[Coco] 2 Sides in VCC

[Robert Gault]

Luis Fernández wrote:
> Curiously in the VCC is can create virtual disks of 2 faces, and 80 track, although only be formatted 35 track and 1 Sided
> I have 2 questions
> 1) when we create 2 Sided the VCC adds 2 bytes at beginning of disk, with & h12 y & h02, of course are 18 sectors per track and two sides. when one side is selected, adds nothing.
> I wonder if this method is standard or VCC is somewhat unique, in other words, it uses some other emulator, or a disc burning utility real? someone uses this header 2 bytes?
> 2) even if the disc do 80 track (written with zeros) Vcc always formatted only 35 tracks 18 sectors per track (filled with 255), and when both sides are jumping the track 36 sectors, this is logical as this represent both sides in other virtual disks,

You need to understand the structure of a virtual double sided disk. No virtual 
disk can have two sides as it is a file not a disk. That means you need to 
choose one of two formats to simulate two sides. You can either have the first 
side the first half of the file or interleave tracks from each side so that the 
first side of the disk is every odd track and the second side is every even 
track in the file.
The virtual disks for all Coco emulators use the every other track to emulate 
double sided disks.

> Now,
> What is the impediment to the DECB can use 80 track and / or 2-sided
> Let's see: 80 track physically need the appropriate controller and drive 5 1/4 or 3 1/2 right. DECB level but I see no impediment, would track 80 - 1 for directory => 79 * 2 = 158 clusters, with the dir in the track 17, the fat would not have disabilities and you can use up to 191 cluster smoothly, with regarding dir entries (currently 9 sectors * 256/32 = 72 entries), this is easily expandable to 14 * 256/32 = 112, or twice if you use only 16 bytes per entry, but let in 112 for compatibility
> I mean this in no nitros DECB, the basic internal buffers if would not be sufficient.
> In some emulators are used drive 0 = Side A and Side B = Drive1 when are two sides, and 36 sectors per track, I have some confusion in all this
> I think the controller J-DOS can handle disks of 80 track and need a DOS different
> But the truth not seen him impediment of that DECB or RGBDOS or HDB-DOS maintain 80 track

Any double sided drive and controller that accesses double sided drives can be 
used with DECB with a caveat. The drive sides are used as individual disks not 
true double sided drives.
The problem with using true double sided disks it the DECB code and the required 
disk variables and data tables in low RAM, $600-$927. You would need a complete 
rewrite of the DECB code to handle true double sided disks.

With the advent of DECB hard drives and emulators, there is little advantage to 
using true double sided disks with DECB. It is fairly easy to write ml programs 
to access disks formatted with 512-byte sectors and track/sector ID numbers 
outside the range of normal Coco disks. That was used in some copyright schemes 
and my disk editor program will read these disks.

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> Excuse my English, I use google translator, my language is Spanish, I'm Spanish but I live in Venezuela
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