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Re: [avr-libc-dev] gcc-3.3 branch

From: Marek Michalkiewicz
Subject: Re: [avr-libc-dev] gcc-3.3 branch
Date: Wed, 15 Jan 2003 00:01:47 +0100 (CET)


sorry for late reply - I've just looked at the issue of indirect 16-bit
writes not working with 16-bit I/O registers (because high byte should
be written first).  I'm afraid it's not that simple - if you look at
out_movhi_mr_r, you will see that there are several places that would
need to be changed.  If we fix just one case, people will assume that
using pointers for 16-bit I/O is safe, when it still isn't.

In some cases, where post-increment is used (most likely with the X
pointer), writing high byte first would make less efficient code
(when writing to normal SRAM, where order doesn't matter) - we lose
the reg_unused_after optimizations.  So, my opinion is that it would
be best to simply document the fact that 16-bit I/O register access
works correctly only with constant addresses (which is the common case
for most I/O register accesses anyway).  If you need to do a 16-bit
write high byte first to non-constant addresses, better don't depend
on GCC, but write a piece of inline asm, like this (not tested):

static inline void
write_high_byte_first(volatile unsigned int *addr, unsigned int val)
        asm volatile (
                "std %a0+1,%B1" "\n\t"
                "st %a0,%A1"
                : /* no outputs */
                : "b" (addr), "r" (val)

After the 3.3 release you can still try to convince me, of course... :)

Hope this helps, time for me to go to sleep(18000);

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