backup registers

A student writes:

	Professor,

	   What is the difference between backup registers in vector processors
	and caches in others machines? Do they access the main memory in the
	same way? Are backup registers content addressable?

	<name withheld to protect the student who wants to know the difference>

The purpose of the backup registers is the same as a small cache.  That is,
a place to store additional values beyond what can be stored in registers.

The big difference is that a cache is transparent to the software.  That is,
the program accesses memory, and if one gets a cache hit, the data is delivered
in one cycle (for exanple), but if one gets a miss, the data does not arrive
for many cycles.  Thus the time for the LD instruction is non-deterministic.
You can not count on the data arriving in one cycle, although most of the
time it does.  Seymour Cray did not like non-determinism.  Ergo, no cache.
The back-up registers are accessed under program control.  Therefore, they
have a fixed latency and the program knows specifically when the data will
be available for subsequent use.

Some have called the back-up registers a "software-managed" cache.

Hope this helps.
Yale Patt