I need to reverse a byte

[Tomi]

Hi All,

I know this is an old topic but maybe the results (execution speed) are interesting for others...

Reversing a byte by using Look Up table.

Additionally  enabling / disabling the debugging option influences the execution time heavily  when the method "bit shift operations" are used for byte reversing.. 

See below  LV7.1

Regards

Tomi

[altenbach]

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Tomi wrote:
Reversing a byte by using Look Up table.

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Tomi,

I agree, a lookup table is always the way to go, even for 16 bits, assuming that you need to convert more often than the size of the lookup table. 😉

Typically, you would encapsulate this in a subVI and place the lookup table in an uninitialized shift register. Use a case connected to a "first call?" primitive so the lookup calculation is done only once (Since the lookup table is folded into a constant by the compiler anyway, the calculation for the lookup table will happen even before you run the VI so most likely there won't even be a penalty at the first call :)).

For some related examples, have a look at the code for my tic tac toe program posted at the following link:

http://forums.ni.com/ni/board/message?board.id=170&view=by_date_ascending&message.id=247044#M247044

Here I use a similar method to e.g. calculate the number of bits set in a given U16 integer (see U16BitsSet.vi) or find the unique transform of a position (UniqueTransformCached.vi).

For an 8bit table, you could just calculate it once and then place it as a diagram constant. It will only increase the size of the VI by ~256bytes... peanuts! 😉 Still, I prefer explicit code for the table generation, because it is easier to tell if there is a mathematical error.