rng for 6502

random:
    ;note: all arithmetic should be unsigned
    ;a, b, c, and d are the input variables
    ;a', b', c', d' are the outputs

    ;e = a - 8bit_rotate_left(b)
    lda RndB
    asl a
    adc #$80
    eor #$FF
    sec
    adc RndA
    tay
    ;a' = b eor nibble_swap(c)
    lda RndC
    asl a
    adc #$80
    rol a
    asl a
    adc #$80
    rol a
    eor RndB
    sta RndA
    ;b' = c + d
    lda RndC
    clc
    adc RndD
    sta RndB
    ;c' = d + e
    tya
    clc
    adc RndD
    sta RndC
    ;d' = a' + e
    ;(also this is the result)
    tya
    clc
    adc RndA
    sta RndD
    rts