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4 Debugging Your BSP

4.3 Advanced Debugging Techniques

127

Software Breakpoints

The type of breakpoints typically used during application development are

software breakpoints. With software breakpoints, the debugger takes an opcode

from RAM and replaces it with a special instruction that causes a debug exception,

and sets up a handler to be executed when the debug exception occurs. When the

handler runs, it checks to see if the developer has a breakpoint set at that location

at the time. If the developer does not have a breakpoint set, the handler executes

the saved opcode and continues execution of the program. However, if the

developer set a debugging breakpoint to stop the application from executing, the

exception handler turns control over to the debugger. At this time, the developer

can perform whatever action is required.

Hardware Breakpoints

Hardware breakpoints allow similar functionality to software breakpoints, but

they do so without replacing the opcode as is done by software breakpoints. To use

this type of breakpoint, the processor must support hardware breakpoints.

Different processors support different hardware breakpoint functionality, but it is

common for the hardware breakpoint support to be limited to one or two

breakpoints.

Because the hardware breakpoints have access to the processor internals, they can

also provide additional functionality that is not available with software

breakpoints. For example, on some processors, it is possible to set a hardware

breakpoint to cause a debug exception when a given memory location is modified,

or even when a given memory location is read. This allows watchpoints and other

capabilities without the cumbersome mechanisms required to implement that

functionality using software breakpoints.

For the remainder of this discussion, these additional features are not important.

Nevertheless, these additional capabilities should be kept in mind while

debugging.

Overview of the Boot Procedure as it Relates to OCD

The nature of breakpoints is important when debugging BSPs. Recall that on

powerup, the processor starts executing at the reset vector address, usually

contained in flash. The code contained in the flash copies itself to RAM and begins

execution. Often, the boot loader then loads another image into a different location

in RAM and starts that image executing.

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