Re: Mystery DOS 3.3 mod.

[dempson@actrix.gen.nz (David Empson)]

Linards Ticmanis <ticmanis@coli.uni-sb.de> wrote:

> I found the following program under the name of "COPYA.ST" on a 1 Jan
> 1983 DOS 3.3 system disk image from the net:
> 
> 10 POKE 47397,24 (i.e. B925- 18 CLC)
> 20 POKE 47398,96 (i.e. B926- 60 RTS)
> 30 PRINT  CHR$ (4)"RUN COPYA"
> 
> Is that on the original 1983 DOS 3.3 Disk? And, dear DOS hackers,
> what's its purpose?

(I thought it was about time someone got around to answering this
question.)

I don't have a real machine handy to check the exact details, but a
quick look through Beneath Apple DOS reveals that these locations are
inside the READ routine in RWTS, somewhere near the end of it.

Carry is clear on exit if there is no error, so I expect this patch is
disabling one or more of the checks done by RWTS to validate the sector
it just read.  For example, it might be disabling the check of the
epilogue bytes and/or the checksum.

This patch is probably to allow the copying of a disk which has very
light copy protection.


I don't recall seeing a "COPYA.ST" on any standard DOS 3.3 system disk
from Apple, but I haven't looked at one since the late 80's.