Logical Shift Right
mtodorov at alu.hr
Sat Dec 8 14:59:27 CET 2001
On Sat, 8 Dec 2001, Frank Heckenbach wrote:
> Mirsad Todorovac wrote:
> Yes, I've thought about this when I translated the MD5 unit for GPC.
> Currently it does (left) rotation as
> (w shl s) or (w shr (32 - s))
> (which is translated from similar C code).
> The backend seems to have support for rotation, so it might be easy
> to add it in GPC (I haven't checked yet, but it appears so).
> If we do it, I think it should not be operators (because that would
> change the syntax more than necessary), but rather simple predefines
> functions (`RotateLeft', `RotateRight'?).
On the other hand, it belongs to the same family of operations as ``shl''
and ``shr'', doesn't it?
> > Of course, if a range of bytes is rotated, it's very useful to preserve
> > carry flag somewhere - but I guess one has to do asm for that, doesn't he?
> I'm not even sure how easy it is in asm. At least on IA32, if my
> information is correct, "The Carry Flag will contain the value of
> the last bit rotated out", but the old value of CF is not rotated
> in, so this would take some extra work, anyway.
On some processors there are both flavors of rotation (through carry,
where old carry is rotated in - and rotation where the bit rotated out is
rotated directly in).
If i wanted to scroll a large b&w bitmap, I would do it with one
instruction per 32 points, while extracting the bit that travels between
32-bit words manually would take at least two additional instruction
(masking it out from first word and or-ing it into second), which can be
But this is perhaps only a specific example.
> So I guess for the (few, I assume) situations where someone wants to
> rotate more than one work (not byte), they'll have to do some manual
> coding, anyway, whether in Pascal or asm. Am I missing something?
well, if I had RORC: (which even ancient Z80 had)
+-> | | -> | | | | | | | | | -+
| +--+ +--+--+--+--+--+--+--+--+ |
Then I could do form of loop unrolled shifting of neghboring words (for
example a bitmap picture), sort of:
(in VAX-like assembler) Now, address register is auto-incremented, while
carry flag transfers bits that travel between words.
But this is digression from PASCAL thematics - since I can't think of
efficient and elegant syntax and semantics for carry flag in pascal.
But calculating a checksum that would be xor-ed and rotated with each new
word isn't hard ot imagine, and isn't that rare use.
Marvin (I feel better as Marvin, my nickname from college than my real
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