[Coco] Sierra Xmas 86 demo

[Gene Heskett]

On Monday 10 December 2012 00:15:39 Stephen H. Fischer did opine:

> Hi,
> 
> What should DNS be for my 80 Track 2 sided 5.25" Disks?
> 
> If you look at the definition for 3.5" disks, the DNS should be DNS = 3.
NO!  not for a 3.5" disk,
> 
>  I had A 80T2S 3.5" drive and a 5.25" 80T2S which I swapped /D0 and /D1
> with a switch.

Bad show, see below.
 
> But most people having only 3.5" 80T2S drives might well have DNS =1.

Which is 100% correct.
> 
> What a mess Charlie Brown. Your rule "It should dns=3 for 3.5 and dns=1
> for 5.25" just does not work.
> 
> 
That is NOT how it works Stephen.  This whole dns tomfoolery was because 
5.25" drives came in 48 tpi for DS40 (360k) disk drives, and 96 tpi for the 
80 track (720k) drives.

DNS=03 is ONLY for use against 80 track 5.25" disks.  The idea being that 
should a 48 track disk be inserted into the 96 tpi disk drive, and the 
driver finds a DNS=01 disk in this drive when the drive descriptor is set 
DNS=03, it is smart enough to double step the drive and thereby READ that 
40 track disk in the 80 track drive.

DO NOT, REPEAT, DO NOT EVER WRITE to that disk, it will be trashed beyond 
recovery because the 96 tpi head doesn't cover the full track width, 
leaving scrambled data behind.  One can occasionally rescue the data by 
backing it up to a real 48tpi drive from that drive, but unless that 80 
track drive is spotlessly clean & well lubed so the head placement is very 
repeatable, the chances aren't great.

This one item I have been thinking about fixing, because when the wide 
track diskette is put in the narrow track drive, in addition to double-
stepping the drive, the driver is a ducking fummy for not also setting that 
disk as write protected, thereby preventing such disasters.  That might be 
one of my projects this winter.

3.5" drives have only one track density, 135 tpi, so there is not a double 
track density available, so ALL 3.5" drive descriptors should be set for 
DNS=01.

Cheers, Gene
-- 
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
My web page: <http://coyoteden.dyndns-free.com:85/gene> is up!
Intel Inside, Idiot Outside.
I was taught to respect my elders, but its getting 
harder and harder to find any...