CAM-1 r6 Annex A, PC BIOS m

MGauthier MGauthier at
Wed Mar 8 22:15:01 PST 1995

Subject:     CAM-1 r6 Annex A, PC BIOS mappings for capacity

Rev. 6 of the CAM working draft includes an informative Annex (A) that
describes, via a little C routine, how IBM PC BIOSes should map a SCSI
disk's capacity to an INT 13h geometry (number of cylinders, heads and
sectors).  This function returns a failure code if the returned number
of cylinders is zero.  However, there is no mention of what conditions
will cause a zero number of cylinders.  I endeavoured to find out, and
it turns out a zero cylinders is only possible if the disk's capacity
is itself zero.  This bit of information is likely useful for many,
and it also means the test made by the function is rather superfluous
(it is just as easy to test for a zero capacity before calling the
function).  I have an outline of a proof of this conclusion, if anyone
is interested.  Can anyone verify this?  Is this already known?

It might be mentioned in the next CAM revision (whether CAM-1 or CAM-3?).


Marc E. Gauthier
Software Engineering Lab, Institute for Information Technology (SEL,IIT)
National Research Council Canada, Bldg M-50,  Ottawa ON Canada  K1A 0R6
+1 613 991 6975   fax:  +1 613 952 7151   email:  mgauthier at

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