> On Nov 30, 2015, at 8:12 PM, Johnny Billquist <b...@update.uu.se> wrote:
> 
> ...
> DECtape never did interleaving that I know of. 

Sure it does.  The DOS format, which was adopted by RSTS, has 4 way 
interleaving.  If you write a 500 block file, it writes every 4th block 
forward, then fills in one set of gaps reverse, then forward and backward 
again, resulting in finally all blocks used.

This is a software function, of course, and actually implemented in the file 
system, but it's certainly interleaving.  It doesn't apply to contiguous files 
(supported in DOS but not RSTS), which is why RSTS V4A sysgen with output to 
DECtape took so long -- writing a contiguous CIL file, in block order, madly 
seeking back & forth.

The reason for the interleaving on DECtape is the start/stop time.  To run 
non-interleaved at high speed you have to leave the tape running (no "stop" 
commands) and you have to issue the next command quickly.  RT-11 could do that; 
DOS could not.

        paul


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