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

[Johnny Billquist]

On 2012-10-09 20:49, glen herrmannsfeldt wrote:
> Paul Sture <nospam at sture.ch> wrote:
>
> (snip)
>
>> 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 haven't heard about that but, just like floppy disks, I suppose
> you could do block interleaving. As well as I understand it, DECtape
> is direct access in the same way floppy disks are, with block headers
> identifying the block number.

Correct.
But I too have not heard of anyone actually doing this kind of 
semi-interleaving. Most machines I've seen or used don't have a problem 
keeping the DECtape running at full speed across several blocks.

But you could definitely do it. But unless the controller managed all 
the reverse twiddling required, it hardly pays to read in reverse. 
You'll get the words in the wrong order, meaning you need to DMA from 
last address to first. And since DECtape do data 3 bits at a time, you 
also get each group of 3 bits in the wrong order compared to forward 
movement. And finally, since you are reading in reverse, all bits also 
gets inverted.

	Johnny