

>RISC stands for Reduced Istruction Set Computer.
True, this is true.  But the choice of the RISC acronym was unfortunate.
Reducing the number of instructions is incidental.  I know that the Berkeley
people emphasized this point in the beginning, but I think they (Patterson
and Katevenis (yes, I know he is out of the country)) would no longer
emphasize this aspect of simple machines.

>Features common to RISC machines:
>[a list of features]
Sigh, RISC machines have whatever features are necessary to achieve an
instruction rate of close to one per cycle and achieve a good compiler
interface.

>The Purpose of Register Windows was to remove the need for a cache.  As a
>student project, a real cache design was deemed too hard.  In practice, I
>think the Sun/3's and similar machines are proving that windows are not
>the "only-true-way" to get good performace.
Who ever said that windows were the only true way to get good performance?

>This 6/10/6 division of register is arbitrary, but is kept fixed to insure
No, not arbitrary. The amount allocated (in this case 16 regs) must be a
power of two.

>maximum speed.  To make it dynamic, would require a size-register
>(effectively nargs() value) probably *IN* the window (otherwize it would
>not be possible to unwind the call-stack).
Well, you can use dynamic links too.  (This makes the protocol for variable
sized windows more efficient.)

>Lets build a *FAST* machine (I-cache, and DATA-cache) with
>windows, and a memory system that's wide enough to fill a full window of
>registers in one (memory access + split-transfer) time unit.  Now what
Wow.  That's a wide bus, but I like the idea!  Bandwidth or bust!

>happens when you want to run a new process???   The state you must save is
>not just your current window, but your *WHOLE* stack of windows!  Windows
>are great for running benchmarks, but are not so good for doing context
>switching.   Anyone know how Pyramid solved this???  How many users (vi,
>csh, compiles, etc.) does it take to bring a Pyramid to its knees??
I worked at Pyramid, my thesis details a machine with variable sized windows,
and I have build a simple machine with variable size windows (er, activation
records that is).  Windows are good for running benchmarks *and* real
programs.  Variable sized windows are just better.  You are right, the entire
active portion of the register file must be swapped on a full context switch.
However, you are completely ignoring the fact that swapping the register file
is only a part, a small part, of the work that must be done to perform a
context switch.  The number of users needed to bring a Pyramid to its knees
is not correlated to the fact that it has register windows.  To be sure,
however, a real-time machine might be better off without windows (latency
vs. throughput).

Context switch overhead is mittigated by variable sized windows:  at least
the registers you are swpping are holding useful data.  With fixed size
windows, there is a high probability that some of the registers you are
swapping are not being used.  A smaller register file allowing variable
sized windows achieves the performance of a larger register file with fixed
sized windows.

    bcase
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