arminterruptstm32f4discovery

Return Address from Interrupt in STM32F

I want to know the return address from Interrupt in STM32F, especially arm processor.

In normal function, return address is saved in Link Register, but in interrupt, I found that the value of Link Register is 0xFFFFFF9.

So I found the Ref. manual of armv6, but I still don't know where the return address is saved.

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I attached part of the Ref. Manual.

I guessed that ReturnAddress(ExceptionType); is the place where the return address is saved.

So I checked [SP, #24] but the value was not the return address.

Solution

  • It is all documented, all you have to do is try it.

    .thumb_func
.global _start
_start:
stacktop: 
.word 0x20001000
.word reset 
.word hang
.word hang
.word hang
.word hang
.word hang
.word hang
.word hang
.word hang
.word hang
.word svcall
.word hang
.word hang
.word hang
.word hang


.thumb_func
.globl TEST
TEST:
    mov r0,#0
    mov r1,#1
    mov r2,#2
    mov r3,#3
    svc 0
    bx lr

void svcall ( void )
{
...
}

    dumping the stack

    from arm docs

    MemA[frameptr,4]     = R[0];
MemA[frameptr+0x4,4] = R[1];
MemA[frameptr+0x8,4] = R[2];
MemA[frameptr+0xC,4] = R[3];
MemA[frameptr+0x10,4] = R[12];
MemA[frameptr+0x14,4] = LR;
MemA[frameptr+0x18,4] = ReturnAddress();
MemA[frameptr+0x1C,4] = (xPSR<31:10>:frameptralign:xPSR<8:0>);

    compiled svcall has a stack frame

    080001e8 <svcall>:
 80001e8:   b570        push    {r4, r5, r6, lr}

    covering the first part of the stack

    00000003 
00000000 
00000000 
FFFFFFF9  lr

    then the exception portion of the stack that the logic uses.

    00000000 r0
00000001 r1
00000002 r2
00000003 r3
00000000 r12
08000267 lr
0800005E return address
21000000 psr


08000054 <TEST>:
 8000054:   2000        movs    r0, #0
 8000056:   2101        movs    r1, #1
 8000058:   2202        movs    r2, #2
 800005a:   2303        movs    r3, #3
 800005c:   df00        svc 0
 800005e:   4770        bx  lr

    and that is the expected return address.

    Not sure why you would care what the return address is. Note that that is not the address you would branch nor return to (as documented) it would be 800005f in this case, of course. But you would also have to restore the registers manually (r0,1,2,...) if you want to return to that address.