Installing PDP-11 UNIX w. no tape - solution

Allison J Parent allisonp at
Sat Jan 31 00:46:31 AEST 1998

<I get some error messages with restor (``Missing address (header) block''
<which I believe are to do with the 10,240 byte record requests from resto
<My code expects 512-byte requests, and I'm doing 20 a time to fulfill th
<10,240 request, but still problems.

Try slowing down.  You may be overflowing the input buffer.  This was
a common problem on TU58s hooked to the 2nd DL on some systems at speeds
above either 4800 or 9600.  It only happend in the TU58 to host direction
(read) as the opposite path expected a handshake every 128 bytes(to allow 
the tu58 to actually do the write to tape).  It seems the tu58 would send 
a 512byte block as 4 128byte packets at a sustained rate fast enough to 
overrun the PDP-11 host input buffer; before it could be emptied.  You 
may be emulating a similar problem.  PCs do not service interrupts all 
that fast and OS overhead can make that longer.

Note PDP-11s can have enough overhead and higher priority stuff ahead of 
the 2nd DL that it cannot take data at greater than 4800 baud (sustained 
rate) without some kind of handshake to allow processing in between.
If the system is basically unloaded like my minimal 11/23 it can run at 
38.4!  The most likely time when this overrun can happen is while doing 
processing to write (via file system) to disk and recieving data at the 
same time.


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