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Weird SPSR behaviour

Juha Aaltonen

I was trying to write a register context saving/restoring when I came across a weird behaviour.

My code (sorry, tried to format tens of times, but the editor WANTS to make asm a table):

asm volatile (
...
"pop {r0 - r3}"
"push {r0 - r3}"
"mov r0, r3"
"bl dbg_out" - outputs 60000013
"pop {r0 - r3}"
"msr cpsr_fsxc, r2"
"@dsb"
"@isb"
"msr spsr_fsxc, r3" - set value
"@dsb"
"@isb"
"mov lr, r1"
"mov sp, r0"
"push {r0 - r4, lr}"
"mov r0, lr"
"bl dbg_out"
"mov r0, sp"
"bl dbg_out"
"mrs r2, cpsr"
"mrs r3, spsr" - read value
"mov r0, r2"
"bl dbg_out"
"mov r0, r3" - outputs 00000002
"bl dbg_out"
...
);

When the exception is returned from, the calling function:

asm volatile ("svc #0\n\t");

    msg = "returned from SVC\r\n";

    serial_io.put_string(msg, util_str_len(msg)+1);

    asm volatile (

"mrs %[retreg], cpsr\n\t"
:[retreg] "=r" (tmp1) ::

    );

    msg = "cpsr = ";

    serial_io.put_string(msg, util_str_len(msg)+1);

    util_word_to_hex(scratchpad, tmp1);

    serial_io.put_string(scratchpad, 9);

    serial_io.put_string("\r\n", 3);

outputs "returned from SVC" and "cpsr = 60000013".

Why the "00000002"? the barriers don't seem to have any effect.

Parents
  • 0 in reply to Juha Aaltonen

    If you wrote the entire dbg_out yourself, it probably won't change r2 and r3.

    What I'm most worried about is that if it's written in C, the C-compiler will optimize it at some later point, so it changes r2 and r3.

    If dbg_out calls another C-routine, which you didn't write, then it's very much in danger of being unpredictable, regarding which registers it uses.

    If you've written dbg_out in assembly language, then begin the routine by saving r0-r3 and r12 (since it's a debugging routine), then you won't have the problem at a later point).

    -But it's great to see you found the error; this one is difficult to spot.

Reply
  • 0 in reply to Juha Aaltonen

    If you wrote the entire dbg_out yourself, it probably won't change r2 and r3.

    What I'm most worried about is that if it's written in C, the C-compiler will optimize it at some later point, so it changes r2 and r3.

    If dbg_out calls another C-routine, which you didn't write, then it's very much in danger of being unpredictable, regarding which registers it uses.

    If you've written dbg_out in assembly language, then begin the routine by saving r0-r3 and r12 (since it's a debugging routine), then you won't have the problem at a later point).

    -But it's great to see you found the error; this one is difficult to spot.

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