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.
r4-r11
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.
r12
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.
r13
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.
r14
Register r14 is the link register, lr. If you save the return address, you can use r14 for other purposes between calls.
r15
Register r15 is the program counter, pc. It cannot be used for any other purpose.
The first four registers r0-r3 (a1-a4) are used to pass argument values into a subroutine and to return a result value from a function. They may also be used to hold intermediate values within a routine (but, in general, only between subroutine calls).
r12
Register r12 (IP) may be used by a linker as a scratch register between a routine and any subroutine it calls (for details, see Use of IP by the linker). It can also be used within a routine to hold intermediate values between subroutine calls.
r11
In some variants r11 (FP) may be used as a frame pointer in order to chain frame activation records into a linked list.
r9
The role of register r9 is platform specific. A virtual platform may assign any role to this register and must document this usage. For example, it may designate it as the static base (SB) in a position-independent data model, or it may designate it as the thread register (TR) in an environment with thread-local storage. The usage of this register may require that the value held is persistent across all calls. A virtual platform that has no need for such a special register may designate r9 as an additional callee-saved variable register, v6.
r4-r8, r10, r11
Typically, the registers r4-r8, r10 and r11 (v1-v5, v7 and v8) are used to hold the values of a routine's local variables. Of these, only v1-v4 can be used uniformly by the whole Thumb instruction set, but the AAPCS does not require that Thumb code only use those registers.
A subroutine must preserve the contents of the registers r4-r8, r10, r11 and SP (and r9 in PCS variants that designate r9 as v6).
r12-r15
In all variants of the procedure call standard, registers r12-r15 have special roles. In these roles they are labeled IP, SP, LR and PC.
