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gccassemblyinline-assemblypowerpc

Registers modified by systemcall invoked through gcc's extended asm

I am using gcc's extended asm to invoke a system call. I am working on a proprietary RTOS on a PowerPC (Freescale MPC5200B).

According to gcc's documentation I should add all registers that the assembly code uses - and that are neither input nor output - to the clobbers list, because gcc does not analyse the assembly code and thus does not know which registers are being altered.

The problem is that I don't know which registers the system call alters. In fact I'm observing a case where the system call alters a register holding a pointer. After the system call has returned, the pointer in the register is being used, which leads to an invalid memory access.

How should I deal with this situation?

Solution

  • For future readers:

    The general answer is that you can find the registers altered by a system call in the documentation of the system's ABI.

    For my system (Freescale MPC5200B) I found the answer is the IBM Application Note 'Developing PowerPC Embedded Application Binary Interface (EABI) Compliant Programs'.

    So I added the registers marked volatile (namely R3..R12, F0..F13 and the flags register) to the clobbers list.