Testing for a particular bit in a byte value

DavidM

Hi,

How do I test for particular "BIT" of a value is set to 1 or 0??

e.g. I have a 4 bit value ..

Value := $0111

I want to test if a particular bit of any of its 4 bits) , to see if its a 1 or a 0

Lets say I want to check the 3rd bit ( starting from the left )

How do I do this? Do I use BITWISE DECODE?? or maybe bitwise AND?

Thanks


Dave M

stevenmess2004

First of all do a shift left and then do a bitwise and with one. This will give you a 1 or a 0 which can be use as true/false. In most things in SPIN you can just use 0 for false and anything else for true.

DavidM

Hi,

Would this be what I need to do ( I don't see why I would have to shift left at all)

e.g. To test the first ( Leftmost) bit

MyValue := %1010
IF MyValue & %1000 == %1000

This if statement above would test TRUE as the first bit is 1


or to test the 2nd bit from the left

MyValue := %1010
IF MyValue & %0100 == %0100

would return FALSE as the 2nd bit is 0

etc..

Regards


Dave M

stevenmess2004

You could do that as well Now that we have the spin source we can tell which is quicker but I bags not doing it(could just use the timer values to figure it out). The advantage with my way is that it should be easier if you want to use the routine for different bits. Also, since I am using 1 I think that there is a spin opcode for pushing 1 onto the stack which will save a couple of bytes and speed things up slightly.

DavidM

Ok Steve,

Your idea of shifting left by the number of bits makes sense so you can see what bit you are testing for as well as keeping the code generic.


Thanks

Dave M

Ron Sutcliffe

@steve
wouldn't this do,

IfNot Myvalue>>4

stevenmess2004

Depends on whether there are other bits set. For example if you had
myValue:=%0100
and you did
if myValue>>2

then it would work but if you had
myValue:=%1100
then it wouldn't work.

You could do

if (myValue>>2)&1

You would have to check to see exactly what 'ifnot' checks for as well.

CardboardGuru

DavidM said...
MyValue := %1010
IF MyValue & %1000 == %1000

This if statement above would test TRUE as the first bit is 1

Hi David,

In 99% of cases that's exactly what you do.· Better yet, set up a named constant for the %1000.

If you did have to be adaptable to check any bit, SPIN has an operator that sets a single bit by number.

|<0 gives %1
|<3 gives %1000
|<31 gives %10000000_00000000_00000000_00000000

So you could do:

MyValue := %1010
IF Myvalue &·|<3



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Ron Sutcliffe

@steve
So what you first said was :-
Mybit :=(myvalue>>Mybit)&1

I think.

regards

Ron

Peter Jakacki

How about this:

pub testbit(value,bit)
  result := value & |<bit

Any nonzero result is true though you could promote it to a true true [noparse]:)[/noparse] with:

  result OR= 0

use it like this:

  if testbit(mybits,2)
    do_this_because_its_set

The above statement does not need to compare testbit to a true or false because the result of testbit is true or false. If the bit is set then the result is true.

*Peter*

DavidM

HI,

Thanks everyone!


I think that BITWISE DECODE is the go for this!

I just have to remember that the first bit ( starting from the RIGHT ) is bit 0 not 1

|<0 First bit starting from the right
|<1 2nd bit
|<2 3rd bit
|<4 4th bit etc..


So..

IF MyValue |<3

would result in TRUE if the 4th bit from the right is set to 1

is this correct?


regards


Dave M

DavidM

Sorry I left out the "&"

that should be..

IF MyValue & |<3

would result in TRUE if the 4th bit from the right is set to 1

Dave M

Paul Baker

Correct

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.