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Path: utzoo!mnetor!seismo!lll-lcc!pyramid!amdahl!amdcad!tim
From: tim@amdcad.UUCP (Tim Olson)
Newsgroups: comp.arch
Subject: Re: Available no. of registers
Message-ID: <14444@amdcad.UUCP>
Date: Thu, 22-Jan-87 21:37:18 EST
Article-I.D.: amdcad.14444
Posted: Thu Jan 22 21:37:18 1987
Date-Received: Fri, 23-Jan-87 21:22:41 EST
References: <3810002@nucsrl.UUCP> <926@mips.UUCP> <759@vaxb.calgary.UUCP> <1029@cuuxb.UUCP>
Organization: AMDCAD, Sunnyvale, CA
Lines: 33
Summary: legal is not necessarily correct...

In article <1029@cuuxb.UUCP>, mwm@cuuxb.UUCP (Marc W. Mengel) writes:
+---------------------------------------
| Unfortunately, you don't have to take the address of a given variable
| to use it, you merely have to take the address of a variable near it
| and add an offset to it.  The way C is defined, this is quite legal.
| 
| For example, suppose I have a function f, declared as follows:
| 	f(p)
| 		struct { int a, b, c; } *p;
| 	{
| 	...
| 	}
| And I call it as follows:
| 	b()
| 	{
| 		int a, b, c;
| 
| 		f( &a );
| 	}
| According to our venerable friends kerningham&ritchie, this is legal;
| it is also used to some great extent in the older (v6 & v7) unix kernels.
+---------------------------------------

What is legal is not necessarily correct.  The above will not run on Vaxen
with pcc (or, for that matter, any machine/compiler combination where stacks 
grow in a direction opposite that of global variables).  Anyone who uses
such a construct should be put out of our misery.

[ By the way, I first thought that this wouldn't work on any machine I knew
of, but it *actually works* on an IBM RT-PC!  Really! ]

	-- Tim Olson
	Advanced Micro Devices