BBC BASIC Programmers' Reference

This page is read only. You can view the source, but not change it. Ask your administrator if you think this is wrong.
=====Generating pseudo-random numbers=====

//by Richard Russell, August 2006//\\ \\  BBC BASIC supplies pseudo-random numbers in three formats:\\ \\ 

  * **RND** supplies a pseudo-random 32-bit integer
  * **RND(N)** supplies a pseudo-random positive integer in the range 1 to N
  * **RND(1)** supplies a pseudo-random floating-point number in the range 0.0 to 1.0
\\  This article contains routines to provide similar facilities for your assembly-language programs. They are short and very fast, yet the pseudo-random numbers they supply are of an equivalent quality to those supplied by RND.\\ \\ 
==== RND ====
\\  The subroutine below provides a direct equivalent to RND. Each time it is called it returns a pseudo-random 32-bit integer in the eax register:\\ \\ 
        .seed
        dd 0
        db 1
        ;
        ; Rnd - return a pseudo-random 32-bit integer
        ;   Inputs - None
        ;  Outputs - eax = result
        ; Destroys - eax, cl, edx, flags
        ;
        .Rnd
        mov  eax,[seed]
        mov  cl,[seed+4]
        mov  edx,eax
        shr  cl,1
        rcr  edx,1
        rcl  cl,1
        shl  eax,12
        xor  edx,eax
        mov  eax,edx
        shr  eax,20
        xor  eax,edx
        mov  [seed+4],cl
        mov  [seed],eax
        ret
This subroutine will generate the same sequence of numbers every time it is used. You are likely to want to //randomise// the sequence in order to make the results less predictable. You can simply do that using the following code, which uses the time as a seed:\\ \\ 
        .Randomise
        call "GetTickCount"
        mov  [seed],eax
        ret
You can test the code from BASIC as follows:

<code bb4w> 
        CALL Randomise
        FOR I% = 1 TO 20
          PRINT ~ USR(Rnd)
        NEXT
</code>

==== RND(N) ====
\\  The subroutine below provides a direct equivalent to RND(N), where N is a positive integer greater than 1. Each time it is called it returns a pseudo-random integer in the range 1 to N, in the eax register:\\ \\ 
        ;
        ; RndRange - return a pseudo-random integer in the range 1 to N
        ;   Inputs - ebx = N
        ;  Outputs - eax = result
        ; Destroys - eax, cl, edx, flags
        ;
        .RndRange
        call Rnd
        xor  edx,edx
        div  ebx
        mov  eax,edx
        inc  eax
        ret
The parameter N is passed to the subroutine in the ebx register. If you prefer to return a number in the range 0 to N-1 just delete the **inc eax** instruction.\\ \\  You can test the code from BASIC as follows:

<code bb4w> 
        CALL Randomise
        FOR I% = 1 TO 20
          B% = 100
          PRINT USR(RndRange)
        NEXT
</code>

==== RND(1) ====
\\  The subroutine below provides a direct equivalent to RND(1). Each time it is called it returns a pseudo-random 64-bit floating-point value (double) in the range 0.0 to 1.0:\\ \\ 
          ;
          ; RndFloat - return a pseudo-random float in the range 0.0 to 1.0
          ;   Inputs - ebx = memory address of 64-bit float (double)
          ;  Outputs - result stored at [ebx]
          ; Destroys - eax, ecx, edx, flags
          ;
          .RndFloat
          call Rnd
          bsr  ecx,eax
          ror  eax,cl
          add  ecx,991
          shld ecx,eax,20
          shl  eax,20
          mov  [ebx],eax
          mov  [ebx+4],ecx
          ret
The value is returned in memory at the address passed in the ebx register.\\ \\  You can test the code from BASIC as follows:

<code bb4w> 
        CALL Randomise
        FOR I% = 1 TO 20
          B% = ^R#
          CALL RndFloat
          PRINT R#
        NEXT
</code>