Synchronous External Abort on ARM

exception arm abort
Adhe Widianjaya picture Adhe Widianjaya · Dec 16, 2014 · Viewed 10.5k times · Source

I was building a bare metal application on ARM Cortex A9 Pandaboard, and I got Instruction Fetch Abort frequently. When I dump IFSR Register I got 0x1008. I've read the reference manual, and I understand that 1008 was Synchronous External Abort. The problem is what synchronous external abort means and where does it come from? Thanks for your help.

Answer

Notlikethat picture Notlikethat · Dec 16, 2014

The ARMv7 ARM section "VMSA Memory aborts" covers this as thoroughly as one would expect (given that it's the authoritative definition of the architecture), but to summarise in slightly less than 14 pages;

An abort means the CPU tried to make a memory access, which for whatever reason, couldn't be completed so raises an exception.

An external abort is one from, well, externally to the processor, i.e. something on the bus. In other words, the access didn't fault in the MMU, went out onto the bus, and either some device or the interconnect itself came back and said "hey, I can't deal with this".

A synchronous external abort means you're rather fortunate, in that it's not going to be utterly hideous to debug - in the case of a prefetch abort, it means the IFAR is going to contain a valid VA for the faulting instruction, so you know exactly what caused it. The unpleasant alternative is an asynchronous external abort, which is little more than an interrupt to say "hey, something you did a while ago didn't actually work. No I don't know what is was either."

So, you're trying to execute instructions from something that you think is memory, but isn't. Without any further details, the actual cause could be anything from a typoed hard-coded address, to dodgy page tables, stale TLB entries, cache coherency, etc. etc.

Related questions

How can I safely create a nested directory?

What is the most elegant way to check if the directory a file is going to be written to exists, and if not, create the directory using Python? Here is what I tried: import os file_path = "/my/directory/filename.…

Read More
Manually raising (throwing) an exception in Python

How can I raise an exception in Python so that it can later be caught via an except block?

Read More
How do you assert that a certain exception is thrown in JUnit 4 tests?

How can I use JUnit4 idiomatically to test that some code throws an exception? While I can certainly do something like this: @Test public void testFooThrowsIndexOutOfBoundsException() { boolean thrown = false; try { foo.doStuff(); } catch (IndexOutOfBoundsException e) { thrown = true; } assertTrue(thrown); } I …

Read More