[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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