Register Usage for Linux/PA-RISC
  
  [ an asterisk is used for planned usage which is currently unimplemented ]
  
  	General Registers as specified by ABI
  
  	Control Registers
  
  CR 0 (Recovery Counter)		used for ptrace
  CR 1-CR 7(undefined)		unused
  CR 8 (Protection ID)		per-process value*
  CR 9, 12, 13 (PIDS)		unused
  CR10 (CCR)			lazy FPU saving*
  CR11				as specified by ABI (SAR)
  CR14 (interruption vector)	initialized to fault_vector
  CR15 (EIEM)			initialized to all ones*
  CR16 (Interval Timer)		read for cycle count/write starts Interval Tmr
  CR17-CR22			interruption parameters
  CR19				Interrupt Instruction Register
  CR20				Interrupt Space Register
  CR21				Interrupt Offset Register
  CR22				Interrupt PSW
  CR23 (EIRR)			read for pending interrupts/write clears bits
  CR24 (TR 0)			Kernel Space Page Directory Pointer
  CR25 (TR 1)			User   Space Page Directory Pointer
  CR26 (TR 2)			not used
  CR27 (TR 3)			Thread descriptor pointer
  CR28 (TR 4)			not used
  CR29 (TR 5)			not used
  CR30 (TR 6)			current / 0
  CR31 (TR 7)			Temporary register, used in various places
  
  	Space Registers (kernel mode)
  
  SR0				temporary space register
  SR4-SR7 			set to 0
  SR1				temporary space register
  SR2				kernel should not clobber this
  SR3				used for userspace accesses (current process)
  
  	Space Registers (user mode)
  
  SR0				temporary space register
  SR1                             temporary space register
  SR2                             holds space of linux gateway page
  SR3                             holds user address space value while in kernel
  SR4-SR7                         Defines short address space for user/kernel
  
  
  	Processor Status Word
  
  W (64-bit addresses)		0
  E (Little-endian)		0
  S (Secure Interval Timer)	0
  T (Taken Branch Trap)		0
  H (Higher-privilege trap)	0
  L (Lower-privilege trap)	0
  N (Nullify next instruction)	used by C code
  X (Data memory break disable)	0
  B (Taken Branch)		used by C code
  C (code address translation)	1, 0 while executing real-mode code
  V (divide step correction)	used by C code
  M (HPMC mask)			0, 1 while executing HPMC handler*
  C/B (carry/borrow bits)		used by C code
  O (ordered references)		1*
  F (performance monitor)		0
  R (Recovery Counter trap)	0
  Q (collect interruption state)	1 (0 in code directly preceding an rfi)
  P (Protection Identifiers)	1*
  D (Data address translation)	1, 0 while executing real-mode code
  I (external interrupt mask)	used by cli()/sti() macros
  
  	"Invisible" Registers
  
  PSW default W value		0
  PSW default E value		0
  Shadow Registers		used by interruption handler code
  TOC enable bit			1
  
  =========================================================================
  Register usage notes, originally from John Marvin, with some additional
  notes from Randolph Chung.
  
  For the general registers:
  
  r1,r2,r19-r26,r28,r29 & r31 can be used without saving them first. And of
  course, you need to save them if you care about them, before calling
  another procedure. Some of the above registers do have special meanings
  that you should be aware of:
  
      r1: The addil instruction is hardwired to place its result in r1,
  	so if you use that instruction be aware of that.
  
      r2: This is the return pointer. In general you don't want to
  	use this, since you need the pointer to get back to your
  	caller. However, it is grouped with this set of registers
  	since the caller can't rely on the value being the same
  	when you return, i.e. you can copy r2 to another register
  	and return through that register after trashing r2, and
  	that should not cause a problem for the calling routine.
  
      r19-r22: these are generally regarded as temporary registers.
  	Note that in 64 bit they are arg7-arg4.
  
      r23-r26: these are arg3-arg0, i.e. you can use them if you
  	don't care about the values that were passed in anymore.
  
      r28,r29: are ret0 and ret1. They are what you pass return values
  	in. r28 is the primary return. When returning small structures
  	r29 may also be used to pass data back to the caller.
  
      r30: stack pointer
  
      r31: the ble instruction puts the return pointer in here.
  
  
  r3-r18,r27,r30 need to be saved and restored. r3-r18 are just
      general purpose registers. r27 is the data pointer, and is
      used to make references to global variables easier. r30 is
      the stack pointer.