Subject:
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Re: The standard NXT firmware & the case of the missing opcodes
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Newsgroups:
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lugnet.robotics.nxt.nxthacking
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Date:
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Wed, 21 Mar 2007 17:34:27 GMT
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Viewed:
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16786 times
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In lugnet.robotics.nxt.nxthacking, Jason J Railton wrote:
> > //Family: Bit manipulation
> > #define OP_CMNT 0x0A // dest, src
> > #define OP_LSL 0x0B // dest, src
> > #define OP_LSR 0x0C // dest, src
> > #define OP_ASL 0x0D // dest, src
> > #define OP_ASR 0x0E // dest, src
> > #define OP_ROTL 0x0F // dest, src
> > #define OP_ROTR 0x10 // dest, src
> Having used shifts in Z80 assembly before, I can see the uses of the different
> types. However, I can't see how I'd want anything other than arithmetical in
> the NBC opcodes. The only times I used logical, rotating, or with-carry-bit
> shifts was for manipulating bitmap graphics, and I probably wouldn't be trying
> to do that sort of thing in NBC.
>
> As I understand it, << is an arithmetical shift left (shift bits up, fill with
> 0s) and >> is an arithmetical shift right (shift bits down, fill with whatever
> the top bit was to preserve the sign for signed integers, or fill with 0s for
> unsigned integers).
>
> Again, for graphics it was always easier to use the <<= operation, to shift in
> place. But, on the odd occasion where I have used shifts in arithmetic, I'd
> write the expression in the x=y<<z style. Whether this makes any difference
> once the expression is compiled to opcodes, I couldn't say. If programming
> directly in opcodes, I guess you just take whatever you can get.
>
> Jason R
I guess what I'm saying is '<<=', 'OP_ASL' and 'OP_LSL' are all the same thing
(zero fill).
'>>=' means either 'OP_ASR' (smear fill) or 'OP_LSR' (zero fill) depending on
whether your target is signed or not.
If you wanted direct equivalents with C, you would have to say '>>=' means
'OP_ASR', and 'OP_ASR' should know the difference between signed (smear fill)
and unsigned (zero fill).
Jason R
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