How to resolve the algorithm Knuth shuffle step by step in the 6502 Assembly programming language

Published on 12 May 2024 09:40 PM
#6502 assembly

How to resolve the algorithm Knuth shuffle step by step in the 6502 Assembly programming language

Table of Contents

Problem Statement

The   Knuth shuffle   (a.k.a. the Fisher-Yates shuffle)   is an algorithm for randomly shuffling the elements of an array.

Implement the Knuth shuffle for an integer array (or, if possible, an array of any type).

Given an array items with indices ranging from 0 to last, the algorithm can be defined as follows (pseudo-code):

(These are listed here just for your convenience; no need to demonstrate them on the page.)

Let's start with the solution:

Step by Step solution about How to resolve the algorithm Knuth shuffle step by step in the 6502 Assembly programming language

Source code in the 6502 programming language

define sysRandom $fe
define tempMask $ff
define range $00
define tempX $01
define tempY $02
define tempRandIndex $03
define temp $04
CreateIdentityTable:
    txa
    sta $0200,x
    sta $1000,x
    inx
    bne CreateIdentityTable
;creates a sorted array from 0-255 starting at addr $1000
;also creates another one at $0200 for our test input

lda #1
sta range

ConstrainRNG:
ldx #255
;max range of RNG
lda range
bne outerloop
jmp end

outerloop:
cpx range
bcc continue	;if X >= range, we need to lower X
pha
  txa
  sta tempX

  lsr
  cmp range
  bcc continue2

  tax
pla
jmp outerloop

continue2:
pla
ldx tempX

continue:
ldy range

KnuthShuffle:
lda sysRandom
and $1000,x	;and with range constrictor
tay

lda $0200,y
sty tempRandIndex
sta temp
ldy range
lda $0200,y
pha
    lda temp
    sta $0200,y
pla
ldy tempRandIndex
sta $0200,y
dec range
jmp ConstrainRNG

end:
brk

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