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From: chris@mimsy.UUCP (Chris Torek)
Newsgroups: comp.arch
Subject: Re: Available no. of registers
Message-ID: <5151@mimsy.UUCP>
Date: Thu, 22-Jan-87 14:48:59 EST
Article-I.D.: mimsy.5151
Posted: Thu Jan 22 14:48:59 1987
Date-Received: Thu, 22-Jan-87 21:45:53 EST
References: <3810002@nucsrl.UUCP> <926@mips.UUCP> <759@vaxb.calgary.UUCP> <1029@cuuxb.UUCP>
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
Lines: 49

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.

Legal indeed, but the result is undefined.  Taking the address of
an adressable object is all right, and offsetting that is fine;
but no mention is made of where the resultant pointer points, if
anywhere.

>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;

How so?

>it is also used to some great extent in the older (v6 & v7) unix kernels.

The kernels have many instances of code such as

	struct {	/* ioctl arguments, e.g. */
		int	fd;
		int	cmd;
		caddr_t	addr;
	} *uap;

The kernel, however, is one of those inherently machine specific
things; it is allowed to cheat.  This cheating is (generally) very
carefully controlled so as not to cause trouble with the compiler.

There is nothing in C that requires that independently declared
variables be adjacent in terms of pointer arithmetic.  Arrays must
be so, but on, e.g., a Pyramid, the thirteenth scalar stack variable
is nowhere near the first twelve:

	f()
	{
		int a, b, c, d, e, f, g, h, i, j, k, l;	/* registers */
		int m, n, o;	/* stack */
		...

As the registers in the Pyramid are addressable, the compiler simply
puts the first twelve variables that fit into the twelve free local
registers.  (There are twelve standard parameter registers as well.)
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7690)
UUCP:	seismo!mimsy!chris	ARPA/CSNet:	chris@mimsy.umd.edu