doRNG:
  ;run this during vblank for best results.
  JSR SYS_READ_CALENDAR
  ;gets the calendar.
  ;MAME uses your computer's time for this.
  MOVE.L BIOS_HOUR,D0   ;D0 = HHMMSS00
  LSR.L #8,D0                     ;shift out the zeroes.
  MOVE.L frame_timer,D1           ;this value is incremented by 1 every vBlank (i.e. just before this procedure is run)
  NOT.L D1                        ;flip all the bits of D1
  MULS D1,D0
  MULU D1,D0
  MOVE.L JOYPAD1,D1               ;get the most recent button presses.
  CloneByte D1                    ;copy this byte to all 4 bytes of D1
  EOR.L D1,D0
  MOVE.L RNGout_32,D2         ;look at last time's results.
  AND.B #1,D2                     ;check if it's odd or even
  BNE SwapRNGifEven
    SWAP D0                 ;if even, swap the low and high words of D0
SwapRNGifEven:
  MOVE.L D0,RNGout_32
  rts