The general rule is this:
Unless otherwise documented, the return code is 4 bytes wide. That means
use of LTGR is incorrect in the general case..
Unless otherwise documented, the high halves of GPRs 0, 1, and 15 (and ARs
0, 1, 14 and 15) are unpredictable on return from any service.
I am unaware of any exceptions to this.
Not that I tend to be a slave to what is documented versus what is not,
from the assembler services guide:
2.1 Saving the Calling Program's Registers
Unless otherwise defined by the individual interface, the calling program
should expect, upon return, that
The low halves (Bits 32-63) of GPRs 2 through 13 are unchanged
The high halves (Bits 0-31) of GPRs 2 through 14 are unchanged
ARs 2 through 13 are unchanged
FPRs 8 through 15 are unchanged; The Floating Point Control (FPC)
Register is unchanged with the exception of two fields: the IEEE
exception flags and the data exception code (DXC).
When return information is provided in GPR 0, 1, and/or 15 (for
example return and reason codes), only bits 32-63 of the register
contain the returned value.
Peter Relson
z/OS Core Technology Design
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