Sector Interleave

Fred Cisin cisin at
Tue Dec 1 11:43:20 CST 2015

On Tue, 1 Dec 2015, Paul Koning wrote:
> On the subject of DECtape, and "keeping good track of things" -- DOS 
> format DECtape has 510 bytes per tape block, the other two bytes are 
> used as the link word.  It's a bit like MSDOS FAT format (or CDC 6000 
> series, which did it 20 years earlier), but with the links in the blocks 
> rather than in a separate region.  The directory points to the first 
> block, and you get the next block address when you read the first.

Take a look also at Apple-DOS!
"Beneath Apple DOS" seemed to be the canonical reference to it.
IFF I'm remembering correctly, . . . 
DIRectory on track 17.
256 bytes per sector.
Each sector within a file has 252 bytes of data and a 4 byte pointer to 
the track and sector number of the next sector
GCR (with a different GCR pattern for the 13 sector V 16 sector disks)

It had a sector interleave, but inefficiencies in the OS (copying bytes 
to and from a buffer) resulted in the need to change the interleave a few 

There are prob'ly some here who remember all of the details.

I had little to no experience with it, other than writing the file 
system software to go with a crude flux-transition board ("Apple 
Turnover") to copy files from 13 and 16 sector Apple DOS, Apple CP/M, 
Apple Pascal, and Pro-DOS.
It never worked well.

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