lfsr.c

#define POLYMASK_32 0xb4bcd35c
#define POLYMASK_31 0x7a5bc2e3

#include <stdio.h>
#include <stdint.h>

uint32_t lfsr32;
uint32_t lfsr31;

int shift_lsfr(uint32_t* lfsr, uint32_t polynomial_mask)
{
    int feedback = *lfsr & 1;
    *lfsr >>= 1;
    if (feedback) {
        *lfsr ^= polynomial_mask;
    }
    return *lfsr;
}

void init()
{
    lfsr32 = 0xabcde;
    lfsr31 = 0x23456789;
}

uint32_t get_random()
{
    shift_lsfr(&lfsr32, POLYMASK_32);
    uint32_t a = shift_lsfr(&lfsr32, POLYMASK_32);
    uint32_t b = shift_lsfr(&lfsr31, POLYMASK_31);
    return (a ^ b) & 0xffff;
}

int main()
{
    int histo[256] = {};
    init();

    for (int i = 0; i < 256*256*8; i++) {
        uint32_t r = get_random() & 255;
        histo[r]++;
        printf("%d\n", r & 255);
    }

    printf("histogram\n");
    for (int i = 0; i < 256; i++) {
        printf(" %03x", histo[i]);
        if ((i & 15) == 15) {
            printf("\n");
        }
    }
}

lfsr_6510.asm

// Original C version: https://gist.github.com/nurpax/d32529b017f71fd0e77c89a9b5a5e328
.const POLYMASK_32 = $b4bcd35c
.const POLYMASK_31 = $7a5bc2e3

// Initial seed
lfsr32: .byte $de, $bc, $0a, $00
lfsr31: .byte $89, $67, $45, $23

.macro shift_lsfr(lfsr, mask) {
    lda lfsr+0
    and #1
    tax
    lsr lfsr+3
    ror lfsr+2
    ror lfsr+1
    ror lfsr+0
    cpx #1
    bne skip_xor
    .for (var i = 0; i < 4; i++) {
        lda lfsr+i
        eor #((mask>>(i*8)) & $ff)
        sta lfsr+i
    }
skip_xor:
}

// return 16-bits of random in A/X
rng: {
    shift_lsfr(lfsr32, POLYMASK_32)
    shift_lsfr(lfsr32, POLYMASK_32)
    shift_lsfr(lfsr31, POLYMASK_31)
    // Note: we only xor the 2 least-significant bytes as we only want 16-bit
    // random out.
    lda lfsr32+0
    eor lfsr31+0
    tax
    lda lfsr32+1
    eor lfsr31+1
    rts
}

C code from https://www.maximintegrated.com/en/app-notes/index.mvp/id/4400