Re: Mystery DOS 3.3 mod.

[mjmahon@aol.com (Michael J. Mahon)]

David Empson answered:

>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.

Exactly.  I looked in my handy RWTS disassembly and find that this
patch eliminates checking of the data checksum and epilogue bytes.

To bypass checking both address and data checksum and epilogue
bytes, patch B942: 18 (CLC).  (This also bypasses all other read checks,
so the data buffer may be unchanged, if, for example, the address cannot
be found or the data prologue bytes fail to match.)

-michael

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