[Coco] DECB DOS Command

[Arthur Flexser]

I would say that the weird use of SWI3 is because the DOS command, unlike
any other part of any of the CoCo ROMs, was written for Disk Basic 1.1 by
Mark Siegel at Tandy (not Microsoft), who basically didn't know what he
was doing, as you can see from the extraordinarily sloppy and convoluted
code of that command.

The bug you mention may just possibly be a "feature" that was a one-byte
last-minute change added to allow the DOS command to operate on a drive
other than zero by such workarounds as DIR 1:DOS.  Or, it may indeed be a
bug, as it is labelled in the Unravelled series.  (Booting OS-9 from a
drive other than 0 also requires a patch to the relevant OS-9 routine.)

Art

On Sat, 13 Dec 2003, (tim lindner) wrote:

> I was just reading the Disk BASIC unravelled book and came away with the
> following question:
> 
> Why does the 'DOS' command first issue a SWI3 instruction?
> 
> Disk BASIC sets up the SWI3 vector to do nothing. It just returns to the
> caller. 
> 
> Also there is a bug noted in the DOS command: It will read track 34 from
> the last drive used. It is documented to only read track 34 from drive
> zero. But drive zero is just what DECB defaults to.
> 
> -- 
> tim lindner
> tlindner at ix.netcom.com                                            Bright
> 
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