Setting up Interrupt Vector Table, ARMv6

Question

I'm trying to use usermode and SVC in my ARMv6 bare metal application, but for this I need to set up the SVC entry of the ARMv6 interrupt vector table to branch to my interrupt handler. But, I can't find a good example on how to do this (ie: what memory address exactly I need to set, and to what). I have done similar things in the past, but always with a more comprehensive bootloader (RedBoot) that set up some of this for me. Any help would be appreciated.

I am testing my application using:

 qemu-system-arm -M versatilepb -cpu arm1176

Answer 1

Are you talking about the SWI interrupt? Or one of the others (FIQ, IRQ). In either case I think I know what the problem is. Qemu is for running linux, your binary is not loaded at address 0x00000 so your entry points are not used by qemu for handling exceptions.

I have an example that uses qemu and implements a solution. Go to the qemu directory of http://github.com/dwelch67/yagbat. The qemu example is not really related to the gba thing in the yagbat repo, the gba is a 32 bit ARM thing so it was easy to borrow code from so I stuck it there.

The example was specifically written for your question as I tried to figure out how to use qemu in this manner. It appears that the address 0x00000000 space is simulated as ram, so you can re-write the qemu exception table and have the exceptions call code in the 0x10000 address space that your binary loads.

A quick and dirty solution is to make the entry point of the binary (that qemu loads to 0x10000) resemble a vector table at address 0x00000. The ldr pc instruction is relative to the program counter, the disassembly might show that it is loading an address at 0x10000 but it is really relative to the pc and the disassembler used the pc assuming the linked address being used.

.globl _start
_start:
    ldr pc,start_vector_add
    ldr pc,undef_vector_add
    ldr pc,swi_vector_add


start_vector_add: .word start_vector
undef_vector_add: .word undef_vector
swi_vector_add: .word swi_vector

Then before you want to cause any interrupts, in the example I use the swi instruction to cause an swi interrupt. You copy enough of the code from 0x10000 to 0x00000 to include the exception table and the list of addresses that it loads into the pc. by linking your program to 0x10000 those addresses are in the 0x10000 range. When the interrupt occurs, the exception handler that you have now modified will load the 0x10000 based address into the pc and your handler in the 0x10000 range will get called.

Using this command line to run the binary in my example

qemu-system-arm -M versatilepb -m 128M -kernel hello_world.bin

and then ctrl-alt-3 (not F3 but 3) will switch to the serial console and you can see the output, and close that window to close out of qemu and stop the simulation.