E5frog: Created page with "Sometimes you want to add a random number to your programming but you will have to make your own code to produce it.
A common way to make a pseudo-random number generator..."

2020-01-03T11:56:30Z

Created page with "Sometimes you want to add a random number to your programming but you will have to make your own code to produce it. A common way to make a pseudo-random number generator..."

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Sometimes you want to add a random number to your programming but you will have to make your own code to produce it. 
A common way to make a pseudo-random number generator (PRNG) is do use a Linear Feedback Shift Register (LFSR). 
This will give you a series of numbers from 0 to 255 in a seemingly randomized order. 

You will however get the same series of values in the same order every time, your "seed" number will choose the start position. 

Here's an example of a subroutine with 8-bit LFSR:

<pre>
; In this example the pseudo random number is stored in register 1
; Also uses register 0 for temporary storage

LFSR: ; Linear Feedback Shift Register
clr
as 1
bz doEor
lr 0, A ; Save original number in r0
sl 1 ; shift one step left
lr 1, A
bz noEor

; Do XOR if b7 was 1, workaround for missing carry on "sl"
lr A, 0
ni %10000000
bz noEor

doEor:
lr A, 1
xi $2b ; Do the XOR thing
lr 1, A
noEor:
pop

</pre>

The value used with XOR-function is $2B here, one of these other values can be used to give a different 256 byte non-repeating chain of values. 
$1d, $2b, $2d, $4d, $5f, $63, $65, $69, $71, $87, $8d, $a9, $c3, $cf, $e7, $f5 

When using $2B as XOR value the data will be this: 
[[File:LFSR_%242B.png]]

 
When using $71 as XOR value the data will be this: 
[[File:LFSR_%2471.png]]

 
[https://codebase64.org/doku.php?id=base:small_fast_8-bit_prng CodeBase64 used for reference]

Snippet:Pseudorandom numbers - Revision history

E5frog: Created page with "Sometimes you want to add a random number to your programming but you will have to make your own code to produce it. A common way to make a pseudo-random number generator..."

E5frog: Created page with "Sometimes you want to add a random number to your programming but you will have to make your own code to produce it.
A common way to make a pseudo-random number generator..."