bitwise..

[cmk128]

Hi
C++ should provide us bit shift but fill with 1 rather than 0.
Or i am wrong, c++ have it???

if 1 left shift 2, i want 111 rather than 100

thanks
from Peter ([email protected])

 

[Mirek Fidler]

Hi
C++ should provide us bit shift but fill with 1 rather than 0.
Or i am wrong, c++ have it???

if 1 left shift 2, i want 111 rather than 100

I am afraid that best you can do is something like (x & 1) | (x << 1).
AFAIK, most CPUs do not have single instruction for this anyway....

Mirek

 

[Karl Heinz Buchegger]

Hi
C++ should provide us bit shift but fill with 1 rather than 0.

Why 'should'?

Or i am wrong, c++ have it???

Not everything you or I will ever need os provided by the language. But the
language is powerful enough that you can synthesize what you need from what
is existing.

result = ( value << 1 ) | 0x01;

does what you want. First shift one position to the left, then
set the leftmost bit to 1

For 'shift values' other then 1, I think the easieast approch is
to use a lookup table:

unsigned int MyShiftLeft( unsigned int What, unsigned int How )
{
static unsigned int OnePattern[] = { 0, 0x01, 0x03, 0x07, 0x0F, 0x1F, 0x3F, 0x7F };

return (What << How ) | OnePattern[How];
}

This works for values of 'How' up to 7. If that is not enough, enlarge the table.

 

[Gernot Frisch]

Hi
C++ should provide us bit shift but fill with 1 rather than 0.
Or i am wrong, c++ have it???

if 1 left shift 2, i want 111 rather than 100

thanks
from Peter ([email protected])

Maybe like this?

inline unsigned int ASL_FILL(unsigned int num, int sh)
{
return (num<<sh) | ((1<<sh)-1);
}

int main()
{
int test = 0x20;
test = ASL_FILL(test, 3);
}

 

[Andrew Koenig]

C++ should provide us bit shift but fill with 1 rather than 0.
Or i am wrong, c++ have it???

if 1 left shift 2, i want 111 rather than 100

To shift x to the left n bits, filling with 1, you can write ~(~x << n)

 

[Mirek Fidler]

I am afraid that best you can do is something like (x & 1) | (x << 1).
AFAIK, most CPUs do not have single instruction for this anyway....

Mirek

Oops, sorry, I thought you want to copy LSB... (like arithemtic shift in
inverse direction).

Mirek