[Info-vax] TK50 - this is annoying...

Alfred Falk falk at arc.REMOVE.ab.ca
Thu Oct 18 14:52:01 EDT 2012


Johnny Billquist <bqt at softjar.se> wrote in
news:k5p9uo$d52$1 at Iltempo.Update.UU.SE: 

> On 2012-10-18 16:44, glen herrmannsfeldt wrote:
>> George Cornelius <cornelius at eisner.decus.org> wrote:
>>
>> (snip, someone wrote)
>>
>>>> I once met someone who had used DECtapes and he had been very
>>>> impressed by them in their day, but he was at least 20 years my
>>>> senior.  IIRC he described some mechanism where they skipped
>>>> alternate blocks when reading or writing so that the tape speed
>>>> could be higher, and those skipped blocks were used when the tape
>>>> was travelling in the opposite direction.
>>
>>> I had heard that the data was written twice and always assumed (I
>>> suppose correctly) that the extra blocks were for redundancy.  That
>>> they might have been written in reverse bit order never occurred to
>>> me.
>>
>> The data is written twice, in parallel. Three data bits on 10 tracks.
> 
> I think it's time we kill the just created myth of blocks written 
> backwards, and what not. That has, as far as I know, never been done.
> 
> Yes, data is written twice on the tape, on separate tracks, just like 
> you write, Glen.
> And that is a big reason why data is rather safe on DECtape. You can 
> even punch holes in the tape, and it will still work.
> 
> Writing blocks backward would be more headaches that it would be
> worth, not to mentioning, as I did before, that computers can keep the
> tape spinning without missing blocks, so there is no need from an 
> optimization point of view.

If I recall correctly, KM-9 (the sort-of OS DEC offered for the PDP-9) 
wrote alternate blocks to DECtape, and wrote the skipped blocks 
backwards.  Thus, if you imagine a ten-block tape it might be written in 
this order: 0 2 4 6 8 9 7 5 4 3 2 1.  The odd-numbered blocks would be 
processed backwards.

> If people actually thought about it... I mean, a PDP-11 could keep a 
> disk fed with data without missing blocks, and disk blocks pass by
> much faster than tapes...
> (Heck, I could probably keep a disk fed data on a PDP-8 as well, if I 
> thought about it.)
> 
>      Johnny

Interleaving of disk blocks used to be pretty common.  The bus tranfer 
rates of some systems couldn't keep up with the disk itslf.  I first 
recall seeing interleaving discussed in a CDC 6400 (or maybe Cyber 63) 
manual.  Data General used to do it for some of their drives when 
connected to microNova systems that had (IIRC) 2-bit buses. (Controlled 
by a switch or jumper on the drive or controller.)



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