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From: cjdb@sphinx.UUCP
Newsgroups: comp.sys.ibm.pc
Subject: DOS Function 42h (int 21h)
Message-ID: <1262@sphinx.UChicago.UUCP>
Date: Thu, 12-Mar-87 10:02:59 EST
Article-I.D.: sphinx.1262
Posted: Thu Mar 12 10:02:59 1987
Date-Received: Fri, 13-Mar-87 22:08:37 EST
Organization: University of Chicago Library Computer Systems
Lines: 21


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." (Programmer's Guide, p. 310). Ray Duncan
implies that with method codes 1 or 2, a byte offset from either the
current location or the end of file may be given as a positive or
_negative_ double integer (Advanced MSDOS, p. 345). He goes on to say,
"Using method 1 or 2, it is possible to set the file pointer to a
location that is before the start of the file." (p. 346) To me this
implies that the pointer can be moved backwards. 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.


-- 
"... ain't nobody's business if I do."		..!ihnp4!gargoyle!sphinx!cjdb  
	-- Billie Holiday     			    PMRCJDB@UCHIMVS1.Bitnet