2.4 Changing between AArch64 and AArch32 states

The processor must be in the correct execution state for the instructions it is executing.

A processor that is executing A64 instructions is operating in AArch64 state. In this state, the instructions can access both the 64-bit and 32-bit registers.

A processor that is executing A32 or T32 instructions is operating in AArch32 state. In this state, the instructions can only access the 32-bit registers, and not the 64-bit registers.

A processor based on the Arm®v8 architecture can run applications built for AArch32 and AArch64 states but a change between AArch32 and AArch64 states can only happen at exception boundaries.

Arm Compiler toolchain builds images for either the AArch32 state or AArch64 state. Therefore, an image built with Arm Compiler toolchain can either contain only A32 and T32 instructions or only A64 instructions.

A processor can only execute instructions from the instruction set that matches its current execution state. A processor in AArch32 state cannot execute A64 instructions, and a processor in AArch64 state cannot execute A32 or T32 instructions. You must ensure that the processor never receives instructions from the wrong instruction set for the current execution state.

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