>  > >what difference does true-parity RAM versus psuedo-parity RAM make in the
>  > >PC world?

It allows you to use the simm in a machine which >requires< parity to
be presented to the memory socket.  Naturally, you do not get any parity
protection.  I suspect that this would work in your sparc if you don't
care about parity protection.

Pseudo parity is implemented by gates that calculate the parity from
the data already there, but not actually store it.  It assumes a PC
standard parity scheme, and will not work with anything else (see
below).

>  > True parity can do ECC with the 430HX (Triton II) chipset.

True but IMHO irrelevant to the original question.

> Maybe I'm missing something here but surely parity ram will add an
> extra bit which will catch any single bit error and any odd number of
> errors but not even numbers of bit errors, and this would truly be
> parity.

The bit you're missing is that parity RAM merely contains one parity
bit per 8 memory bits in the simm, the parity generation/checking
hardware is in the PC.

Now, some modern PCs do something slightly different, and treat memory
in 64bit chunks, whereupon you now have 8 "parity" bits rather than
one, and you can do your Hamming with them.

So, originally, you could do one bit error detection in 8, now you can
do one bit correction/two bit detection in 64.  Swings and roundabouts.

You can get the gory details of the coding by perusing the 430HX manual
which is lurking somewhere on developer.intel.com.
-- 
Ronald Khoo <ronald@demon.net> Voice: +44 181 371 1000 Fax: +44 181 371 1150