[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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