[RC5] keyrate stabilizing?

Test Man mr_nobby at yahoo.com
Mon Oct 22 14:53:18 EDT 2001


Ahh, I remember those undocumented SLL instructions!

--- Stephan Wonczak <sws at sherlock.pc.uni-koeln.de>
wrote:
> On Mon, 22 Oct 2001, Bruce Wilson fell across the
> keyboard, resulting in:
> 
> > A shift is useful for multiplying and dividing by
> powers of two, a
> > rotate is not.
> > 
> > 8-bit examples:
> > 
> > 10010111	Sample
> > 00101110	left shift (carry would be set)
> > 00101111	left rotate (carry unaffected)
> > 01001011	right shift
> > 11001011	right rotate
> 
>   Starting to pick some nits: I still remember
> programming in
> Z80-Assembler ... am I that old already? There were
> a whole bunch of
> shifting and rotating instructions, some affecting
> the carry-bit, some
> not. If I remember correctly, there were the
> following variations:
> 
>   Shifts  bit7 bit0 carry
> 
>    left    b6   0     b7   usual shift, multiply by
> 2 
>    left    b6   C     b7   shift, add carry (use to
> emulate rotate)
>   right     0   b1    b0   usual shift, divide by 2
>   right    b7   b1    b0   'arithmetic' shift,
> keeping the sign
>   right     C   b1    b0   shift, add carry (use to
> emulate rotate)
> 
>  Rotates
> 
>    left    b6   b7     X   8-bit rotate, carry
> unaffected
>    left    b6   b7     b7  8-bit rotate, carry
> affected
>    left    b6   C      b7  9-bit rotate
>   right    b0   b1     X   8-bit rotate, carry
> unaffected
>   right    b0   b1     b0  8-bit rotate, carry
> affected
>   right     C   b1     b0  9-bit rotate
>   
>   I have used the rotate-instruction in some of my
> programs (nothing to
> do with encryption!) to save calculation time, but
> it is certainly
> possible to get by without it. 
> 
> 		C U, Stephan
> 		
>    "I haven't lost my mind; I know exactly where I
> left it."
>    "The meaning of my life is to make me crazy"
> 
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