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Path: utzoo!mnetor!seismo!rutgers!mit-eddie!interlan!backman
From: backman@interlan.UUCP (Larry Backman)
Newsgroups: comp.sys.ibm.pc
Subject: Re: DOS Function 42h (int 21h)
Message-ID: <124@interlan.UUCP>
Date: Mon, 16-Mar-87 09:10:04 EST
Article-I.D.: interlan.124
Posted: Mon Mar 16 09:10:04 1987
Date-Received: Tue, 17-Mar-87 01:40:41 EST
References: <1262@sphinx.UChicago.UUCP>
Reply-To: backman@interlan.UUCP (1014-Larry Backman)
Organization: MICOM-Interlan, Boxborough, MA
Lines: 29
Keywords: FSEEK, INT 21, FUNCTION 42

In article <1262@sphinx.UChicago.UUCP> cjdb@sphinx.UChicago.UUCP (Charles Blair) writes:
>
>Simple question: has anyone used this function to move a file pointer
>backwards? Peter Norton says "The byte offset in CX:DX is a 32-bit
>unsigned long integer."
>However, I've tried
>moving the pointer a few bytes backwards with method 1, and although
>the negative sign flag was on, DOS function 3Fh (read file) seems
>never able to find any more data to read (that is, it's as if the file
>pointer had been moved past the end of file).
>
>Any enlightenment as to how this really works would be welcome.
>
>
>

	Through a glass darkly ....

	A few years ago I had reason to write a set of file system primitives
	that randomly accessed a file.  I remember having problems moving
	the pointer backwards, getting disgusted with DOS, cursing , and
	resorting to brute force.. I ended up always moving the file pointer
	to byte 0 of the file and moving forwatd to the appropriate place
	within the file.  This was under DOS 2.0 and 2.1, I too am curious..
	you should be able to back up a file pointer! 


						Larry Backman
						Micom - Interlan, Inc.