Re: Disk II Checksum

["Peter Watson" <paw77@N_O_hotmail_S_P_A_M.com>]

"Alex Freed" <alexf@mirrow.com> wrote in message 
Gt-dnVKgR7i7jFDYnZ2dnUVZ_u2mnZ2d@comcast.com">news:Gt-dnVKgR7i7jFDYnZ2dnUVZ_u2mnZ2d@comcast.com...
> I'm not an expert at all, but as I recall it CRC was invented
> because the odds of finding an error are much better than
> using a simple 16 bit checksum.

It's worth keeping in mind that speed was an absolute requirement/necessity 
for the Disk ][ checksum algorithm. On the hardware of the day, a CRC was 
very expensive, while the simple EOR checksum was very fast (and small).

And yes, it probably resulted in the occasional undetected corrupt block. I 
can certainly remember back to the odd occasion where a previously working 
program suddenly stopped working, and some research and/or comparison 
against a backup would show that one block was subtlely "different". It 
never occurred to me at the time, but it makes a lot of sense that it may 
have been a disk corruption (common enough back then) combined with a 256:1 
checksum non-failure.
-- 
Peter Watson
-- Write to MS-DOS disks on the Apple IIgs?
-- Impossible!  ;-)