[Coco] Access to Double Sided Disks with standard Floppy

[Barry Nelson]

Hello Art! Glad to see you are still kicking around! :) I thought I would add that some time ago, Art sent me a copy of the original ADOS for the CoCo 1/2 which retrieved completely and without error from the aging floppy disk and created a disk image of it, with his permission. ADOS and ADOS3 are available for download from http://www.colorcomputerarchive.com/coco/Disks/Operating%20Systems/
ADOS can be loaded from disk on a CoCo 1/2 with 64K of RAM or ADOS3 can be loaded on a CoCo 3. It comes with a configuration program to setup how the double sided drives are accessed. 

On Apr 21, 2015, at 1:18 AM, coco-request at maltedmedia.com wrote:

> Date: Tue, 21 Apr 2015 01:06:32 -0400
> From: Arthur Flexser <flexser at fiu.edu>
> To: CoCoList for Color Computer Enthusiasts <coco at maltedmedia.com>
> Subject: Re: [Coco] Access to Double Sided Disks with standard Floppy
> 	Drives on the CoCo?
> Message-ID:
> 	<CA+LuDceC8rTZbMgtTpedhzNsTXvTTioYSzDQaGOUf6jE1z9Rpw at mail.gmail.com>
> Content-Type: text/plain; charset=UTF-8
> 
> A problem with the simple 2-poke method of getting double-sided drives is
> this:  Disk Basic still maintains a 4-byte table that keeps track of what
> track the drive head is on for 4 drives.  Since there are in fact only 2
> drives, Disk Basic is unaware that the head position for Drive 2 is always
> identical to that for Drive 0 and similarly for Drives 1 and 3.  This
> causes the drive head to sometimes bump against the Track 0 stop in trying
> to get to a new track, because the system is working with erroneous
> information about what track the head was previously on.
> 
> Because of a bug in the controller chip's logic, when this happens, both
> the current track and (inappropriately) the destination track get set to
> zero.  Since the two match, the system thinks the head has reached its
> destination and the operation gets performed on Track 0 instead of the
> appropriate track.  A typical outcome is that the GAT gets written to Track
> 0 Sector 2 instead of Track 17 Sector 2, and shortly thereafter, you see a
> ?FS ERROR from Basic when you try to access a file.
> 
> Happens only now and then, but enough to trash a significant number of
> diskettes if you're not careful.  (Recovery of the diskette may be possible
> by copying the GAT from Track 0 to where it belongs.)
> 
> A proper implementation (such as is found in ADOS-3) uses a 2-byte drive
> table instead of a 4-byte one.
> 
> Art