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+===============================================
+Power Architecture 64-bit Linux system call ABI
+===============================================
+
+syscall
+=======
+
+syscall calling sequence[*] matches the Power Architecture 64-bit ELF ABI
+specification C function calling sequence, including register preservation
+rules, with the following differences.
+
+[*] Some syscalls (typically low-level management functions) may have
+    different calling sequences (e.g., rt_sigreturn).
+
+Parameters and return value
+---------------------------
+The system call number is specified in r0.
+
+There is a maximum of 6 integer parameters to a syscall, passed in r3-r8.
+
+Both a return value and a return error code are returned. cr0.SO is the return
+error code, and r3 is the return value or error code. When cr0.SO is clear,
+the syscall succeeded and r3 is the return value. When cr0.SO is set, the
+syscall failed and r3 is the error code that generally corresponds to errno.
+
+Stack
+-----
+System calls do not modify the caller's stack frame. For example, the caller's
+stack frame LR and CR save fields are not used.
+
+Register preservation rules
+---------------------------
+Register preservation rules match the ELF ABI calling sequence with the
+following differences:
+
+r0:         Volatile.   (System call number.)
+r3:         Volatile.   (Parameter 1, and return value.)
+r4-r8:      Volatile.   (Parameters 2-6.)
+cr0:        Volatile    (cr0.SO is the return error condition)
+cr1, cr5-7: Nonvolatile.
+lr:         Nonvolatile.
+
+All floating point and vector data registers as well as control and status
+registers are nonvolatile.
+
+Invocation
+----------
+The syscall is performed with the sc instruction, and returns with execution
+continuing at the instruction following the sc instruction.
+
+Transactional Memory
+--------------------
+Syscall behavior can change if the processor is in transactional or suspended
+transaction state, and the syscall can affect the behavior of the transaction.
+
+If the processor is in suspended state when a syscall is made, the syscall
+will be performed as normal, and will return as normal. The syscall will be
+performed in suspended state, so its side effects will be persistent according
+to the usual transactional memory semantics. A syscall may or may not result
+in the transaction being doomed by hardware.
+
+If the processor is in transactional state when a syscall is made, then the
+behavior depends on the presence of PPC_FEATURE2_HTM_NOSC in the AT_HWCAP2 ELF
+auxiliary vector.
+
+- If present, which is the case for newer kernels, then the syscall will not
+  be performed and the transaction will be doomed by the kernel with the
+  failure code TM_CAUSE_SYSCALL | TM_CAUSE_PERSISTENT in the TEXASR SPR.
+
+- If not present (older kernels), then the kernel will suspend the
+  transactional state and the syscall will proceed as in the case of a
+  suspended state syscall, and will resume the transactional state before
+  returning to the caller. This case is not well defined or supported, so this
+  behavior should not be relied upon.
+
+
+vsyscall
+========
+
+vsyscall calling sequence matches the syscall calling sequence, with the
+following differences. Some vsyscalls may have different calling sequences.
+
+Parameters and return value
+---------------------------
+r0 is not used as an input. The vsyscall is selected by its address.
+
+Stack
+-----
+The vsyscall may or may not use the caller's stack frame save areas.
+
+Register preservation rules
+---------------------------
+r0: Volatile.
+cr1, cr5-7: Volatile.
+lr: Volatile.
+
+Invocation
+----------
+The vsyscall is performed with a branch-with-link instruction to the vsyscall
+function address.
+
+Transactional Memory
+--------------------
+vsyscalls will run in the same transactional state as the caller. A vsyscall
+may or may not result in the transaction being doomed by hardware.