Page 263 - Embedded Microprocessor Systems Real World Design

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A
RTOS BWFER

C
BUFFER
RTOS MAILBOX
------
MESSAGE
0 MESSAGE
RTOS OLEUE

BUFFER 1
FULL

BUFFER 2
FULL
Ll
BUFFER 3
FILLING

4
BUFFER

Figure 9.3
RTOS Communication.

Scheduling Tasks

A task may be ready immediately after an event occurs or it may be scheduled to
start later. As already mentioned, a task may be scheduled to start when a sema-
phore is set, perhaps as a result of a hardware event. It also may be scheduled to
start after a number of ticks have elapsed or at a specific time of day in systems that
maintain time of day.

Memory Management

As I mentioned in the section about buffers and queues, a process requests memory
from the RTOS when it needs a buffer. This allows the system to get by on less
memory than otherwise would be required.
In the first example mentioned in this chapter, a system used either Ethernet or
RS232 to communicate with a host PC. Say that receive data needs 256K of memory
for each interface and transmit needs the same. In a non-RTOS system, the

244 Embedded Microprocessm Systems