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Re: [avr-gcc-list] Re: [avr-libc-dev] [bug #29774] prologue/epilogue sta

From: Matthew
Subject: Re: [avr-gcc-list] Re: [avr-libc-dev] [bug #29774] prologue/epilogue stack pointer manipulation not interrupt safe in XMega
Date: Tue, 08 Jun 2010 11:31:58 -0600
User-agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US; rv: Gecko/20100317 Thunderbird/3.0.4

On 6/8/2010 10:31 AM, Bob Paddock wrote:
There are several places that handle SP adjustment:

avr.c: output_movhi, expand_prologue, expand_epilogue
avr.md: movhi_sp_r_irq_off, movhi_sp_r_irq_on (both generated in
libgcc.S: __prologue_saves__, __epilogue_restores__

For changes in libgcc.S to take effect, appropriate builtin-defines must
be provided so that the right multilib variants will be generated.
When I was patching my libgcc.a for the avrxmega7 lib, the only place
that I saw, that turned off the IRQs were the epilogue/prologue.
Is there some reason they don't need turned off in the other contexts?
  Seems like such a problem would have shown up long before now?

So here's what I have found so far. When executing SEI, one more instruction can execute before the first interrupt is serviced. When enabling interrupts by restoring the status register, interrupts can occur immediately. When writing to SPL, interrupts are automatically disabled until the next memory write (up to 4 cycles).

Therefore, the prologue and epilogue do not need to disable interrupts at all. A write to SPL followed by a write to SPH should be interrupt safe. I have not tested this yet.

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