Page 277 - Embedded Microprocessor Systems Real World Design

Protected Mode Programming Intel x86 processors, from the 386 on, imple-
ment protected mode programming. In the 8086/8088 and the 80186/188, the
only memory model available is the Real mode. The addressing scheme for these
CPUs permits addressing of 1 MB of memory (20-bit address) , in 16 64K segments.
The 386 and higher CPUs use a different addressing scheme that permits access to
4 GB of memory using 32-bit addresses.

Hardware Memory Management In the Real mode, every task has access to the
entire 1 MB memory space. Any task can read or write to any address. In the Pro-
tected mode, the memory partitions may be protected so that a task cannot access
memory outside its own segment. A segment of memory even can be shared
between two tasks, so that one task can both write and read to the common area,
while another task can only read it. Attempts by a task to execute or access memory
outside its bounds cause the hardware to generate an exception condition that can
be handled by the operating system.

Hardware Debug Registers The 386 and higher processors have registers that
simplify debugging. The four address registers can be set to break on read, write,
or read/write access. Debuggers can take advantage of these onchip resources to
simplify debugging.

PO Control The x86 architecture has a 64KI/O space, separate from the memory
space, that can be accessed with unique instructions. In the Protected mode, the
1/0 space can be protected so that only the operating system has access to 1/0
ports. This forces applications to use the operating system resources to access I/O.

Drawbacks of Using a PC Platform

So, if it is this easy to design a PGbased system, why doesn’t every system use a PC?
This section discusses the few drawbacks to using a PC.

Product Cost
This may not be an issue for low-volume applications or for systems in which the
embedded control components are a small part of a much larger system (such as
an automated assembly line), but for consumer and other cost-sensitive applica-
tions it is an issue. Imagine a microwave oven that must be sold with a PC attached.
Enough said.
Another thing that drives product costs higher in PC systems is that you must
pay for everything that comes with the PC architecture. If your application needs

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