3.4.4. Accessing Hi registers in THUMB state

In THUMB state, registers R8 – R15 (the hi-registers) are not part of the standard register set. However, the assembly language programmer has limited access to them, and can use them for fast temporary storage.

A value may be transferred from a register in the range R0 – R7 (a lo-register) to a hi-register, and from a hi-register to a lo-register, using special variants of the MOV instruction. Hi-register values can also be compared against or added to lo-register values with the CMP and ADD instructions. See the ARM Architecture Reference Manual (ARM DDI 0100) for details on hi‑register operations.

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