Assembly Language for Beginners

1.23 Linear congruential generator.

Clear specific bit (known at compile stage)

Just applyANDoperation with the inverted value:

Listing 1.312: C/C++

value=value&(~0x40);

Listing 1.313: x86

AND REG, 0FFFFFFBFh

Listing 1.314: x64

AND REG, 0FFFFFFFFFFFFFFBFh

This is actually leaving all bits set except one.

ARM in ARM mode hasBICinstruction, which works like theNOT+ANDinstruction pair:

Listing 1.315: ARM (ARM mode)

BIC R0, R0, #0x40

Clear specific bit (specified at runtime)

Listing 1.316: C/C++

value=value&(~(1<<n));

Listing 1.317: x86

; CL=n
MOV REG, 1
SHL REG, CL
NOT REG
AND input_value, REG

1.22.7 Exercises

• http://challenges.re/67


• http://challenges.re/68


• http://challenges.re/69


• http://challenges.re/70

1.23 Linear congruential generator as pseudorandom number

generator

Perhaps, the linear congruential generator is the simplest possible way to generate random numbers.

It’s not in favour nowadays^156 , but it’s so simple (just one multiplication, one addition and AND operation),
that we can use it as an example.

#include <stdint.h>

// constants from the Numerical Recipes book
#define RNG_a 1664525
#define RNG_c 1013904223

static uint32_t rand_state;

(^156) Mersenne twister is better