> On Nov 5, 2017, at 5:48 PM, Johnny Billquist <[email protected]> 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
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