ARM DS-5 Using the Debug Hardware Configuration Utilities

Debugging with your debug hardware unit8.6 Software instruction breakpointsFor processors that do not support hardware instruction breakpoints, or in cases where you haveused up all the available hardware breakpoint resources, you can use software instructionbreakpoints. Software breakpoints modify the instruction in memory to create a special valuethat causes the processor to enter debug state when executed. The value written to memorydepends on the processor you are using. For ARM processors, one of the following schemes isused, depending on the architecture and processor revision:• An undefined instruction is written to memory, and a hardware breakpoint resource isused to spot this instruction being executed. The processor enters debug state when thehardware breakpoint unit spots the undefined instruction entering the execute pipelinestage.• An ARMv5 BKPT instruction is written to memory, and a hardware breakpoint resource isused to spot the instruction being executed. The processor enters debug state when thehardware breakpoint unit spots the BKPT instruction entering the execute pipeline stage.• An ARMv5 BKPT instruction is written to memory. When this instruction is executed, theprocessor automatically enters debug state.Where a hardware breakpoint unit is used to spot software instruction breakpoints, only a singlehardware resource is used, no matter how many software instruction breakpoints are set. If youhave difficulty setting software instruction breakpoints, you might have to free up a hardwarebreakpoint resource first.Software breakpoints cannot be used to debug code in Flash or ROM, and can be unreliable inself-modifying code.NoteWhen viewing memory or disassembly, debug hardware reports the actual contents of memory.Prior to running, any software breakpoints are written to memory. When the processor halts, thesoftware breakpoints are removed from memory. On a number of processors, it is not possibleto access memory while running. This means that if you disconnect debug hardware from theprocessor while the target is running, the breakpoints are left in memory. If the processorsubsequently executes one of the instructions, then (depending on the processor architecture)the processor either stops at the software breakpoint or causes the processor to take an undefinedexception.8.6.1 See alsoConcepts• Hardware breakpoints on page 8-8• Processor exceptions on page 8-10• Breakpoints and the program counter on page 8-11• Interaction between breakpoint handling in the debug hardware and your debugger onpage 8-12• Problems setting breakpoints on page 8-14.ARM DUI 0498F Copyright © 2010-2012 ARM. All rights reserved. 8-9ID021112Non-Confidential