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From: stauffer@enme.UCalgary.CA (Ken Stauffer)
Newsgroups: comp.sys.tandy
Subject: Tandy 6000 stack probes - question
Message-ID: <1990May31.173301.25856@calgary.uucp>
Date: 31 May 90 17:33:01 GMT
Sender: news@calgary.uucp (Network News Manager)
Reply-To: stauffer@enme.UCalgary.CA (Ken Stauffer)
Organization: U. of Calgary, Calgary, Alberta, Canada
Lines: 34


	Does anyone understand how the stack-probes are implemented
	on a Tandy 6000 running Xenix? In particular how does
	Xenix increase the stack space of a process, and how
	does the 'tstb' instruction (see below) help xenix
	accomplish this? Also, is the stack space of a process
	fixed, or can it grow as large as available memory?

	Below is assembly of the header of a C function, generated
	by the C compiler. The highlighted line is the stack probe.
	C functions which are compiled with the -K option
	(no stack probes), do not generate this line.

	The constant LM1, seems to be related to the amount of stack
	space needed by the function for calling other functions.

		        .data
		        .text
		        .globl  _f
		_f:
		        link    a6,#-.LF1
>>>>>>>>	        tstb    -.LM1-132(sp)	<<<<<<<<<<<
		        moveml  #.LS1,-(a7)

	I am writting an alloca() function which increases the
	stack frame size of the caller of alloca(), I am running
	into a problem with large alloca requests which get
	a segmentation violation. I was thinking that I
	could have my alloca routine do the nessesary
	'tstb' equivilant, and thus ensure proper stack growth...

	Thanks,

	Ken Stauffer.