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There are restrictions on the use of SP and LR as general-purpose registers.
With the exception of ARMv6-M and ARMv7-M based processors, there are 30 (or 32 if Security Extensions are implemented) general-purpose 32-bit registers, that include the banked SP and LR registers. Fifteen general-purpose registers are visible at any one time, depending on the current processor mode. These are R0-R12, SP, LR. The PC (R15) is not considered a general-purpose register.
SP (or R13) is the stack pointer. The C and C++ compilers always use SP as the stack pointer. Use of SP as a general purpose register is discouraged. In Thumb, SP is strictly defined as the stack pointer. The instruction descriptions mention when SP and PC can be used.
In User mode, LR (or R14) is used as a link register to store the return address when a subroutine call is made. It can also be used as a general-purpose register if the return address is stored on the stack.
In the exception handling modes, LR holds the return address for the exception, or a subroutine return address if subroutine calls are executed within an exception. LR can be used as a general-purpose register if the return address is stored on the stack.
When using the
--use_frame_pointer option with armcc, do not
R11 as a general-purpose register.