Bad Register Name

[FunnyGuy9796]

I am trying to write to a register and want to be able to set the register as a variable. However, when I try to use inline assembly in order to set the register to a variable I get an error saying "Bad register name '%0'"

Here is the code:

Code: Select all

void write_reg(uint32_t addr, uint32_t val) {
    __asm__ volatile("mov %0, %1");
}

I believe I am doing it correctly by setting the register and value as variables using "%0" and "%1". Help would be much appreciated!

[klange]

You haven't specified any inputs or outputs. Without them, the "%0" syntax does not work and an actual register name is expected.

[FunnyGuy9796]

Pardon my ignorance but wouldn’t ‘addr’ and ‘val’ translate to ‘%0’ and ‘%1’?

[klange]

Arson my ignorance but wouldn’t ‘addr’ and ‘val’ translate to ‘%0’ and ‘%1’?

No. You need to pass them as inputs, and also mark clobbers. They are not associated with your arguments by default - and your arguments don't exist in registers by default in x86-32 anyway as they are passed on the stack. You need explicitly ask for them to be put into registers for use by the inline assembly.

Here's a quick link to the manual for reference: https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html

[Octocontrabass]

Do you need inline assembly in the first place? This is the sort of thing the volatile keyword is intended for.

Code: Select all

void write_reg( size_t addr, uint32_t val )
{
    *(volatile uint32_t *)addr = val;
}

[FunnyGuy9796]

Oh, ok! I had seen this used before but could never really figure out the use or need for it. Thank you!