9 track tapes and block sizes
imp at bsdimp.com
Mon Oct 5 16:06:50 CDT 2020
On Mon, Oct 5, 2020 at 9:25 AM Chuck Guzis via cctech <cctech at classiccmp.org>
> On 10/4/20 10:51 PM, J. David Bryan via cctech wrote:
> > On Sunday, October 4, 2020 at 16:00, Chuck Guzis via cctalk wrote:
> >> A 16MB tape block is impossibly large in any case.
> > The HP 3000 mag tape diagnostic attempts to write a single record from
> > to EOT, which unfortunately fails under simulation due to the 16 MB
> > limitation. In hindsight, it would have been better to accommodate
> > lengths corresponding to the highest density and longest reel length,
> > I think would need 28 bits. Four bits for metadata identifier would
> > have been be good enough, and one of those should have been dedicated to
> > "private data" that would appear invisible to programs running under
> > simulation (and could be used to include information about the tape
> > with the tape image).
> That's interesting. Looking at my own code for .TAP files, I see the
> #define TAP_FILEMARK 0x0 // 0 = filemark
> #define TAP_EOM 0xffffffff // -1 = end of medium
> #define TAP_ERASE_GAP 0xfffffffe // -2 = erase gap
> #define TAP_ERROR_FLAG 0x80000000 // error flag bit
> #define TAP_LENGTH_MASK 0x00ffffff // mask for length
> So increasing the mask for block length wouldn't seem to be a problem,
> assuming that SIMH could support it. There may be other high-order bit
> meanings assigned, but I've not run into them.
The other way to handle it, should we not be able to steal bits which
should be plan A, is to have a metadata type that says 'append this to the
prior record' which would remove the limit entirely.
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