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Path: utzoo!watmath!clyde!floyd!harpo!seismo!hao!hplabs!sri-unix!mknox@ut-ngp.ARPA
From: mknox@ut-ngp.ARPA
Newsgroups: net.micro.cpm
Subject: SELDSK bug in CP/M-86
Message-ID: <16898@sri-arpa.UUCP>
Date: Sun, 19-Feb-84 14:36:43 EST
Article-I.D.: sri-arpa.16898
Posted: Sun Feb 19 14:36:43 1984
Date-Received: Fri, 2-Mar-84 12:45:39 EST
Lines: 22


I just encountered a rather interesting bug in the IBM-PC BIOS implementation
of CP/M-86 (found it the hard way, of course).

   Perform a SELDSK BIOS call (BIOS call number 9) to a disk (it doesn't
   matter what one), specifying that it is a 'new disk'.  It will work
   correctly.

   Without doing any disk READ or WRITE functions, now do another SELDSK
   call for the same disk (again specifying that it is a 'new disk').
   The DISK parameter block returned will UNCONDITIONALLY specify that the
   disk is SINGLE DENSITY!!!

-----

Why would anyone do two disk selects in a row to the same drive?  One case
is an application program that selects a disk upon startup, and is then
instructed by the user to 'log in a new diskette in that drive'.  Since
the density and allocation map may have changed a fresh SELDSK is necessary.

Curiously, any read/write between the SELDSK function fixes the problem.
Other BIOS calls do not.