Registers are R0 to R15
Each is 32 bits wide
R0 to R12 are user registers
R13 to R15 are special
R13: stack pointer register(SP)
R14:Link register(LR)
R15:Program Counter(PC)
When a call is made to a function the return address is automatically stored in the link register and is immediately available on return from the function. This allows quick entry and return into a 'leaf’ function (a function that is not going to call further functions). If the function is part of a branch (i.e. it is going to call other functions) then the link register must be preserved on the stack (R13).
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