Re: Questions on Disk ][ error checking

[hatespam@invalid.invalid (Zardo Zapp)]

dog_cow@macgui.com (D Finnigan) wrote:

>I have a question about the error-checking mechanism used on the Disk ][.
>
>As I understand it, checksums are used to verify that the data read from the
>disk are valid and error-free. However, would there be a way to skip these
>checks? Is there any software that does so?
>
>I ask because I have more than one disk which has some data and programs
>which can be read successfully, and some data which cannot. These disks are
>not copy-protected. Would it be worth my time to make a disk image of a disk
>with only partially good data? Is this even possible?

I worked with Sheppy a while back to get a version of ImageMaker that
can do this. It worked well for me.

Yes, it's worthwhile. The corruption may not be on an area of the disk
you care about, and so you can salvage the rest.

What would be nice, though, is to get a list of the damaged sectors so
you know where the trouble may be. So far, I don't have a good way to
do that.

>Also, would there ever be a circumstance where the disk has become damaged
>in an area such that the Disk ][ cannot tell whether a bit is 1 or 0 and
>thus refuses to read that disk?

The hardware isn't that sophisticated. It'll read a 1 or a 0 no matter
what - whether that's what supposed to be on the disk is anothe story.
The checksum is only 8 bits, so if the disk is so bad that you get
essentially random data, then on average 1 out of 256 reads the
checksum will still match. In other words, the checksum only does a
fair job of detecting data corruption.