Path: utzoo!yunexus!geac!geaclib!daveb
From: daveb@geaclib.UUCP (David Collier-Brown)
Newsgroups: comp.arch
Subject: Lisp "future" instruction in 88k hardware.
Message-ID: <3385@geaclib.UUCP>
Date: 5 Nov 88 15:37:07 GMT
Article-I.D.: geaclib.3385
References: <7472@winchester.mips.COM>
Organization: GEAC Computers, Toronto, CANADA
Lines: 41

From article <7472@winchester.mips.COM>, by cprice@mips.COM (Charlie Price):
> From the architecture spec:
> 
>   Multiply and divide operations are performed by a separate,
>   autonomous execution unit.  After a multiply or divide operation
>   is started, execution of other instructions may continue in parallel.
>   The multiply/divide unit continues to operate during cache miss and
>   other delaying cycles in which no instructions are executed.
> 
>   The number of cycles required for multiply/divide operations is
>   implementation-dependent.  The MFHI and MFLO instructions are
>   interlocked so that any attempt to read them before operations
>   have completed will cause execution of instructions to be delayed
>   until the operations finishes.

  Some of you may recognize this set of behavior as substantially
identical to the "future" function in experimental lisp(s):
  Future takes a computation and returns a function which, when
called later will do one of:
	1) return the result immediately, because it has been computed
	   concurrently and the computation has completed, or
	2) block until the concurrent computation is finished, or
	3) compute it itself if it could do it concurrently.

  The significance of this should be apparent: here is a RISC
instruction which can be decomposed into two, independently useful
instructions:
	1) floating accumulator move (of course)
	2) wait for concurrent computation to complete...

  If the manufacturers see fit to write a generalized MFHI/MFLO
instruction, they have a compiler-visible "hook" for not just
special-purpose coprocessors, but general-purpose parallellism.
  Shall we call it MAHI/MALO? Or FUTU?

--dave (them 88k guys is smart, harold) c-b
-- 
 David Collier-Brown.  | yunexus!lethe!dave
 Interleaf Canada Inc. |
 1550 Enterprise Rd.   | HE's so smart he's dumb.
 Mississauga, Ontario  |       --Joyce C-B