From: Michele Manzato <aragorn AT dei DOT unipd DOT it>
Newsgroups: comp.os.msdos.djgpp
Subject: Creating and using a protected stack area: how?
Date: Fri, 10 Apr 1998 13:25:02 +0200
Organization: BBS - Universita' di Padova - Italia
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To: djgpp AT delorie DOT com
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I am writing a multithreading library for DJGPP, so I have to manage stack
switches from a thread to another thread. Obviously the stacks are private
to the thread and are allocated via malloc(). Stack switching works
very vell... as long as I use DS as SS.

Now I want to have a protected stack for every thread, so I want to set up
a selector for the thread's private stack. This is the code I wrote, I
hope it is self explaining: 

	// stack base is stack_base (long *)
	// stack length is _stackLength (int)
	// stack pointer is stack_pointer (long *)

	unsigned long	baseaddr;
	short		sel;
        int		r;
	unsigned short	SS;
	unsigned long	ESP;
                        
        r = __dpmi_get_segment_base_address(_my_ds(), &baseaddr);
        baseaddr +=  (unsigned long) (stack_base);
        sel = __dpmi_create_alias_descriptor(_my_ds());
        r = __dpmi_set_segment_base_address(sel, baseaddr);
        r = __dpmi_set_segment_limit(sel, _stackLength);
        
        SS = sel;
	ESP = (long *) ((long) stack_pointer - (long) 
                             (stack_base));
        
then when I have to switch the stack I simply copy SS to %ss and ESP to
%esp (with an assembly procedure). No way! Debugging the program I can
see that the stack is OK (it contains what it must) and the program
runs correctly until it finds a 'retf' instruction. Then it returns... to
nowhere, the debugger only shows rubbish. Please note that without using
the new SS:ESP it works fine. I suspect that changing SS influences CS
as well, or at least it influences the way 'retf' works... but I don't
know how.

Can anybody help me? Any suggestion will be greatly appreciated.

	Ciao (bye)

					Michele

-- Michele Manzato -------------------------------------------------------
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