ARM9 SVC_STACK out of range?

Question

I'm debugging some odd ARM exceptions in an embedded system using the IAR workbench toolchain. Sometimes, when an exception is trapped the SVC_STACK is reported as out of range (very out of range!) Is this relevant, or just an artifact of the J-Link JTAG debugger? What is the SVC_STACK used for? It is set to 0x1000 size, but when it is out of range, it is way up in our heap area. Thanks!

Answer 1

ARMs SVC mode is entered when an exception occurs (not an IRQ or FIQ - fast IRQ). It can also be entered directly by code executing in non-user mode by setting the CPRS register, but I think this is uncommon except for when initializing the system.

When an exception occurs, the processor switches to the SVC stack, which has to be set up very early in the initialization of the system. I'm guessing that your initialization code is not properly setting up the SVC stack, or it's possible that one of the exception handlers is not coded properly and is trashing the stack.

A third possibility is that you're using an RTOS that sets up the ARM stacks the way it wants (basically overriding the SVC stack that the IAR's initialization code might set up). If this is the case, it's possible that everything is OK, but the IAR debugger thinks the SVC stack is out of range - the debugger will get its information from the linker config file - but if something changes the stack to another area of memory, then the debugger will get confused.

This happened to me all the time with the user mode stack in IAR when using an RTOS - the stacks were allocated based on task control blocks which were not in the CSTACK segment the debugger thought it should be in, and the debugger would issue irritating warnings. There was some project configuration setting that could be used to quiet the warnings, but I don't recall off the top of my head what it was - we rarely bothered with it, and just lived with the noise.

You'll need to verify that the the stack 'way up in the heap' area is valid - if you don't have some bit of code explicitly doing this, it's likely that it's wrong (or maybe you'll need to ask your RTOS vendor).

The ARM Architecture Reference Manual (ARM ARM) is freely available from arm.com and goes into excruciating detail about how the ARM stacks work. Another good reference is the ARM System Developer's Guide by Andrew Sloss, et al.