RTEMS CPU Architecture Supplement
A called function may clobber all registers, except RETS, R4-R7, P3-P5, FP and SP. It may also modify the first 12 bytes in the caller’s stack frame which is used as an argument area for the first three arguments (which are passed in R0...R3 but may be placed on the stack by the called function).
RTEMS CPU Architecture Supplement
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