On Nov 22, 2007, at 20:29:11, Alan Cox wrote:
Most architectures are unable to perform unaligned memory
accesses. Any unaligned access causes a processor exception.
Not all. Some simply produce the wrong answer - thats oh so much
more exciting.
As one example, the MicroBlaze soft-core processor family designed
for use on Xilinx FPGAs will (by default) simply forcibly zero the
lower bits of the unaligned address, such that the following code
will fail mysteriously:
const char foo[] = { 0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07 };
printf("0x%08lx 0x%08lx 0x%08lx 0x%08lx\n",
*((u32 *)(foo+0)),
*((u32 *)(foo+1)),
*((u32 *)(foo+2)),
*((u32 *)(foo+3)));
Instead of outputting:
0x00010203 0x01020304 0x02030405 0x03040506
It will output:
0x00010203 0x00010203 0x00010203 0x00010203
Other embedded architectures have very similar problems. Some may
provide an "unaligned data access" exception, but offer insufficient
information to repair the damage and resume execution.
Cheers,
Kyle Moffett
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