Byte sizes (was Re: 2.8M 3.5' floppy

William Donzelli aw288 at osfn.org
Tue Mar 15 17:10:27 CST 2005


> That depends on whether you trust the datapath between the memory and
> the CPU.  In the 1620, the memory is a separate cabinet, connected by
> cables to the CPU cabinet.  A wise designer would run parity on that
> interconnect.  That's still true: high end "system on a chip" designs
> have ECC memory AND parity (at least) on the buses -- even if they
> only run inside the chip.

Most IBMs do quite a lot of checking on just about every datapath. I think
this all started with the S/360. Many even have duplicate ALUs for
checking. 
 
> Oh, and of course processors don't ever figure memory parity in
> software, only in hardware, so "waste of CPU" can't apply.
> 
> As for sign bit per digit, in the 1620 the "sign" bit serves two
> purposes -- on the least significant digit it's the sign of the
> number, on the most significant digit it's the "this is the last
> digit" marker.

But the original argument was that sixbit is great for numeric data. It is
not, no matter how you look at it. How the 1620 popped up is still a
little odd.

Later IBM though dispose of the sign/marker bit - variable length words 
(BCD or character) were treated with a start address and a stop address
(or a length). One has to be careful or a large addition could overwrite
itself. 

William Donzelli
aw288 at osfn.org




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