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From: tsc2597@acf4.UUCP (Sam Chin)
Newsgroups: net.micro.pc
Subject: Re: Summary about DOS and Interrupts
Message-ID: <1050033@acf4.UUCP>
Date: Sun, 12-May-85 21:19:00 EDT
Article-I.D.: acf4.1050033
Posted: Sun May 12 21:19:00 1985
Date-Received: Tue, 14-May-85 07:54:52 EDT
References: <84@tekig5.UUCP>
Organization: New York University
Lines: 32

<>
Regarding the question about calling interrupts from within interrupts:


Officially you can't because DOS keeps some local variables and is thus *not
reentrant*.  Even the bios is non reentrant (on the AT too so XENIX has to
completely bypass the BIOS).  Consider this bios example.  Two processes A
and B are doing access to the screen through interrupt 10h.  In order for
the bios to work, it must keep track of the current cursor position and
update the appropriate location in screen ram.  It must therefore have local
variables Row and Column indicating where the current cursor is.  If both
process A and B are updating the screen and are time sliced such that
process A loses control in the middle of a screen update, process B will
come along and clobber the Row and Column variables such that when process A
gains control it is no longer at the place it was supposed to be.  Another
example, you write an interupt routine that gets triggered whenever a
character is recieved on the serial port.  In the middle of a DOS interrupt,
DOS itself gets interrupted by a character arriving and your interrupt
routine gets control.  When DOS was interrupted it was in the middle of
doing some operations and was in the middle of updating some local variables
it keeps.  Now your interrupt routine issues the DOS interrupt again and
this clobbers those buffers which DOS was in the middle of processing.  When
you issue the IRET you get a branch back to where DOS was executing but its
local variables were clobbered so your original program may be in a strange
state.  If you are not dealing with hardware interrupts, it may be OK but
you cannot be guarenteed that it will work for sure.  The 8086/88
architechture allows for nested interrupts by saving return addresses on a
stack but the limitation is with MS-DOS and the BIOS.

Sam Chin
allegra!cmcl2!acf4!tsc2597
tsc2597.acf4@nyu