ARM DS-5 Using the Debug Hardware Configuration Utilities

Debugging with your debug hardware unit8.3 Adding an application SVC handler when using debug hardwareMany applications require their own SVC handlers in addition to semihosting SVCs. To ensurethat the application SVC handler cooperates with the debug hardware semihosting mechanism,use your debugger to:1. Install the application SVC handler into the vector table.2. Modify the value of SEMIHOST_VECTOR to point to a location that is only reached if yourhandler does not recognize the SVC, or recognizes it as a semihosting SVC.For example, a particular SVC handler might detect if it has failed to handle a SVC and branchto an error handler. An example of a basic exception handler is shown in the following example.Example 8-1 Basic SVC handler; r0 = 1 if SVC handledCMP r0, #1; Test if SVC has been handled.BNE NoSuchSVC; Call unknown SVC handler.LDMFD sp!, {r0}; Unstack SPSR...MSR spsr_cf, r0; ...and restore it.LDMFD sp!, {r0-r12, pc}^ ; Restore registers and return.You can modify this code for use in conjunction with debug hardware semihosting as shown inthe following example.Example 8-2 SVC handler with debug hardware link; r0 = 1 if SVC handledCMP r0, #1; Test if SVC has been handled.LDMFD sp!, {r0}; Unstack SPSR...MSR spsr_cf, r0; ...and restore it.LDMFD sp!, {r0-r12, lr} ; Restore registers.MOVEQS pc, lr; Return if SVC handled.Semi_SVCMOVS pc, lrYou must then set up the SEMIHOST_VECTOR with the address of Semi_SVC. The instruction at thisaddress is never actually executed because the debug software returns directly to the applicationafter processing the semihosted SVC. Using a normal SVC return instruction ensures that theapplication does not crash if the semihosting breakpoint is not set up.If the application is linked with the semihosted ARM C library, and therefore uses the C librarystartup code, you must change the contents of SEMIHOST_VECTOR before the application installs itsown handler, typically by setting a breakpoint in the main code. This is because, ifSEMIHOST_VECTOR is set to the fall-through part of the application SVC handler before theapplication starts execution, the semihosted SVCs that are called by the library initialization cantrigger an unknown breakpoint error. At this point, the SVC vector has not yet had theapplication handler written to it, and might still contain the software breakpoint bit pattern. Thistriggers a breakpoint that the debug software does not know about, because the SEMIHOST_VECTORaddress has moved to a place that cannot currently be reached.NoteIf semihosting is not used by your application, including the startup code, you can simplify thisprocess by setting SEMIHOST_ENABLED to zero.ARM DUI 0498F Copyright © 2010-2012 ARM. All rights reserved. 8-5ID021112Non-Confidential