A frame pointer helps mitigate problems with the limited size of the bit field that specifies register-displacement-addressing offset. The frame pointer typically points to a fixed frame offset in the RSA or Local and Temporaries areas of the stack frame, but the pointer can point to other offsets within the frame. To more efficiently access data in large stack frames, a routine can establish another frame pointer.

• A routine does not need to set up a stack frame unless it needs to save permanent registers, or to allocate space for locals or outgoing argument areas that are bigger than four words. The stack pointer and frame pointer addresses align on 4-byte boundaries.
  
• If a routine has alloca() locals, the ARM specification requires a separate frame pointer register to access incoming arguments and locals.
  R11 is the assigned frame pointer for ARM, and R7 is the assigned frame pointer for THUMB.
  A leaf routine can use any free integer register as the frame pointer. A nonleaf routine must use a permanent register. The routine must not modify the frame pointer register between the prolog and epilog.
  
• If a routine uses alloca(), everything in the frame at a lower address than the alloca() area is referenced relative to R13 and never contains a defined value at the time of an alloca() call. Thus, the alloca() operation never needs to copy this part of the stack frame, and no data relocation problems arise.
  Everything in the frame at an address higher than the alloca() area is referenced relative to the frame pointer, R11 for ARM or R7 for THUMB.
  

Concepts