Register usage and linkage
The following list shows the register use and linkage.
GPR1-3 => arguments (depending upon type)
GPR4 => the caller's stack frame in the downward-
growing stack. This is biased and actually points to 2048
bytes before the real start of the stack frame.
GPR5 => the called routine's environment pointer
GPR6 => the entry point in the called routine if
the call was made by a BASR instruction
GPR7 => the return point in the caller's routine. The return point
also contains information to determine if the call
was made via BASR or branch relative.
GPR8-15 => preserved
FPRs => arguments (depending upon type)
VR24-31 => arguments (depending upon type)