[nfbcs] a little brain teaser

[Doug Lee]

That's nearly what I thought up on reading the original message, but mine was this:

Load R1 with x
Shift R1 left once
Load R2 with R1
Shift R1 left two more times
Add R2 into R1
Store R1 in y

On Wed, Jul 20, 2016 at 04:49:45PM -0700, NFBCS mailing list wrote:
Well, using assembler psuedo code, I'd do something like...

     put x into register 1
     put x into register 2
     logical shift register 1 left 3 times
     logical shift register 2 left 1 times
     Add register 2 to register 1

You'd have to detect register overflow, but otherwise this works.

Depending on how you're doing your big O calculations, it's now O(6)
instead of O(10)

Jim

On Wed, Jul 20, 2016 at 06:12:12PM -0500, david hertweck via nfbcs wrote:
> OK this is a CS group have a little CS puzzle / question for anyone.  I work
> down in the micro controller realm and memory and speed is still something
> that a programmer must deal with.
> 
> if one wanted to calculate  y = x*10 such that it took the min time / cpu
> cycles how would you do it.  There is no math co-processor such that * is a
> single operation.
> 
> I have one answer I thought it might be fun / interesting what people could
> come up with.   Will post the best answers.
> 
> have fun.
> 
> 
> 
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-- 
Doug Lee                 dgl at dlee.org                http://www.dlee.org
SSB BART Group           doug.lee at ssbbartgroup.com   http://www.ssbbartgroup.com
"Nearly all men can stand adversity, but if you want to test a man's
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