best method to extract and compare single bits?

Jimmy W.

What is everyone using to extract single bits from longs? I know they were some talks of using ctrb and whatnot, I am currently unhappy with the way I have been doing it for a while now, shift left till the bit is MSB then right shift it to LSB then compare against 1, it is a very inefficient process and I am looking to squeeze a bit more speed out of these comparisons.

Forgot to mention, this is in spin, thanks!

Thanks, and happy 4th!

Jimmy

Post Edited (Jimmy W.) : 7/5/2008 2:54:48 AM GMT

stevenmess2004

You can use the test instruction with wz something like this

mov x,#123
test x,testBit wz
'code to use the bit...

testBit long %0010

The test instruction is simply an and that doesn't write the result.

Jimmy W.

I should have been more specific when talking about a chip that you can write high level or low level code for. I am using this test in spin [noparse]:)[/noparse]

hippy

I'm not entirely clear what your exact situation is but if you want to check or use bit N (0 to 31) I'd use ...

  ' Test if bit N is set or clear

  if longVar & ( |< N )
    BitIsSet
  else
    BitIsClear

  ' Optimised when N is a constant

  if longVar  & constant( |< N )
    BitIsSet
  else
    BitIsClear

  ' To get bit N ( as a 0 or 1 )

  result := ( longVar >> N ) & 1

stevenmess2004

What you've got is probably about the most efficient. Something like one of the following

bit:=(var>>bitNumber)&1

or

if var&|<bitNumber

Don't know which is quicker. If it was always the same value you could use a precalculated value instead which may speed things up. If speed is marginal there are a couple of things you can do such as making sure the two variables are in either the first 8 object variables or the the first 7 method variables. Also, if possible keep any constants small so that they will fit in a byte or word rather than a long which will also make it slightly faster (there's three less hub accesses needed for a byte than a long constant in spin).