[Info-vax] TK50 - this is annoying...
glen herrmannsfeldt
gah at ugcs.caltech.edu
Thu Oct 18 15:39:59 EDT 2012
Alfred Falk <falk at arc.remove.ab.ca> wrote:
> Johnny Billquist <bqt at softjar.se> wrote in
(snip regarding DECtape)
>> 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.
(snip)
> 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.
I think you mean 0 2 4 6 8 9 7 5 3 1. Yes, as I understand
it that is what we are discussing, though, until now without
any reference to an actual system.
> 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.)
For 8 inch floppies, it was usual that they came preformatted.
CP/M, at least, did interleaving with a table that mapped
logical sector numbers to physical sector numbers.
For 5.25in floppies, it was usual that they didn't come
preformatted, (until the IBM PC got popular enough).
In this case, it was easy enough to renumber the physical
sector headers, so that is what was (usually) done.
Low-level formatting programs offered one to select
the interleave to be used.
-- glen
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