Breakpoint type

There are generally two types of breakpoints: hardware (HW) breakpoints and software (SW) breakpoints. Arm® DS / DS-5 also has HW breakpoints and SW breakpoints. It is possible to separate the points.

The icon shown in Figure 1 makes it possible to distinguish whether the currently set breakpoint is HW or SW. Alternatively, you can check it by displaying a dedicated screen (breakpoint view).

*For Arm® DS, the breakpoint view is not displayed on the standard screen layout. Add it from the menu "Window → Show View → Breakpoint" at the top of the Arm® DS screen.

How to specify breakpoints

Figure 2 shows how to specify breakpoints. Basically, click the left side of the source code (or the left side of the disassemble screen). If you want to check and change the specified breakpoints all at once, we recommend using the breakpoint view in Figure 3.

*When setting a breakpoint from the right-click menu, the name of the menu differs between Arm® DS and DS-5. For Arm® DS, select the item you want to specify from below "DS Breakpoint".

Among the operation menus in the Breakpoints view, "Disable Breakpoint" and "Go to File" are particularly useful and recommended.

"Disable breakpoints" can temporarily disable registered breakpoints simply by switching ON/OFF the check Box on the left side of the list. While disabled, the CPU does not break even if breakpoint processing is executed.
In addition, the "Go to file" menu allows you to display the relevant line of the source code registered as a breakpoint at once, which is useful when debugging complicated programs.

There are other useful functions, so please check them by actually operating the debugger. (You can also specify complex break conditions by changing breakpoint properties.)

Difference between HW break and SW break

The table below summarizes the differences and features of HW breaks and SW breaks.

The biggest difference between HW break and SW break is whether or not the program is rewritten.

SW breakpoints involve rewriting the program. Therefore, it cannot handle cases where the program is reloaded during debugging, or cases where the program memory around the breakpoint is destroyed due to a bug. In addition, it cannot be used in systems that cannot rewrite the program area, such as the memory protection function and direct execution of programs on Flash (XIP).

HW breakpoints do not involve rewriting the program, so there are no restrictions like SW breakpoints.

HW breakpoints are functionally better, but there is an upper limit to the number of breakpoints that can be registered. Therefore, when there are many breakpoints to be registered, both SW break and HW break are used.

Use case 1: Capture the moment an exception occurs

Setting breakpoints on interrupt vectors in preparation for the occurrence of abnormal exception interrupts such as resets and aborts is extremely useful when investigating where problems occur.

As a result of abnormal software operation, read/write access to an illegal address may occur and an exception interrupt such as a data abort may occur. You can catch the moment when an exception occurs.

By also using the trace function introduced in the article below, you can go back and check the processing that the software performed immediately before the exception occurred.

Reference: Arm DS Utilization Techniques - How to use the trace function

Figure 4 shows how to set a breakpoint on an interrupt vector.

Use case 2: Specifying break conditions

The breakpoint function of Arm® DS/DS-5 allows you to set a break condition in addition to breaking unconditionally when the code at the specified location is executed.

For example, it is useful when you want to debug behavior after a process has been executed 100 times. Specifically, if 99 is specified for the "Ignore times" of the breakpoint stop condition, the break will occur at the 100th time.

It is also possible to create complex conditions by giving conditional expressions other than the number of times, so we recommend that you try each setting once.

Figure 5 shows how to specify break conditions.

In this article, we have introduced how to use the breakpoint function. Breakpoints themselves are a common function of a debugger, but Arm® DS / DS-5 has many useful functions such as specifying complex break conditions. We hope that these functions can be used to improve the efficiency of software verification work and defect investigations.

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