Path: utzoo!attcan!uunet!bu.edu!purdue!mentor.cc.purdue.edu!l.cc.purdue.edu!cik
From: cik@l.cc.purdue.edu (Herman Rubin)
Newsgroups: comp.arch
Subject: Re: Computer Architecture methodology
Summary: Some examples
Keywords: A Series, B6700
Message-ID: <2328@l.cc.purdue.edu>
Date: 9 Jul 90 02:44:15 GMT
References: <14279@burdvax.PRC.Unisys.COM> <844@tredysvr.Tredydev.Unisys.COM> <3627@auspex.auspex.com>
Distribution: comp.arch
Organization: Purdue University Statistics Department
Lines: 64

In article <3627@auspex.auspex.com>, guy@auspex.auspex.com (Guy Harris) writes:
> >Users can make use of weird instructions.
> 
> If users can make use of any arbitrary "weird instruction", what
> prevents the desired instruction set of a machine from being unbounded
> above?  Eventually you have to choose *some* that you won't implement,
> as a machine that implements every possible operation as a single
> instruction is clearly unimplementable (few infinite items are
> constructable in the real world :-)).

			.............................

> >You will find that there are good applications uses of most of them.

		[Example of weird instruction omitted.]

> If you can't, then perhaps there are *some* "weird instructions" that
> have no good application uses, and therefore, you do need some way of
> choosing which "weird instructions" should go in anyway and which should
> be deleted.  Perhaps a practical suggestion of exactly such a way might
> have more effect on the designers of hardware and HLLs than a series of
> complaints that they're just not doing things right with no real
> suggestions as to how they might do them better?

Admittedly there are weird instructions which are at least extremely difficult
to justify.  In fact, I would even argue that some of the "instructions" on
floating point chips, such as the transcendental functions, are nothing more
that programs encoded in microcode.  But there are lots of reasonable hardware
instructions which have either disappeared or were rarely implemented.

Examples of simple instructions in hardware, much more expensive in software,
and for which I know of "reasonable" applications.

	Multiplication of integers with both most and least significant parts
	of the product available

	Division with quotient and remainder simultaneously

	Division of floating point numbers, with integer quotient and
	floating point remainder

	In the above two operations, allowing the choice of which quotient
	and remainder, depending on the signs of the arguments.

	Obtaining the spacing between the ones in a bit sequence.  In the
	algorithms I would produce, this can be a major operation.

	The use of overflow and carry tests.

	Fixed point arithmetic.

	Multiplication of a floating point number by a power of two, not
	using the multiply unit

	Better conversion between integer and floating point.  There is one
	major computer company which produces huge numbers of mainframes which
	do not have any hardware conversion capabilities whatever.  Not only
	that, there was plenty  of room for additional instructions when the
	line was started, and the company's own scientific people complained
	about the problem.
-- 
Herman Rubin, Dept. of Statistics, Purdue Univ., West Lafayette IN47907
Phone: (317)494-6054
hrubin@l.cc.purdue.edu (Internet, bitnet)	{purdue,pur-ee}!l.cc!cik(UUCP)