Sector Interleave

[Johnny Billquist]

On 2015-12-01 02:19, Paul Koning wrote:
>
>> On Nov 30, 2015, at 8:12 PM, Johnny Billquist <bqt at 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.

Oh. You mean that the software decided to use blocks 0,4,8,12,...

Yes, that would be doable. I was thinking of interleaving at the format 
level.

But such interleaving means the software have to keep rather good track 
of things...

> 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.

Yes. Having run a PDP-8 booted from DECtape, I've seen plenty of rocking 
back and forth to start/stop. :-)

	Johnny

-- 
Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol