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 17:26:55 GMT
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Viewed:
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16567 times
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In lugnet.robotics.nxt.nxthacking, John Hansen wrote:
> There are several opcodes defined in c_cmd_bytecodes.h which are missing from
> the VM implementation in c_cmd.c
>
> Here are the ones that are not implemented:
>
> //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
>
> #define OP_CMPSET 0x13 // dest, src, testsrc, testsrc
> #define OP_TSTSET 0x14 // dest, src, testsrc
>
> #define OP_WAIT 0x34 // milliseconds
>
> The bit manipulation set seem to have odd comments following them. I guess the
> original plan was to shift dest by src and store the result in dest. Kind of
> like a x <<= 3 sort of operation in C or C++. I'd prefer to have these opcodes
> be implemented as binary operators like x = y << 4 where the opcode would take a
> dest and two sources. If the first source happened to be the destination then
> you would have the <<= or >>= version of the operation.
>
> Can anyone from NI or LEGO comment on the intended meaning of each of these
> opcodes? Would anyone care to post a short snippet of C code which demonstrates
> what you think each of these opcodes should do? I'm not entirely sure whether
> << is a logical or an arithmatic shift or how to distinguish between a rotate
> left, a logical shift left, or an arithmatic shift left. Does anybody else
> think that having three different types of shifts is a bit more than we really
> need?
>
> The CMPSET and TSTSET opcodes seem to be intended to set dest equal to src if
> the comparison or the test is true. That would allow for a more optimal code
> generation of a simple if statement such as if (x < y) y = 4; Sounds pretty
> simple to implement and useful to have available to use in NBC/NXC.
>
> The wait opcode will be really nice to have implemented in the NXT firmware.
> I'm inclined to think it will be the hardest to implement, though. We'll need
> to come up with a nice way to mark the "clump" as being in the waiting state and
> store the wakeup tick in the clump record then when the VM tries to execute code
> in the waiting clump it will do nothing until the current tick is >= to the
> wakeup tick and then it will go on to the next instruction. It is kind of like
> calling a subroutine and then continuing on in the code once the subroutine
> returns.
>
> Other opcodes that will be nice to add to the NXT firmware include the OP_ABS
> and OP_SIGN opcodes which are implemented in the NBC compiler right now.
>
> Any other opcodes you all would like to have added to the NXT VM? Another set I
> have been thinking about would be for doing fixed-point arithmetic using a
> cluster type that would define the fixed point number. Maybe some
> bitset/bitclear/bittest opcodes would be nice to have as well.
>
> Your thoughts or comments?
>
> John Hansen
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
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