GetRandomNumber:
; Uses a 16-bit RAM variable called RandomNumberGeneratorWord
; Returns an 8-bit pseudo-random number in a
    push hl
        ld hl,(RandomNumberGeneratorWord)
        ld a,h         ; get high byte
        rrca           ; rotate right by 2
        rrca
        xor h          ; xor with original
        rrca           ; rotate right by 1
        xor l          ; xor with low byte
        rrca           ; rotate right by 4
        rrca
        rrca
        rrca
        xor l          ; xor again
        rra            ; rotate right by 1 through carry
        adc hl,hl      ; add RandomNumberGeneratorWord to itself
        jr nz,+
        ld hl,$733c    ; if last xor resulted in zero then re-seed random number generator
+:      ld a,r         ; r = refresh register = semi-random number
        xor l          ; xor with l which is fairly random
        ld (RandomNumberGeneratorWord),hl 
    pop hl
    ret                ; return random number in a