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+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
+
+=========================================================================
+
+The PA-RISC architecture defines 7 registers as "shadow registers".
+Those are used in RETURN FROM INTERRUPTION AND RESTORE instruction to reduce
+the state save and restore time by eliminating the need for general register
+(GR) saves and restores in interruption handlers.
+Shadow registers are the GRs 1, 8, 9, 16, 17, 24, and 25.
+
+=========================================================================
+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.
+