Sector Interleave

[Chuck Guzis]

On 12/01/2015 09:25 AM, Charles Anthony wrote:

> 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.

A particularly cute one was employed with early versions of Harvard 
Graphics for the PC.  A protection key was encoded in the gap between 
sectors.  Since the PC 765 controller writes individual sectors (i.e. 
looks for a IDAM, then a DAM, then turns on Write Gate), it was very 
difficult (but not impossible) to replicate this stuff, as using a 
regular write command would cause the data separator to lose sync in the 
write splice area.  On the other hand, it was pretty simple for a 
WD-17/27 type FDC to duplicate this.

--Chuck