Page 303 - Embedded Microprocessor Systems Real World Design

is required with only 256 bytes of on-chip RAM. The microchip debug capability is
enabled by programming a bit when the microcontroller EPROM is programmed.
Many microprocessors implement a joint test action group (JTAG) interface for
debugging. ThepAG interface is a standardized serial interface that permits auto-
matic test equipment to serially read and write the contents of internal registers in
the IC. TheJTAG interface standard (IEEE 1149) is flexible enough that it also can
support on-chip debugging capabilities.
The AMD SC520 uses theJTAG interface to provide debug support. An internal
memory stores trace information about program execution. Of course, with a
serial interface, there is no way to track every instruction in real time, so the trace
information is partial. The software in the host PC must do some of the work of
decoding the debug information from the chip.

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Memory Management Hardware

Many advanced microprocessors include hardware for memory management. The
features provided by memory management include the following.

Memory Protection
As mentioned in Chapter 4, there is nothing to prevent a berserk program from
writing all through memory. In a system with a memory management unit (MMU),
each program is limited to its own area of memory and cannot corrupt memory
allocated to other programs.

Write Protection
Using an MMU, certain areas of memory can be set aside as read-only, even though
they are physically implemented as RAM. The MMU detects any attempts to write
to those memory areas.

Relocation
A program may be written with absolute branch addresses and it may access
absolute memory locations. Such programs cannot be relocated because the
addresses would all be wrong. The MMU can translate the addresses, allowing
the programs to be executed Erom any location in memory.

Supervisor
Processors that have an MMU also have multiple privilege levels. Supervisor is one
of these. Among other things, the supervisor level allows the MMU to be pro-

284 Embedded Mimopfomsm Systems