Registers
R0 .. R12 (general purpose)
SP  (R13 = stack pointer)
LR  (R14 = link register)
PC  (R15 = program counter)

Functions:
Parameters: first 4 in: R0, R1, R2, R3
 for more than 4: use stack
Return value: R0 (32bits) or R0/R1 (64 bits) or stack (bigger)
If registers R4-12 are used, then save/restore on stack
Return address: LR