[Info-vax] DEC Multia (UDB) issues

[glen herrmannsfeldt]

In comp.os.vms Johnny Billquist <bqt at softjar.se> wrote:

(snip)
>>> 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.

>> No, but 8 extra bits for a 64 bit word is enough for ECC.

> Right. If I remember right, you need five bits of ECC for 32 bits of data.

> Exactly how many bits you need for 64 bits I don't know, but I would 
> have suspected that 8 bits would be enough. :-)

I believe log(n+1)+1 bits, were n is the number of bits, including
the ECC bits.  You need to be able to correct all errors, including
errors in the ECC bits.  Then you need to be able to detect double
bit errors, so one additional bit.

So, eight ECC bits for 64.

-- glen