Page 254 - Embedded Microprocessor Systems Real World Design

Real-Time Operating Systems 9

The theory and use of a real-time operating system (RTOS) can and has taken
entire books. This chapter provides an overview.
As embedded systems grow in complexity, they start to look more and more like
their personal computer (PC) cousins. Software development for an embedded
system often is complicated by the need to control system resources. In addition,
some embedded systems need to connect to Ethernet interfaces, harddisk drives,
and other PC-like peripherals. If all the software is written from scratch, code must
be written to interact with every device. For many standard interfaces, this is a dupli-
cation of the effort already expended by some other software engineer.
In a typical embedded system, each function or process handles its own
resources, somewhat independent of the others. A process that interfaces to a host
system over an Ethernet link, for example, has memory allocated for its buffers. A
similar process may have code and buffers for an RS232 connection. The polling
loop gives each process control, one at a time, and each checks for data to or from
its respective interface. But say that, in this example system, the host uses either
Ethernet or RS-232, never both. In that case, the system really does not need both
pieces of code and both sets of buffers active at the same time. This system could
get by with less RAM by managing the memory, allocating whichever buffer is not
needed to other purposes.
In addition to memory management, all embedded systems must schedule
processes in some manner. The polling loop method, sometimes called sequential
or ruund-robin scheduling, is probably the most common. In the pool timer example,
each task (motor on/off control, time rollover handling, keypad processing) is
given control one at a time. When motor control is finished, it passes control to
the time rollover process, which subsequently hands it off to the keypad (mode
control) code, which returns to the motor control code. Task scheduling is one
big loop.
Although this method works well for simple systems like the pool timer, it has
some drawbacks. In the pool timer example, each task runs until it is finished. The
keypad processing code takes as much time as is needed to handle user inputs from

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