From: "greg andruk" <gja@meowing.net>
To: <9fans@cse.psu.edu>
Subject: RE: [9fans] Random Question
Date: Tue, 21 May 2002 06:05:41 -0400 [thread overview]
Message-ID: <000001c200af$13da8500$0400a8c0@k0deZ> (raw)
In-Reply-To: <8f6cf824.0205201813.34db153f@posting.google.com>
Don wrote:
> Heyas,
> I just finished writing most of the MBR
> for my Autumn OS and began reading through FS
> documentation to help me develop my FS. Now.. an
> intereting notion crossed my mind while reading
> the FAT specification by Microsoft. If byte zero
> of a BPB jmp_Boot is 0xEB (the preference is this
> short jmp according to the specs) the third byte
> _must_ be a NOP (0x90).
Right.
> Now, I remember reading
> in several other operating system's comments
> that some BIOS actually bail out if there isn't
> a jmp short followed by a nop in the first three
> bytes of code. Noone seemed to know why the
> BIOS needed to see that nop there.
It's historical identification cruft.
2 bytes of Short jump + Nop -> DOS 3 or later
3 bytes of near jump = DOS 2
DOS 1 didn't have a BPB structure.
> Could this
> have been a tactic used by Microsoft to attempt
> to thwart other Operating Systems from occupying
> an IBM PC?
Doubtful; a documented 3 byte signature isn't much of a hurdle.
> I mean, if the BIOS is designed to check
> for a documented FAT specification, wouldn't that
> be a clue that Microsoft coerced the manufacturers
> to implement the check? Anyone out there have a
> comment on this issue?
Smells more like a buggy attempt to guess at what partitioning scheme
might be in place.
> Don
next prev parent reply other threads:[~2002-05-21 10:05 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2002-05-21 8:47 Don
2002-05-21 10:05 ` greg andruk [this message]
2002-05-22 8:52 ` Don
2014-07-03 11:44 [9fans] random question Nick Owens
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