Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!utgpu!water!watnot!watmath!clyde!rutgers!husc6!seismo!sundc!rlgvax!vrdxhq!tom
From: tom@vrdxhq.UUCP
Newsgroups: comp.os.minix
Subject: Re: TCP/IP on minix
Message-ID: <3070@vrdxhq.UUCP>
Date: Tue, 3-Mar-87 15:23:16 EST
Article-I.D.: vrdxhq.3070
Posted: Tue Mar  3 15:23:16 1987
Date-Received: Fri, 6-Mar-87 04:09:42 EST
References: <1036@wang7.UUCP> <335f0fa7.9540@apollo.uucp> <396@faline.UUCP>
Organization: Verdix Corporation, Chantilly, VA
Lines: 15
Keywords: tcp ip minix
Summary: WAIT and POST


When the IBM AT BIOS does a disk read or write, it first outputs
a command to the command port of the disk controller.  It always waits
for this action (giving the command) to complete.  However, when it
actually starts the read/write operation, it makes a call to a routine
called WAIT (page 5-111) of IBM AT Tech Ref, and that routine does a
INT 15H to signal it's gone into a wait loop.  At this point, save the state
of the machine, and dispatch another task.  Then, when the disk operation
completes, it will issue another INT 15H (with a different code in AX).
Simply (ha ha!) restore the state of the machine and go back to where
you where in BIOS.  Be aware, however, that you need a real operating
system on top of this that can serialize disk requests because, though
some things like BIOS provide a little bit of re-entrancy with the
POSt-and-WAIT operations, the usual PC disk controllers are NOT 
"re-entrant" (i.e. only one I/O channel).