[Info-vax] OT: parity vs ECC on EV4

[John Wallace]

On Oct 19, 4:12 am, Steven Schweda <sms.antin... at gmail.com> wrote:
> On Oct 17, 6:43 pm, John Wallace <johnwalla... at yahoo.co.uk> wrote:
>
> > Some AlphaStations started life with parity memory and it shipped as
> > 36bit parity (aka longword parity).
>
> > Then some HQ genius spotted that in a 21064-based AlphaStation you
> > didn't need all 36 bits. Because all memory reads and writes were
> > longwords or wider, you only needed 33bit memory to make it parity
> > protected. [...]
>
>    Really?  The oldest/lamest Alpha system I ever used was an
> AlphaStation 200 4/233.  Its approved memory was fast-page
> mode, parity.  (According to its "User Information" book,
> "The system requires 72-pin parity SIMMs that have an access
> time of 70 ns (nanoseconds) or 60 ns.")  But those extra
> "parity" bits were aggregated for use as ECC, not for simple
> parity.  One extra bit for each eight data bits means four
> extra bits for a 32-bit word, which is enough for ECC.  The
> last VMS system I saw which used parity memory as parity
> memory (and not as ECC memory) was a MicroVAX II/III or a
> MicroVAX/VAXstation 2000.  I've never seen any machine which
> used standard parity SIMM/DIMMs as anything other than ECC.
>
>    I don't get around much, so perhaps someone, somewhere,
> really did use some non-standard, parity-only,
> wider-than-8-bits, memory module, but I've never seen one,
> and I wouldn't credit such a claim without some better
> evidence than a vague recollection.

Wrt AlphaStation 200: what leads you to think it has ECC? The
"AlphaStation 200 Series: User Information" book at
http://www.compaq.com/alphaserver/download/ek-pcdta-ui-e01.pdf
says, in the Motherboard section in chapter 2, "Memory controller and
data path that connects through a 64-bit wide data bus to main
memory and through a 128-bit wide data bus to second-level cache. The
data is longword (32-bits) parity protected. "