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From: duke@gitpyr.UUCP
Newsgroups: comp.sys.ibm.pc
Subject: Re: How do you do concurrency?
Message-ID: <3384@gitpyr.gatech.EDU>
Date: Fri, 3-Apr-87 23:20:33 EST
Article-I.D.: gitpyr.3384
Posted: Fri Apr  3 23:20:33 1987
Date-Received: Sun, 5-Apr-87 08:52:26 EST
References: <1328@erix.UUCP>
Organization: Georgia Institute of Technology
Lines: 30
Summary: Concurrency on an IBM PC

In article <1328@erix.UUCP>, mike@erix.UUCP (Mike Williams) writes:
> Several systems like MS-WINDOWS or GEM allow users to run several
> processes concurently in MS-DOS. By this I mean allow several processes
> to time share the CPU:
> 
> How is this done? I know the basic principles of time sharing operating
> systems, this question really means, how is this done in MS-DOS? Are
> there any available software procedures which allow one to do this
> oneself? 
> 
 
I can't speak for how Windows does multitasking, but I can describe a 
method which will work.  If you steal an interrupt, such as the timer 
interrupt, and have the interrupt vector point to some machine code you 
have written to save the contents of all registers, then you will 
successfully have saved the context of the currently running process. 
The interrupt will save the contents of the instruction pointer and 
flags on top of the current stack.  To save the contents of all 
registers, simply push them on top of the current stack.  Then you need 
only remember the stack segment (SS) and stack pointer (SP).  To 
restore the context of a process, all you need to do is load the SS 
and SP and then pop all registers off the stack in the reverse order you
pushed them, and then issue a return from interrupt.  On a timer
interrupt, you would simply swap the context of a process to 
one which is currently waiting to execute.  Obviously, this would mean 
you must initialize the stack of a process upon creation to look as 
though it has just had its context saved following an interrupt.  I'm 
not sure if I explained the process very well, but I hope you get the 
general idea.  If anything is unclear or you have any questions, I will
be more than happy to expand on anyting I have said.