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Combined .com executable/DOS driver (Announce)

posted by Ninho E-mail, 12.06.2010, 15:49

>> Actually, when loading a driver, MS-DOS (others ?) check only the
>> segment part (high word) of this double word for the special
>> value FFFFh.

> Interesting. So have you produced or seen a combined SYS/COM previously?
I've "produced" a lot of the kind, they come out handy. Others may have found and used the same trick independently, though I have not seen any.

> Ok, I fully understand. e.g. using EB1Eh, FFFFh instead of FFFFh, FFFFh.
Correct. And re-patch FFFF in place of EB xx before returning to DOS.

> Unfortunately one small point I have noted already is if someone also
> wants to use UPX that UPX (at least v3.05w) flags up the following error
> (as you would expect) if the header on a SYS is not set to FFFFh FFFFh:
> upx: xmgr3.sys: UnknownExecutableFormatException
Good to know. I don't pack my drivers - they've been generally small enough by themselves :=)

> Yup. As the code will be short I might even give this a try this evening.
Y.W.! Watch out the necessary segment adjustments between .sys and .com formats, to compensate for the DOS PSP in the latter.

---
Ninho

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