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From: ast@botter.UUCP
Newsgroups: comp.os.minix
Subject: Re: More on polled keyboards
Message-ID: <1071@botter.cs.vu.nl>
Date: Tue, 17-Feb-87 14:35:07 EST
Article-I.D.: botter.1071
Posted: Tue Feb 17 14:35:07 1987
Date-Received: Thu, 19-Feb-87 06:23:45 EST
References: <1070@botter.cs.vu.nl> <1236@husc6.UUCP>
Reply-To: ast@cs.vu.nl (Andy Tanenbaum)
Distribution: world
Organization: VU Informatica, Amsterdam
Lines: 12

I would be curious to know what kind of kludge is needed to make the following
work in the normal way.  When the operating system wants to read a disk block,
it calls the BIOS.  The BIOS then issues a disk command.  The question is
how does the operating system arrange for the BIOS to release control so
that the OS can switch to another process while the operation is being carried
out?  This is the essence of multiprogramming, after all.  In the IBM Technical
Reference Manual BIOS listing, after the BIOS has issued a hardware command, it 
goes to line 2916 where it sits in a tight little loop (lines 2916 to 2927) 
until the interrupt happens.  It does not give control back to the operating 
system so that another process can be run while waiting for the interrupt.

Andy Tanenbaum