Q22 BUS parity check?

Tony Duell ard at p850ug1.demon.co.uk
Wed Dec 13 16:34:03 CST 2006


> 
> Hello,
>   I read the Q22 specification (search "Q22 specification" on this page:
> http://www.chd.dyndns.org/qbus_ide/ ) but the parity check function is still
> not clear to me. My questions are,
> 1. Is there parity check when interrupt vector is read ?
> 2. Is there parity check if IO page is read?
> 3. What is the polarity of the parity bit?
> 
> Those questions are not answered from the specification. Any expert here?

Assuming it's like Unibus (and I am pretty sure it is), I think your 
confusion coimes from not realising what device actually does the partity 
check.

What it really is is that a memory board can signal an error on read, 
which was often down by a parity check. It _could_ be done by, say, 
having ECC memory and having the error signals if there were too many bit 
errors to correct. It's tbe memory board (or memory controller) that 
checks parity, nnt any part of the CPU hardware.

So  to answer your questions: 

1) There could be, it depends on the memory board where the vector is 
stored. 

2) Not normally, I don't think any standard I/O devices checked parity on 
reads

3) Depends on the memory board, etc.

-tony



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