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What happens with unaligned LW accesses ?

Question asked by RobB. on Jul 4, 2017
Latest reply on Jul 14, 2017 by Jithul_Janardhanan

I have been chasing down an intermittent problem (Host comms via SPI stops working) with our 21469 framework. It seems to be related to interrupts, but post-mortem debugging has not proved very fruitful so far. 


What I did discover is that the illegal condition interrupt was disabled, but an illegal condition was being latched. After enabling the interrupt, I tracked down the illegal access to the following register save on the entry to an ISR written in ASM.. 

< from vector table... >
    push sts; // save ASTAT and MODE1, apply MMASK to MODE1
    jump link_port_rx_service (db);
      dm(save_dma8_px)=px (lw);
      px=r0; // save r0, move all 40 bits (extended floating-point precision!)


< in normal code space ...>


    dm(save_dma8_r0) = px (lw); <<<< THIS INSTRUCTION WAS MISALIGNED


where save_dma8_r0 was an array of length 2, but wasn't aligned on a 64-bit boundary.


What I want to know, is what actually is the effect of this unaligned LW write? As I understand it, the 21469 is able to 'correct' for unaligned accesses when in SIMD normal word mode, but what about when using the LW modifier ? Could it get the odd / even addresses mixed up somehow ? As I say the problem is very intermittent & happens about once a day so I suspect there may some other interaction with DMA or something else which only happens now & again.


Since fixing the unaligned write, I haven't seen the original problem again, but it's only been about half a day, so may still happen...