OPERATION         R0       R1       R2       R3       R4       R5
+-------+      +----------------------------------------------------+
    |
    |          +----------- CONTENTS UNKNOWN -----------------------+
    |              |        |        |        |        |        |
    v              |        |        |        |        |        |
                   |        |        |        |        |        |
INTERRUPT +------+ v +----+ v +----+ v +----+ v +----+ v +----+ v +-+
    |
    |          +---------- SAVE/RESTORE IF NEEDED ------------------+
    |              |        |        |        |        |        |
    v              |        |        |        |        |        |
                   |        |        |        |        v        v
  .INTEN  +------+ v +----+ v +----+ v +----+ v +-------------------+
    |
    |          +---------- SAVE/RESTORE ---------++-- FREE TO USE --+
    |              |        |        |        |        |        |
    v              |        |        |        |        |        |
                   v        v        v        v        |        |
  .FORK   +------------------------------------------+ | +----+ | +-+
    |                                                  |        |
    |          +----------- FREE TO USE -------------+ | +----+ | +-+
    |              |        |        |        |        |        |
    v              |        |        |        |        |        |
                   v        v        v        v        v        v
 .SYNCH   +---------------------------------------------------------+
    |
    |          +----- FREE -----+ +--------- SAVE/RESTORE ----------+
    |              |        |        |        |        |        |
    |              |        |        |        |        |        |
    v              v        v        v        v        v        v          reg