How could we get the value of the misa register?

[zhengxiaolinX]

Hi,

I've tried

int misa;
asm volatile("csrr x0, misa" : "=r" (misa));
printf("%x\n", misa);

and got an An illegal instruction was executed! message in spike. I wonder how can I get the value of this register, or how could we get CPU features of the current simulator, e.g. whether the C extension is supported. Did I get something wrong? I am not so familiar with this, and thanks in advance.

[aswaterman]

misa is only readable in M-mode. If you are running a user program using pk, your program is running in U-mode, so it is impossible to directly read that register.

[zhengxiaolinX]

misa is only readable in M-mode. If you are running a user program using pk, your program is running in U-mode, so it is impossible to directly read that register.

Thank you @aswaterman for replying to this!
Just one more question, I wonder how to detect whether the C extension is supported on the current core by using some instructions. Thanks!

[aswaterman]

There isn't a straightforward way to check. You can try executing a C-extension instruction and see if you get an exception.

On RISC-V Linux, a program's ELF header indicates whether it requires the C extension, and the kernel's ELF loader will refuse to load the program if the processor does not support C.

[zhengxiaolinX]

There isn't a straightforward way to check. You can try executing a C-extension instruction and see if you get an exception.

On RISC-V Linux, a program's ELF header indicates whether it requires the C extension, and the kernel's ELF loader will refuse to load the program if the processor does not support C.

Thank you. I really appreciate your help and the details.