Using a 3.5" 720k disk...

Philip Pemberton classiccmp at
Sun Apr 19 14:13:48 CDT 2009

Chuck Guzis wrote:
> I also considered an "embedded sector" solution wherein a disk would 
> be pre-formatted with a unique pattern) that could be easily detected 
> by external circuitry, conditioned by a  timing window.  (Isn't that 
> called an "Address Mark? :) )

The issues as I see them are:
   - If you use the B-side of a 5.25" DSDD disc, you need something to write 
that formatting
   - If you add something to sync to the index marks and generate a 
hard-sector "emulation" index pattern from that, you can't put a HS disc in 
that drive

What about just detecting the speed of the input index pulse? If they're 
turning up faster than, say, once every quarter-second, then bypass the 
index-pulse multiplier. Basically have a multiplexer that allows the output to 
be sourced from /Index_In, or /Index_Generated.

> Whether or not the system using it would over-write the pattern is 
> another issue.

True. You'd probably have to jack into WRGATE and force-disable it to stop the 
address mark getting overwritten.

And you still need to build something to format these crazy discs. And I'd 
appreciate it if someone was kind enough to document the format :)

> But there's more than one way to skin a cat, eh?

Depends if you want the skin, the meat, or just a Mythbusters-style big 
explosion... <very_big_grin> :)

classiccmp at

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