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From: src@cup.portal.com (Steve R Calwas)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Magic Number Five in turbo C, MS C, and int 21/0E
Message-ID: <22974@cup.portal.com>
Date: 11 Oct 89 18:01:13 GMT
References: <483@schaefer.MATH.WISC.EDU>
Organization: The Portal System (TM)
Lines: 27

In article <483@schaefer.MATH.WISC.EDU> wayne@schaefer.MATH.WISC.EDU (Rick Wayn
e) 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.

RALF@CS.CMU.EDU (Ralf Brown) responds:
>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

I would like to add that the problem finding the actual number of drives on
a system began with DOS 3.xx.  Also, one method to determine how many drives
really exist is to take the returned value from the INT 21h/AH=0Eh function
(i.e. the 5 you keep getting) and try to get the current directory for each
drive.  If the drive does not exist, an error will result.

Steve Calwas                    src@cup.portal.com
Santa Clara, CA                 ...!sun!cup.portal.com!src