* R0 = Client handle for this task.
R1 = Either the ID or client handle for the recipient (if R2 not 0).
R2 ! Block containing the message to send, or 0 to check if the buffer already contains a message.
* R2 ! Message buffer, or 0 if no message is waiting to be delivered.
Purpose: Send a message to another client task.
Interrupts: Interrupt status is undefined.
Fast interrupts are enabled.
Processor mode: Processor is in SVC mode.
Re-entrancy: SWI is not re-entrant.
Use: Send a message to another task. The destination task can be specified
using either it's ID or client handle. If the ID is specified then it
will be offered to each matching task that polls it until it is
acknowledged. A particular message is only offered to each task once.
When sending messages to PC software (ID = 0) the handle should
always be specified.
Note that the message buffer for each task can only contain a single
message to send; if there is already a message waiting to be
delivered then it is overwritten. When multiple clients are used,
this SWI should first be called with R2 = 0 to check if a message
is still waiting. Broadcast messages are never cleared automatically.
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