初期状態               wposが先               rposが先
  +-------------+        +-------------+        +-------------+ 
  |             |        |             |        |/////////////| 
  |             |        |             |        |/////////////| 
  |             |       W+-------------+       R+-------------+ 
  |             |        |/////////////|        |             | 
  |             |        |/////////////|        |             | 
  |             |       R+-------------+       W+-------------+ 
  |             |        |             |        |/////////////| 
RW+-------------+        +-------------+        +-------------+ 


  emp       !emp       !emp      !emp      !emp      !emp      !emp      emp
   +---+     +---+      +---+     +---+     +---+     +---+     +---+     +---+
  5|   |    5|   |     5|   |    5|   |    5| c |    5| c |    5|   |    5|   |
  4|   |    4|   |     4|   |    4|   |    4| b |    4| b |    4|   |    4|   |
  3|   |   W3|   |    W3|   |  RW3|   |   R3| a |  RW3| a |   W3|   |  RW3|   |
  2|   |    2| C |     2| C |    2|   |   W2|   |    2| f |    2| f |    2|   |
  1|   |    1| B |    R1| B |    1|   |    1| e |    1| e |   R1| e |    1|   |
RW0|   |   R0| A |     0|   |    0|   |    0| d |    0| d |    0|   |    0|   |
   +---+     +---+      +---+     +---+     +---+     +---+     +---+     +---+
wr(ABC)    rd(1)      rd(2)     wr(abcde) wr(f)     rd(4)      rd(2)