Endianness

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Endianness refers to the order in which bytes of a multi-byte structure are placed to main memory (RAM). The bit order in any byte is normally from bit seven downto bit zero (little-endian) and remains preserved.

Big- vs. little-endian

Big-endian stores a value in order, little-endian in reversed order of significance. In big-endian the most-significant (left-hand) byte, in little-endian the least-significant (right-hand) byte is stored at the lowest address.
The following table shows the byte number of a 32-bit structure with byte three equals bit 31 downto 24, byte two equals bit 23 downto 16, byte one equals bit 15 downto 8 and byte zero equals bit seven downto zero.

Memory
address
' n ' n+1 n+2 n+3
Big-endian: 3 2 1 0
Little-endian: 0 1 2 3

Little-endian may seem unlogical until one consider that a variable can be used more easily as 8-bit, 16-bit, 32-bit or even 64-bit value without changing its base address. Therefor AMD and Intel use little-endian order.

Big-endian is sometimes refered as network byte order. Big endian order is used f.e. in natural languages or .png-files.

Examples

  • Table
Memory
address
n n+1 n+2 ...
Value =
0x15
Big-endian: 0x15 - - -
Little-endian: 0x15 - - -
Value =
0x0A15
Big-endian: 0x0A 0x15 - -
Little-endian: 0x15 0x0A - -
Value =
0x780A15
Big-endian: 0x78 0x0A 0x15 -
Little-endian: 0x15 0x0A 0x78 -
a.s.o.
  • Let's assume a x86-computer, which means little-endian order. Register eax contains the value 0xCAFEF00D. You store this register at address 0:

Memmory address: 0 contains: 0x0D
Memmory address: 1 contains: 0xF0
Memmory address: 2 contains: 0xFE
Memmory address: 3 contains: 0xCA

Now you read into register ax the value from address 0:

Register ax contains: 0xF00D

  • Let's assume big-endian order. A 32-bit register contains the value 0xCAFEF00D. You store this register at address 0:

Memmory address: 0 contains: 0xCA
Memmory address: 1 contains: 0xFE
Memmory address: 2 contains: 0xF0
Memmory address: 3 contains: 0x0D

Now you read into a 16-bit register the value from address 0:

This 16-bit register contains: 0xCAFE
(Which is the upper-half of the value. You probably wanted the lower-half and therefor must change the base address and read from address: 2.)

  • 578 Pound Sterling

"byte" from 9 to 0:
- big-endian: 578 £
- little-endian: 875 £
"byte" from 99 to 0:
- big-endian: 578 £
- little-endian: 7805 £

Common CPUs

CPU Endianness
x86 little
x86-64 little
MIPS both
Motorola big
68k big
PowerPC both
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