On 2015-12-01 18:09, Paul Koning wrote:

On Nov 30, 2015, at 8:39 PM, Johnny Billquist <b...@update.uu.se> wrote:

On 2015-12-01 02:19, Paul Koning wrote:

On Nov 30, 2015, at 8:12 PM, Johnny Billquist <b...@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).

Yes, the "hardware" interleave is what I was assuming everyone here was talking about. Otherwise there is no point/need to format to get the interleaving... With disks, this is perfectly doable, as the block number is in the block header, and the drive/controller scans headers until the current block passes by. There is no real reason to actually place the disk blocks in consecutive physical order on the disk. Any order will work. That's what soft sectors gets you.

And in addition to the interleaving someone also mentioned that you can stagger blocks between tracks, so that you have time for a track switch and then hit the next sector in optimal time, if you really work at it.

        Johnny

Reply via email to