In Thumb state, ARM registers r8–r15 (the high 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 can be transferred from a register in the range r0 – r7 (a low register) to a high register, and from a high register to a low register, using special variants of the MOV instruction. High register values can also be compared against or added to low register values with the CMP and ADD instructions. See the ARM Architecture Reference Manual for details on high register operations.