[Coco] Strange.... two 40 track disks on one 80 track disk?

[Christopher Hawks]

Fedor Steeman said the following on 10/31/2009 03:50 PM:
> Hi,
> 
> Some time ago I acquired this huge collection of diskettes that were part of
> a user group's CoCo software library. A lot of these are backups of backups
> so there is bound to be a lot of stuff I will be able to recover.
> 
> There are a large number of diskettes that seem to contain a backup of two
> 40 tracks diskettes on one side of an assumed 80 track diskette.
> 
> However, I can only access the first diskette of each of these backup
> floppies. I know because some of them have their contents listed on the
> envelope (or whatever you called the paper bag the diskette is in).
> 
> I have a drive that can be switched from 80 to 40 tracks but that made no
> difference.
> 
> Is there some way in HDB DOS that I am using (or RSDOS for that matter) that
> would enable access to the other 40 tracks and thus the other diskette
> backup?

Fedor:

	It may be that the other tracks are on the other side of the disk (not as in a 
flippy).

	On a Coco 3 (or a 1 or 2 in ram) with DECB 2.1 (1.1), POKE &HD89F, &H40 will 
make drive 2 the backside of drive 0. Likewise POKE &HD8A0,&H41 will make drive 
3 the back of drive 1. There a 4 memory locations that are the drive mask 
(&HD89D - &HD8A0) the relevant bits are:
bit 0 - drive select 0 (&H01)
bit 1 - drive select 1 (&H02)
bit 2 - drive select 2 (&H04)
bit 7 - side select    (&H40)

	These can be manipulated to access any side of any (physical) drive as any 
drive number.

-- 
Christopher R. Hawks
HAWKSoft
---------------------------------------------------------
Never meddle in the affairs of NT. It is slow to boot and quick to crash.
	-- Stephen Harris