Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!att!pacbell.com!decwrl!mcnc!rti!bcw
From: bcw@rti.rti.org (Bruce Wright)
Newsgroups: comp.os.msdos.programmer
Subject: Re: Porting UNIX apps. to MS-DOS
Summary: Stack segments and interrupts
Message-ID: <1990Nov10.223333.4628@rti.rti.org>
Date: 10 Nov 90 22:33:33 GMT
References: <90296.205211DLV101@psuvm.psu.edu> <1990Oct26.223541.26634@NCoast.ORG> <1990Oct31.190520.11526@Octopus.COM>
Organization: Research Triangle Institute, RTP, NC
Lines: 48

In article <1990Oct31.190520.11526@Octopus.COM>, stever@Octopus.COM (Steve Resnick ) writes:
> In article <9449@jarthur.Claremont.EDU> dfoster@jarthur.Claremont.EDU (Derek R. Foster) writes:
> 
> >void StackSwapFunction(void)
> >{
> >  unsigned oldSS;
> >
> >  oldSS = _SS;
> 	
> 	You *MUST* disable interrupts during this operation. If an interrupt
> 	occures during the stack segment transition, you will not return
> 	from the interrupt correctly and crash the machine.
> 
> 
> >  _SS = FP_SEG(ABuffer+sizeof(ABuffer));

This is misleading.  It's not necessary to disable interrupts when
swapping stack segments if you do it right!  The point is that the
80x86 processors disable interrupts for one instruction after loading
the SS register (some of them disable interrupts for one instruction
after loading ANY segment register, but later processors limited it
to SS - I forget the exact details of which processors behave which
way because it's usually not important).  So if the next instruction
IMMEDIATELY after loading the new value into SS loads the new value
into SP, then you're golden and don't need to mess around with manually
disabling interrupts.  Look in any of the processor handbooks for the
Intel processors.  I don't know enough about Turbo C++ to know whether 
it gets in your way and inserts other instructions after the assignment
to SS, but it does work fine in assembler.

On the subject of swapping the stack, another place that it's often
needed is for interrupt service routines running in real mode - in
this mode the 80x86 processor does not perform the interrupt on a
separate stack but on whatever stack the application program is using.
An amazing number of application programs have allocated EXACTLY the
amount of stack that they are going to be using, and there isn't
enough room to even save the registers for the interrupt service
routine.  (Note that they may have taken into account the amount of
stack space required by any BIOS calls that they make, so that your
interrupt may be OK until you interrupt a BIOS call).  Anyway, if you
are writing a real mode interrupt handler it's a real good idea to 
swap stacks to a local stack while using as little of the application
program's stack as possible;  it can be useful to know that when
swapping in either direction you don't have to worry about whether
you have 80x86 interrupts enabled when you move the segment as long
as you do things in the proper order.

						Bruce C. Wright