Sector Interleave

Johnny Billquist bqt at
Tue Dec 1 12:04:44 CST 2015

On 2015-12-01 18:09, Paul Koning wrote:
>> On Nov 30, 2015, at 8:39 PM, Johnny Billquist <bqt at> 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).

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.


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