9 track tapes and block sizes

J. David Bryan jdbryan at acm.org
Sun Oct 4 15:50:48 CDT 2020

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?

                                      -- Dave

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