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

Christopher Hawks chawks at dls.net
Sat Oct 31 21:25:03 EDT 2009

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?


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

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

More information about the Coco mailing list