21.2 Mainframe registers

If you develop your own applications, it may be of use to know something about the mainframe register sets. A full description is in S/390 ELF Application Binary Interface Supplement and zSeries ELF Application Binary Interface Supplement, both available at http://www.linuxbase.org/spec/ELF/zSeries/.

21.2.1 zSeries registers

S/390 has 16 general-purpose registers of 32-bit width; with zSeries, the registers are 64-bit wide. The number of floating-point registers depends on the model. Older systems have only four floating-point registers; recent systems come with 16. The 16 registers were introduced together with the IEEE floating-point implementation. All Linux programs are able to use 16 floating-point registers. The missing 12 on older hardware are emulated within the Linux kernel. The use of the registers on z/Architecture is described in Table 21-2.

Table 21-2. Register usage

Register name	Usage	Call effect
r0, r1	General purpose	Volatile[1]
r2	Parameter passing and return values	Volatile
r3, r4, r5	Parameter passing	Volatile
r6	Parameter passing	Saved[2]
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
Access registers 0, 1	Reserved for system use	Volatile
Access registers 2 15	General purpose	Volatile

^[1] Volatile: These registers are not preserved across function calls.

^[2] Saved: These registers belong to the calling function. A called function will save these registers' values before it changes them, restoring their values before it returns.

The registers in Table 21-3 have assigned roles in the standard calling sequence.

Table 21-3. Standard calling sequence roles

r12	Global Offset Table pointer. If a position-independent module uses cross-linking, the compiler must point r12 to the GOT. If not, this register may be used locally.
r13	Commonly used as the literal pool pointer. If the literal pool is not required, this register may be used locally.
r14	This register will contain the address to which a called function will normally return. r14 is volatile across function calls.
r15	The stack pointer (stored in r15) will maintain an 8-byte alignment. It will always point to the lowest allocated valid stack frame and will grow toward low addresses. The contents of the word addressed by this register may point to the previously allocated stack frame. If required, it can be decremented by the called function.

Signals can interrupt processes. Functions called during signal handling have no unusual restrictions on their use of registers. Moreover, if a signal-handling function returns, the process will resume its original execution path with all registers restored to their original values. Thus programs and compilers may freely use all registers listed above, except those reserved for system use, without the danger of signal handlers' inadvertently changing their values.

Register usage

With these calling conventions, the following usage of the registers for inline assemblies is recommended:

• General registers r0 and r1 should be used internally whenever possible.

• General registers r2 to r5 should be second choice.

• General registers r12 to r15 should be used only for their standard function.