pseudo random 8bit number in A

; returns pseudo random 8 bit number in A. Only affects A.
; (r_seed) is the byte from which the number is generated and MUST be
; initialised to a non zero value or this function will always return
; zero. Also r_seed must be in RAM, you can see why......

rand_8:
    LD    A,(r_seed)    ; get seed
    AND    #B8h        ; mask non feedback bits
    SCF            ; set carry
    JP    PO,no_clr    ; skip clear if odd
    CCF            ; complement carry (clear it)
no_clr:
    LD    A,(r_seed)    ; get seed back
    RLA            ; rotate carry into byte
    LD    (r_seed),A    ; save back for next prn
    RET            ; done

r_seed:
    DB    1        ; prng seed byte (must not be zero)