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Re: Is r25 saved across syscalls?

To: "Kevin D. Kissell" <>
Subject: Re: Is r25 saved across syscalls?
From: David Daney <>
Date: Mon, 10 Sep 2012 18:27:55 -0700
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On 09/10/2012 06:04 PM, Kevin D. Kissell wrote:
On 09/10/2012 05:29 PM, Maciej W. Rozycki wrote:
I do wonder however why we have these instructions to save/restore $25
in SAVE_SOME/RESTORE_SOME. This dates back to 2.4 at the very least.
Ralf, any insights?
Hi, guys.  Maybe the fact that it's used for dispatching PIC calls has
something to do with it?

I don't think so. It is call clobbered in all Linux ABIs. So there is never a need to save it. It even has the pseudonym of $t9 indicating that it will get clobbered.

The other call clobbered registers that happen to be saved are for the nefarious uses of the kernel itself (a0..a6 for system call restarting), I think this is just left over cruft.

David Daney


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