[Simh] Simh Digest, Vol 166, Issue 12

[Paul Koning]

> On Nov 5, 2017, at 5:48 PM, Johnny Billquist <bqt at softjar.se> wrote:
> 
> Ah.
> 
> That sound like a simh problem. The tape file itself have one of a few different formats, none of them even keep any CRC (at least none of the ones I can think of right now).
> The backing file storage just have the tape records. And obviously, since the same backing file storage is used on both emulated tape drives, there is obviously nothing wrong with the file.
> 
> Any CRC indication is purely made up in simh. And that is where you get the error indication. So I guess someone needs to dig into the emulation of the Massbus tape drives.

Another possibility: VMS BACKUP has its own CRC in its data blocks, entirely separate from the tape drive hardware check codes.  If BACKUP reports CRC errors, it's most likely talking about those.  BACKUP also writes redundant blocks, so if only 1 out of N blocks is bad, the bad block can be reconstructed.  Presumably, CRC errors reported by BACKUP are caused by flaws in the original material that was transcribed into SIMH tape images.

	paul