BIT masks

Hi Folks

I was looking for a way to test bit settings in a Hex value.

i.e.

Hex value 0x07
Bit mask  0x05

If at least all bits in mask are set in value then true.

Using expr,
expr 0x07 & 0x05, gives 5

which reading from the man page is as expected.

Now if we do

expr 0x04 & 0x05, gives 4

which is not what I wanted to see but still correct from the description.

The only way I can think of doing what I want is

if {[expr $value & $bitpattern ] == $bitpattern } {
#Then all present
} else {
#Not all there
}

Any body thing of a better way?

Thanks

Derek

Derek <derek.philip@csr.com> wrote:
> The only way I can think of doing what I want is
> if {[expr $value & $bitpattern ] == $bitpattern } {

You're already quite near!
The first argument to "if" is already treated like an expr,
so you can "simplify" this to:
if {($value & $bitpattern) == $bitpattern} { ...
which is likely among the best you can get.

Oh, there is still an alternative, but you would
have to swap the "then" and "else" clause for this:
if {($value ^ $bitpattern) & $bitpattern} {
# some bits were missing in $value
# Incidentally, the expression evaluates to
#  exactly the set of missing bits :-)
} else {
# all required bits were set in $value
}

Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> wrote in message news:<slrnchn7v1.nab.avl@ga
mma.logic.tuwien.ac.at>...
> Derek <derek.philip@csr.com> wrote: 
>
> You're already quite near!
> The first argument to "if" is already treated like an expr,
> so you can "simplify" this to:
>   if {($value & $bitpattern) == $bitpattern} { ...
> which is likely among the best you can get.
>
> Oh, there is still an alternative, but you would
> have to swap the "then" and "else" clause for this:
>   if {($value ^ $bitpattern) & $bitpattern} {
>      # some bits were missing in $value
>      # Incidentally, the expression evaluates to
>      #  exactly the set of missing bits :-)
>   } else {
>   # all required bits were set in $value
>   }

Thanks for the comments Andreas

I did look at the if statement early on but my understanding of what
was happening wasn't clear.

I think I'll use your "if" sugestion with the & and test for equality
as it is slighty easier to understand for the average joe who may look
at the scripts I'm writing.


Derek