On Thu, Aug 27, 2015 at 3:53 PM, sudarshan.rajagopalan <sudarshan.rajagopa...@vecna.com> wrote: > Hey guys, > > I was working on the exception handler for the CPU hard fault. Managed to > register the fatal error user extension to RTEMS and call the user defined > function when an exception occurs. But the pointer to the stacked frame > didn't have the correct register values(esp. the PC register). So I looked > into the assembly code in /cpukit/score/cpu/arm/armv7m-exception-default.c, > where it decides which stack pointer was used (MSP or PSP) before the > exception occurred depending on the error code returned in the Link > Register. After carefully examining all the assembly instructions, I guess > theres a little bug in the code. > > The instruction "cmn r2, #3\n" in line 31 basically compares the Link > Register(lr) to value 0xFFFFFFFD (negative #3, because CMN negates the RHS > and compares with LHS) and chooses MSP or PSP in the following IT block. > This is pretty cool! But, it will not work if you have the floating-point > unit (FPU) enabled in your processor, which is enabled in mine. With FPU > enabled, the error code returned is either 0xFFFFFFE9 or 0xFFFFFFED, for > which the above assembly instruction will not work out and MSP will be > selected always. > > Better way to do is to check the 2nd bit of the error code to determine > which stack pointer was used before the exception happened - "tst lr, #4\n" > and change the IT block from "itt ne" to "itt eq" and the "mov" and "add" > within this IT block. > > Have tested this with the above changes and it works. I have sent the patch > "0001-Fix-exception-handler-for-supporting-FPU.patch" to the devel mailing > list that fixes this problem. :)
Nice. Have you tested this without FPU support too? > > Thanks and Regards, > Sudarshan > _______________________________________________ > devel mailing list > devel@rtems.org > http://lists.rtems.org/mailman/listinfo/devel -- Daniel F. Gutson Chief Engineering Officer, SPD San Lorenzo 47, 3rd Floor, Office 5 Córdoba, Argentina Phone: +54 351 4217888 / +54 351 4218211 Skype: dgutson LinkedIn: http://ar.linkedin.com/in/danielgutson _______________________________________________ devel mailing list devel@rtems.org http://lists.rtems.org/mailman/listinfo/devel
