How to write a C program which exchange the bits in a byte?

[kukurigu]

c program to exchange the nibbles

How to write a C program which exchange the bits in a byte (i.e. mirror the bits)?
There are different approaches but I am looking for the fast one!

 

[brmadhukar]

Re: How to write a C program which exchange the bits in a by

use a look up table

 

[jhbbunch]

I haven't tested this but the geneal idea is right. Should work.


int MirrorInt(value)
{

int lastbit = 15;
int i;

for ( i = 0; i <= lastbit; i++)
(
if ( 1 << i && value)
value = value || 1 << (lastbit - i );
else
value = value && !(1 <<(lastbit - i ));


)

return value;

}

 

[devendra_devgupta]

Kukurigu what you are trying to ask. Do you want to exchange nibbles in a byte or tou want to reverse the number in a byte i.e LSB will be MSB and vice versa

Moreover jhbbunch your program seems to be wrong plz check it out

 

[kukurigu]

Re: How to write a C program which exchange the bits in a by

I want to reverse the bits (i.e mirror) bit7 <-> bit0, bit6 <-> bit1 and so on.

 

[Encrypted]

Re: How to write a C program which exchange the bits in a by

Not my idea!

char reverse (char n) {
  n = n & 0b11110000 >> 4 | n & 0b00001111 << 4;  // efghabcd
  n = n & 0b11001100 >> 2 | n & 0b00110011 << 2;  // ghefcdab 
  n = n & 0b10101010 >> 1 | n & 0b01010101 << 1;  // hgfedcba
  return n;
}

 

[VSMVDD]

Re: How to write a C program which exchange the bits in a by

assign the whole port bits 0-7 as a word
the use modulus

portword= ~portword

this will invert the word then you can shift out what bit you want

bit=(BOOL) (portword<<x) // where x is the bit you want

 

[leomecma]

Re: How to write a C program which exchange the bits in a by

Go to the title Reversing bit sequences

https://graphics.stanford.edu/~seander/bithacks.html#BitReverseObvious

Maybe you can get a good idea ...

leomecma

 

[whatcall`]

Re: How to write a C program which exchange the bits in a by

If what you want is just fast may be look-up-table is a better method. But I think for 8-bits byte we can code in a "stupid" way like this:

void reverse_byte(byte &a)
{
byte b ;

b = *a;
*a=0;
*a += (b&(1<<7))>>7;
*a += (b&(1<<6))>>5;
*a += (b&(1<<5))>>3;
*a += (b&(1<<4))>>1;
*a += (b&(1<<3))<<1;
*a += (b&(1<<2))<<3;
*a += (b&(1<<1))<<5;
*a += (b&(1<<0))<<7;

return;
}

I hope this can work and the compiler can optimize it to simple asm instructions.

 

[Mr.MEB]

void reverse_byte(byte &a)
isn't this some C++ with reference arg?
I don't think it will work with a non-AnsI C compiler for 8 bit mcu.

 

[whatcall`]

Re: How to write a C program which exchange the bits in a by

Sorry.
It should be (byte *a)