T-Kernel Specification (1.B0.02)

128 CHAPTER 4. T-KERNEL/OS FUNCTIONS
Rendezvous operation is explained here using the example in Figure 4.5. In this figure Task A and Task
B are running asynchronously.
• If Task A first calls tk cal por, Task A goes to WAIT state until Task B calls tk acp por. The
state of Task A at this time is WAIT on rendezvous call (a).
• If, on the other hand, Task B first calls tk acp por, Task B goes to WAIT state until Task A calls
tk cal por. The stat of Task B at this time is WAIT on rendezvous acceptance (b).
• A rendezvous is established when both Task A has called tk cal por and Task B has called
tk acp por. At this time Task A remains in WAIT state while the WAIT state of Task B is
released. The state of Task A is WAIT for rendezvous completion.
• The Task A WAIT state is released when Task B calls tk rpl rdv. Thereafter both tasks enter a
run state.
TASK A
TASK B
TASK A
TASK B
tk cal por
wait on call
✲ tk acp por
wait on completion
✛ tk rpl rdv
tk acp por
wait on acceptance
tk cal por ✲
wait on completion
✛ tk rpl rdv
(a) tk cal por called first
(b) tk acp por called first
Figure 4.5: Rendezvous Operation
As an example of a specific method for assigning rendezvous object numbers, the ID number of the task
calling the rendezvous can go in the low bits of the rendezvous number, with the high bits used for a
sequential number.
[Rationale for the Specification]
While it is true that the rendezvous functionality can be achieved through a combination of other
synchronization and communication functions, better efficiency and ease of programming are achieved
by having a dedicated function for cases where the communication involves an acknowledgment. One
advantage of the rendezvous function is that since both tasks wait until message passing is completed,
no memory space needs to be allocated for storing messages.
The reason for assigning unique rendezvous numbers even when the same task does the calling is as
follows. It is possible that a task, after establishing a rendezvous and going to WAIT state for its
completion, will have its WAIT state released due to timeout or forcible release by another task, then
again call a rendezvous and have that rendezvous established. If the same number were assigned to both
the first and second rendezvous, attempting to terminate the first rendezvous would end up terminating
the second rendezvous. If separate numbers are assigned to the two rendezvous and the task in WAIT
state for rendezvous completion is made to remember the number of the rendezvous for which it is
waiting, error will be returned when the attempt is made to terminate the first rendezvous.
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T-Kernel 1.B0.02