On Tue, Dec 1, 2015 at 9:09 AM, Paul Koning <paulkon...@comcast.net> 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). > > A long time ago, I reverse-engineered an Atari 800/400 copy protection 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
