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armv8

Are there unused bits in aarch64 instruction encoding?

As per this link about aarch64 instruction encoding, there are unused bits in some instructions, like x in below listing for LDR instruciton. But I any documentation about unused bits in armv8 manual. Are these unused bits valid according to armv8 manual?

xxx1 1101 x1ii iiii iiii iinn nnnt tttt - ldr Ft ADDR_UIMM12

Solution

  • That link is from 2012, that is when the ARMv8 architecture was released, so there was not a lot of information about it. The 'x' in that case is related to the decoding of the instruction, not sure about how they do it, it does not look correct to me. You can find all the values for the encoding in the ARM Architecture Reference Manual, look at the LDR instructions use immediate values (e.g LDR (immediate) page 693 specifically the Unsigned offset in the next page). You will see there that the two most significant bits are used for the size of the register (size == 10 is for W registers (32 bits) and size == 11 for X registers (64 bits)).

    In the ARM Architecture Reference Manual usually, when there are encodings that are not used it says Unallocated Encoding or Reserve Encoding or something similar.

    Encoding for LDR (immediate) Unsigned offset

    Also, there are plenty of free encodings to be used, probably for future use or for example for the Scalable Vector Extensions module. You can see all the encodings used and free in the following slides by Nigel Stephens at the Hot Chips 28 conference on August 22, 2016, look at slice 8, the grey squares are free unused encodings.