Did you know: Unaligned accesses in ARM

The X86 has always supported unaligned accesses. In the ARM world the first architecture that supported unaligned accesses in hardware was ARMv6. The architecture was implemented in the ARM11 core around the year 2002 and onward. There is an excellent article at ARM Infocenter giving technical details:


The support for unaligned accesses must be enabled in an ARM core explicitly. This is done by setting the bit A in the register SCTLR. Still, unaligned accesses will be allowed only on Normal memory; accesses to  Device memory type are always checked and will throw exceptions on misaligned accesses.

I had to verify that a Cortex-A53 core (ARMv8) correctly implements the support for unaligned accesses. In the beginning this task seemed very simple as only the bit SCTRL.A had to be set, I thought. However, the hidden issue is that all memory is treated as the Device memory type by default! Memory type specification is part of MMU page tables. Each block or page descriptor has a 3-bit field called Attribute Index. This is an index into the Memory Attribute Indirection Register (MAIR), which holds eight descriptors of the memory types used in the system (e.g. normal cacheable, normal non-cacheable, device, etc.). The operating system must know the system memory map and the caching requirements; therefore it can maintain virtual memory tables with correct attributes.

In the end to implement my test case, I had to implement paging tables with a flat mapping of VA to PA, setting the memory types on RAM blocks as needed.


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