Registers Conventions[1]

Register Status Use

r0, r1     General purpose                                       Volatile
r2         Parameter passing and return values                   Volatile
r3, r4, r5 Parameter passing                                     Volatile
r6         Parameter passing                                     Saved
r7 - r11   Local variables                                       Saved
r12        Local variable, commonly used as GOT pointer          Saved
r13        Local variable, commonly used as Literal Pool pointer Saved
r14        Return address                                        Volatile
r15        Stack pointer                                         Saved

f0, f2, f4, f6 Parameter passing and return values Volatile
f1, f3, f5, f7 General purpose                     Volatile
f8 – f15       General purpose                     Saved

Vector Register Conventions

Register Use

VR0 - VR7    Volatile
VR8 - VR15   Bytes 0-7 are non-volatile, Bytes 8-15 are volatile
VR16 - VR23  Non-volatile
VR24 - VR31  Volatile

General Registers[2]

In addition to their use as accumulators in general
arithmetic and logical operations, 15 of the 16 general
registers are also used as base-address and
index registers in address generation. In these
cases, the registers are designated by a four-bit B
field or X field in an instruction. A value of zero in the
B or X field specifies that no base or index is to be
applied, and, thus, general register 0 cannot be designated
as containing a base address or index.

Parameter passing[1]

Values shorter than 64 bits are sign- or zero-extended (as appropriate)
to 64 bits. Arguments not handled in the registers are passed in the parameter
words of the caller’s stack frame.

Dynamic stack space allocation[1]

1. After a new stack frame is acquired, and before the first dynamic space
allocation, a new register, the frame pointer or FP, is set to the value of the
stack pointer.
2. The amount of dynamic space to be allocated is rounded up to a multiple of 8
bytes, so that 8-byte stack alignment is maintained.
3. The stack pointer is decreased by the rounded byte count, and the address of
the previous stack frame (the back chain) may be stored at the word addressed
by the new stack pointer. The back chain is not necessary to restore from this
allocation at the end of the function since the frame pointer can be used to
restore the stack pointer.

[1] http://legacy.redhat.com/pub/redhat/linux/7.1/es/os/s390x/doc/lzsabi0.pdf
[2] https://www.ibm.com/support/pages/zarchitecture-principles-operation