Assembly Language for Beginners

2.8 Endianness.

The instruction is also known as “NSA^22 instruction” due to rumors:

This branch of cryptography is fast-paced and very politically charged. Most designs

are secret; a majority of military encryptions systems in use today are based on LFSRs. In

fact, most Cray computers (Cray 1, Cray X-MP, Cray Y-MP) have a rather curious instruction

generallyknownas“populationcount.” Itcountsthe1bitsinaregisterandcanbeusedboth

to efficiently calculate the Hamming distance between two binary wordsand to implement a

vectorized version of a LFSR. I’ve heard this called the canonical NSA instruction, demanded

by almost all computer contracts.

[Bruce Schneier,Applied Cryptography, (John Wiley & Sons, 1994)]

2.8 Endianness

The endianness is a way of representing values in memory.

2.8.1 Big-endian.

The0x12345678value is represented in memory as:

address in memory byte value

+0 0x12

+1 0x34

+2 0x56

+3 0x78

Big-endian CPUs include Motorola 68k, IBM POWER.

2.8.2 Little-endian.

The0x12345678value is represented in memory as:

address in memory byte value

+0 0x78

+1 0x56

+2 0x34

+3 0x12

Little-endian CPUs include Intel x86.

2.8.3 Example

Let’s take big-endian MIPS Linux installed and ready in QEMU^23.

And let’s compile this simple example:

#include <stdio.h>

int main()
{
int v, i;

v=123;

printf ("%02X %02X %02X %02X\n",

*(char*)&v,

*(((char*)&v)+1),

*(((char*)&v)+2),

(^22) National Security Agency
(^23) Available for download here:http://go.yurichev.com/17008