In z/OS® R2, program entry is in AMODE 31; therefore linkage conventions you have used in S/390® apply, which means passing 4-byte parameter lists and a 72-byte savearea.
A older program changing into AMODE 64 to exploit z/Architecture® instructions should expect to receive 31-bit addresses and the 72-byte save area from its callers. If you are running in AMODE 64 and want to use an address a caller has passed to you, the high-order half of the GPR will probably not be cleared to zeroes. As soon as you receive this address, use the Load Logical G Thirty One Bits (LLGTR) instruction to change this 31-bit address into a 64-bit address that you can use.