9 track tapes and block sizes

Chuck Guzis cclist at sydex.com
Sun Oct 4 18:00:32 CDT 2020

On 10/4/20 1:50 PM, J. David Bryan via cctalk wrote:
> On Friday, October 2, 2020 at 13:51, Al Kossow via cctech wrote:
>> Those would already be broken with Bob's use of large negative numbers
>> for physical end of tape and 'bad block is here' (you don't get to know
>> how big that bad block was, so that is hell with tapes with
>> variable-length records, grumble..)
> I'm not sure I understand.  SIMH metadata markers are treated in the tape 
> library code as unsigned values, so EOM and erase gap are seen as large 
> unsigned values.  The format limits record lengths to 24 bits (so about 16 
> MB maximum per record), reserving the upper 8 bits to indicate the type of 
> the marker, and the bad record marker is the top byte = 0x80.  The record 
> length of a bad block is encoded in the lower 24 bits and indicates how big 
> the bad block was.
> What am I missing?

I don't believe that you're missing anything.   When I process these
files, I mask off the lower 24 bits as the block length.  A 16MB tape
block is impossibly large in any case.


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