Programming Embedded Systems in C and C++

36 Chapter 4: Downloading and Debugging

Like a debug monitor, an emulator uses a remote debugger for its human inter-

face. In some cases, it is even possible to use the same debugger frontend for

both. But because the emulator has its own copy of the target processor it is possi-

ble to monitor and control the state of the processor in real time. This allows the

emulator to support such powerful debugging features as hardware breakpoints

and real-time tracing, in addition to the features provided by any debug monitor.

With a debug monitor, you can set breakpoints in your program. However, these

software breakpoints are restricted to instruction fetches—the equivalent of the

command “stop execution if this instruction is about to be fetched.” Emulators, by

contrast, also support hardware breakpoints. Hardware breakpoints allow you to

stop execution in response to a wide variety of events. These events include not

only instruction fetches, but also memory and I/O reads and writes, and inter-

rupts. For example, you might set a hardware breakpoint on the event “variable

foo contains 15 and register AX becomes 0.”

Another useful feature of an in-circuit emulator is real-time tracing. Typically, an

emulator incorporates a large block of special-purpose RAM that is dedicated to

storing information about each of the processor cycles that are executed. This fea-

ture allows you to see in exactly what ...