On Sun, Sep 09, 2012 at 03:30:08PM -0400, Rich Felker wrote: > The kernel syscall entry/exit code seems to always save and restore > r25. Is this stable/documented behavior I can rely on? If there's a > reason it _needs_ to be preserved, knowing that would help convince me > it's safe to assume it will always be done. The intended usage is to > be able to make syscalls (where the syscall # is not a constant that > could be loaded with lwi) without a stack frame, as in "move $2,$25 ; > syscall". The basic design idea is that syscalls use a calling convention similar to subroutine calls. $25 is $t9, so a temp register which is callee saved. So if the kernel is saving $t9 and you've been relying on that, consider yourself lucky - there's not guarantee for that. Ralf
