Distinguishing between 32-bit and 64-bit A64 instructions

Posted 2021-03-16 aspirs

Distinguishing between 32-bit and 64-bit A64 instructions
Most integer instructions in the A64 instruction set have two forms, which operate on either 32-bit or 64-bit values within the 64-bit general-purpose register file.
When looking at the register name that the instruction uses:
• If the register name starts with X, it is a 64-bit register.
• If the register name starts with W, it is a 32-bit register.

 

When a 32-bit register form is selected:
• Right shifts and rotates inject at bit 31, instead of bit 63.
• The condition flags, where set by the instruction, are computed from the lower 32 bits.
• Writes to the W register set bits [63:32] of the X register to zero.
This distinction applies even when the results of a 32-bit register form would be indistinguishable from the lower 32 bits computed by the equivalent 64-bit register form. For example, A64 includes separate 32-bit and 64-bit register forms of the ORR instructions. A 32-bit bitwise ORR could just as easily be performed using a 64-bit ORR and ignoring the top 32 bits of the result.