cclist at sydex.com
Tue Dec 1 12:09:58 CST 2015
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.
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