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From: ralf@b.gp.cs.cmu.edu (Ralf Brown)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Magic Number Five in turbo C, MS C, and int 21/0E
Keywords: drives, C programming
Message-ID: <6477@pt.cs.cmu.edu>
Date: 11 Oct 89 03:58:36 GMT
References: <483@schaefer.MATH.WISC.EDU>
Organization: Carnegie-Mellon University, CS/RI
Lines: 23

In article <483@schaefer.MATH.WISC.EDU> wayne@schaefer.MATH.WISC.EDU (Rick Wayne) writes:
}determine the number of disk drives on a machine at runtime.  there's a DOS
}interrupt service for this: interrupt 0x21, function 0x0E, "set default
}disk drive".  it's supposed to return the number of drives in the system.
}
}for all the machines we have here (AST Premium 386, Standard 286, Compaq
}Deskpro, Dell 210), we get the answer of...five.  needless to say, these
}disparate machines have differing numbers of drives.  the one number we
}CANNOT come up with, of course, is five.

The value you are seeing is the value of LASTDRIVE= in CONFIG.SYS, which
defaults to 5 if not present.  The value returned by INT 21/AH=0Eh is the
maximum of
	actual drives in system (including virtual drives)
	value of LASTDRIVE
	5


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