1 Dec
2015
1 Dec
'15
9:25 a.m.
On Tue, Dec 1, 2015 at 9:09 AM, Paul Koning <paulkoning at comcast.net> wrote:
scheme which
was based on using multiple sectors with the same sector
number. Apparently the floppy controller put sector number information in a
date preamble when formatting the floppy. The scheme (somehow) overwrote
the preamble on some of the sectors, changing their number; the track
contained something like:
1 2 3 4 5 6 7 4
This meant that a command to "read sector 4" would return whichever sector
4 passed under the head first. If you did 'read sector 2', 'read sector
4'
you would get the first one; 'read sector 6', 'read sector 4', you would
get the second.
Interleaving for obfustication, not efficiency.
-- Charles
On Nov 30, 2015, at 8:39 PM, Johnny Billquist
<bqt at update.uu.se> wrote:
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...
True. Interleaving, as described in this thread, is typically a software
function; the software uses the blocks in an order different from the
"ascending by 1" natural ordering.
I suppose it's possible to do something like interleaving where
consecutive sector addresses are not physically adjacent on the media.
Come to think of it, that's exactly what the MSCP RX50 controllers do,
since MSCP implements the mapping from LBA to physical addresses in the
controller, not the host. But in older systems where the controllers
handle physical addresses and the mapping from LBA is in the driver,
interleave is handled there (or above).
A long time ago, I reverse-engineered an Atari 800/400 copy protection