how to generate random number?

Thanks for giving advice.
However my code runs on 8051 and i just want to generate an integer from 1 to 127. Since there are many methods to do that and my MCU is out of code memory, is there a shortest code that can do it? Thanks very much.

Thanks for giving advice.
However my code runs on 8051 and i just want to generate an integer from 1 to 127. Since there are many methods to do that and my MCU is out of code memory, is there a shortest code that can do it? Thanks very much.

Here is a very small 16-bit LFSR based pseudo random number generator and it is about as basic as you can get.

#include <REG52.H>

//
//  Masks for 8-bit galios LFSR maximum run length generator
//
//  8 stages, 4 taps:  (6 sets)
//
//  [8, 7, 6, 1]
//  [8, 7, 5, 3]
//  [8, 7, 3, 2]
//  [8, 6, 5, 4]
//  [8, 6, 5, 3]
//  [8, 6, 5, 2]
//
//  [8, 7, 6, 1] =
//
//    3                   2                   1                   0
//  1 0 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0
//  0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 0 0 0 0 1
//
//  0 0 0 0 0 0 0 0  = 0xE1
//
//  Masks for 16-bit galios LFSR maximum run length generator
//
// 16 stages, 8 taps:  (406 sets)
//
//  [16, 15, 14, 13, 12, 11, 10, 4]
//  [16, 15, 14, 13, 12, 11, 9, 8]
//  [16, 15, 14, 13, 12, 11, 9, 6]
//  [16, 15, 14, 13, 12, 11, 9, 5]
//  [16, 15, 14, 13, 12, 11, 8, 2]
//  [16, 15, 14, 13, 12, 11, 5, 2]
//  [16, 15, 14, 13, 12, 10, 9, 5]
//  [16, 15, 14, 13, 12, 10, 5, 1]
//  [16, 15, 14, 13, 12, 9, 8, 7]
//  [16, 15, 14, 13, 12, 9, 7, 1]
//  [16, 15, 14, 13, 12, 9, 6, 3]
//  [16, 15, 14, 13, 12, 9, 6, 2]
//  [16, 15, 14, 13, 12, 9, 3, 2]
//  [16, 15, 14, 13, 12, 8, 4, 1]
//  [16, 15, 14, 13, 12, 7, 6, 4]
//  [16, 15, 14, 13, 12, 6, 5, 3]
//  [16, 15, 14, 13, 12, 6, 5, 2]
//  [16, 15, 14, 13, 12, 6, 3, 2]
//  [16, 15, 14, 13, 12, 5, 4, 3]
//  [16, 15, 14, 13, 12, 5, 3, 2]
//  [16, 15, 14, 13, 11, 10, 8, 4]
//  [16, 15, 14, 13, 11, 10, 6, 1]
//  [16, 15, 14, 13, 11, 10, 5, 4]
//  [16, 15, 14, 13, 11, 10, 3, 1]
//  [16, 15, 14, 13, 11, 9, 8, 5]
//  [16, 15, 14, 13, 11, 9, 8, 2]
//  [16, 15, 14, 13, 11, 9, 7, 4]
//  [16, 15, 14, 13, 11, 9, 7, 3]
//  [16, 15, 14, 13, 11, 9, 6, 5]
//  [16, 15, 14, 13, 11, 9, 6, 4]
//  [16, 15, 14, 13, 11, 9, 6, 1]
//  etc...
//
//  [16, 15, 14, 13, 11, 9, 6, 1] =
//
//    3                   2                   1                   0
//  1 0 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0
//  0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 0 1 0 1 0 0 1 0 0 0 0 1
//
//  1 1 1 1 0 1 0 1  = 0xF5
//  0 0 1 0 0 0 0 1  = 0x21
//

#pragma ASM

$REGUSE small_rand( PSW, A, R7 )

#pragma ENDASM

static data unsigned int lfsr = 0xA5A5;

unsigned char small_rand()
{

    #pragma ASM
                                    ;
        MOV     R7,#8               ;
                                    ;
?random_loop:                       ;
                                    ;
        CLR     C                   ;
        MOV     Acc,lfsr+0          ;
        RRC     A                   ;
        MOV     lfsr+0,A            ;
        MOV     Acc,lfsr+1          ;
        RRC     A                   ;
        MOV     lfsr+1,A            ;
                                    ;
        JNC     ?random_x           ;
                                    ;
        XRL     lfsr+0,#0xF5        ;
        XRL     lfsr+1,#0x21        ;
                                    ;
?random_x:                          ;
                                    ;
        DJNZ    R7,?random_loop     ;
                                    ;
    #pragma ENDASM

    return( (unsigned char) lfsr );
}

extern unsigned char small_rand();

main()
{
    data  unsigned int loop;

    data  unsigned int v,target;

    xdata unsigned int a[256];

    loop = 256;

    do
    {

         a[ loop ] = 0;

    } while( --loop != 0 );

    target = small_rand();

    loop = 0;

    do
    {

        v = small_rand();

        a[ v % 256 ]++;

        if( v == target )
        {
            target = v;
        }

    } while( ++loop != 0 );
}

"my MCU is out of code memory"

Would simply using the time be sufficently "random" for your purpose?

Is there some external event/signal that would give you an essentially random number?
eg, the time a user presses some button (any button - doesn't have to be a "generate random number" button), or the instantaneous value of some analogue signal, or some unrelated count, or...