Use registers r0-r3 to pass parameter values into routines, and to pass result values out. You can refer to r0-r3 as a1-a4 to make this usage apparent. See Parameter passing. Between subroutine calls you can use r0-r3 for any purpose. A called routine does not have to restore r0-r3 before returning. A calling routine must preserve the contents of r0-r3 if it needs them again.

Use registers r4-r11 to hold the values of a routine’s local variables. You can refer to them as v1-v8 to make this usage apparent. In Thumb state, in most instructions you can only use registers r4-r7 for local variables.

A called routine must restore the values of these registers before returning, if it has used them.

Register r12 is the intra-call scratch register, ip. It is used in this role in procedure linkage veneers, for example interworking veneers. Between procedure calls you can use it for any purpose. A called routine does not need to restore r12 before returning.

Register r13 is the stack pointer, sp. You must not use it for any other purpose. The value held in sp on exit from a called routine must be the same as it was on entry.

Register r14 is the link register, lr. If you save the return address, you can use r14 for other purposes between calls.

Register r15 is the program counter, pc. It cannot be used for any other purpose.