Little and Big Endian
Endianess
Endianess is the byte order of the number in the computer's memory. The number can have any size of bits, but the most common numbers used are 32 bits (4 bytes) and 16 bits (2 bytes).
There are 2 main types of endianess:
- Little endian - used mostly in Intel machines.
- Big endian - used mostly in Motorola machines.
Little endian
Little endian number is ordered from the least significant byte (LSB) in low memory address to the most significant byte (MSB) in high memory address.
Address offset Data 0 byte0 1 byte1 2 byte2 3 byte3 Example
The hex number 0x0D0C0B0A will be represented in memory:
Address offset Data 0 0A 1 0B 2 0C 3 0D Big endian
Big endian number is ordered from the most significant byte (MSB) in low memory address to the least significant byte (LSB) in high memory address.
Address offset Data 0 byte3 1 byte2 2 byte1 3 byte0 Example
The hex number 0x0D0C0B0A will be represented in memory:
Address offset Data 0 0D 1 0C 2 0B 3 0A Bits order
The order of the bits inside the byte is not changed when converting from little to big or from big to little.
Bi-endianess
Many processors allow to configure the endianess type to little endian or big endian.
Endianess table
Machine / Processor Endianess Intel Pentium / x86 (PC) Little ARM Little / Big PowerPC Big / Little Motorola 68000 Big MIPS Big / Little Sun Sparc Big / Little TI TMS320C6000 Big / Little Analog ADMC / Sharc Big Java Big Network Big PC / Windows Little Mac / OS-X Big / Little Linux Big / Little Unix Big / Little Endian converter
TBD
Big / little endian conversion C code
The conversion is done, simply by swapping the bytes in memory for a 32 bit number.
Byte0 is swapped with byte3. Byte1 is swapped with byte2.
The conversion works both ways - from big to little and from little to big.
32 bits endian conversion of long number
unsigned long EndianSwap32(unsgined long x)
{
unsigned long y=0;
y += (x & 0x000000FF)<<24
y += (x & 0xFF000000)>>24
y += (x & 0x0000FF00)<<8
y += (x & 0x00FF0000)>>8
return x
}
32 bits endian conversion of char array
void EndianSwap32(unsgined char c[])
{
unsigned char tmp;
tmp = c[0];
c[0] = c[3];
c[3] = tmp;
tmp = c[1];
c[1] = c[2];
c[2] = tmp;
}
CPU endianess check C code
Here is a C code function to finds the endianess of the CPU:
typedef enum _endian {little_endian, big_endian} EndianType;
EndianType CheckCPUEndian()
{
unsigned short x;
unsigned char c;
EndianType CPUEndian;
x = 0x0001;;
c = *(unsigned char *)(&x);
if( c == 0x01 )
CPUEndian = little_endian;
else
CPUEndian = big_endian;
return CPUEndian;
}
See also
Online Reference & Tools