Register preservation rules match the ELF ABI calling sequence with the
following differences:
-=========== ============= ========================================
--- For the sc instruction, differences with the ELF ABI ---
+=========== ============= ========================================
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
+=========== ============= ========================================
--- For the scv 0 instruction, differences with the ELF ABI ---
+=========== ============= ========================================
r0 Volatile (System call number.)
r3 Volatile (Parameter 1, and return value.)
r4-r8 Volatile (Parameters 2-6.)