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Path: utzoo!mnetor!seismo!rutgers!princeton!allegra!ulysses!mhuxt!ihnp4!alberta!ubc-vision!fornax!chapman
From: chapman@fornax.UUCP
Newsgroups: comp.sys.m68k,comp.sys.intel
Subject: Re: Recent Motorola ad seen in Byte
Message-ID: <280@fornax.uucp>
Date: Fri, 8-May-87 01:54:36 EDT
Article-I.D.: fornax.280
Posted: Fri May  8 01:54:36 1987
Date-Received: Sun, 10-May-87 09:15:07 EDT
References: <8705030305.AA18195@cory.Berkeley.EDU>
Organization: School of Computing Science, SFU, Burnaby, B.C. Canada
Lines: 58
Xref: mnetor comp.sys.m68k:468 comp.sys.intel:232

> >1. how would you code a string instruction like FFS (find first set bit)
> >   to fit in something like the 68010's single instruction looping
> >   feature/cache?
> 
> 	moveq.l	#31,D0
> 	move.l 	ea,D1
> loop	asl.l	#1,D1
> 	dbcs 	loop
> 
>     Notes: You could just as easily specify 7, and use byte operations to 
>     find the first set bit in a byte.  The routine above checks bits MSB
>     first.  You can, of course, use lsr instead to check bits LSB first.
>     (remember to take 32 - the count afterword for longword operations)
>     You can add an outer loop to check any size item by using
>     (A0)+ for the EA on the MOVE (and setting up A0) when loading the 
>     initial data into D1, followed by, say, a BEQ if you also want to check for
>     an end of string.  Remember that the outer loop would only execute
>     once every 32 cached loops (97% of the instructions are cached) assuming
>     you use longword operations.
> 
.
. 68020 comments
. 
>     P.S. I didn't test this code... but it's almost a children's exercise
>     anyway.
> 
> 					-Matt

Yes, well, the problem with children is they often make mistakes.

1. The code you have produced does not meet the original "challenge"
   I made to the original poster who was wondering why specific
   registers and why a single instruction.  Your code does not all
   fit in the 68010's cache.  Lest you think this is picky I will point
   out that I chose the simplest of the string instructions as an example;
   others would take even more code and use more registers.

2. Duplicating the function has now used up a general purpose register.
    One of the advantages of the "all in one" type instruction is that
   internal temporaries can be used.

3.  You have not implemented FFS, you have implemented a much simpler version.
    FFS is (operand order may be wrong):

    FFS <base addr>,<offset>,<length>,<destination of count>

    <base addr> is a byte address of the bit string origin
    <offset> is a bit offset into the bit string at which the search
             begins; if the source is not a register this may be more than 32
             bits
    <length> is the length of the field to be checked in bits
    <destination of count> is where the bit position of the first set bit
                           is placed

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