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/Documentation/parisc/registers

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  1Register Usage for Linux/PA-RISC
  2
  3[ an asterisk is used for planned usage which is currently unimplemented ]
  4
  5	General Registers as specified by ABI
  6
  7	Control Registers
  8
  9CR 0 (Recovery Counter)		used for ptrace
 10CR 1-CR 7(undefined)		unused
 11CR 8 (Protection ID)		per-process value*
 12CR 9, 12, 13 (PIDS)		unused
 13CR10 (CCR)			lazy FPU saving*
 14CR11				as specified by ABI (SAR)
 15CR14 (interruption vector)	initialized to fault_vector
 16CR15 (EIEM)			initialized to all ones*
 17CR16 (Interval Timer)		read for cycle count/write starts Interval Tmr
 18CR17-CR22			interruption parameters
 19CR19				Interrupt Instruction Register
 20CR20				Interrupt Space Register
 21CR21				Interrupt Offset Register
 22CR22				Interrupt PSW
 23CR23 (EIRR)			read for pending interrupts/write clears bits
 24CR24 (TR 0)			Kernel Space Page Directory Pointer
 25CR25 (TR 1)			User   Space Page Directory Pointer
 26CR26 (TR 2)			not used
 27CR27 (TR 3)			Thread descriptor pointer
 28CR28 (TR 4)			not used
 29CR29 (TR 5)			not used
 30CR30 (TR 6)			current / 0
 31CR31 (TR 7)			Temporary register, used in various places
 32
 33	Space Registers (kernel mode)
 34
 35SR0				temporary space register
 36SR4-SR7 			set to 0
 37SR1				temporary space register
 38SR2				kernel should not clobber this
 39SR3				used for userspace accesses (current process)
 40
 41	Space Registers (user mode)
 42
 43SR0				temporary space register
 44SR1                             temporary space register
 45SR2                             holds space of linux gateway page
 46SR3                             holds user address space value while in kernel
 47SR4-SR7                         Defines short address space for user/kernel
 48
 49
 50	Processor Status Word
 51
 52W (64-bit addresses)		0
 53E (Little-endian)		0
 54S (Secure Interval Timer)	0
 55T (Taken Branch Trap)		0
 56H (Higher-privilege trap)	0
 57L (Lower-privilege trap)	0
 58N (Nullify next instruction)	used by C code
 59X (Data memory break disable)	0
 60B (Taken Branch)		used by C code
 61C (code address translation)	1, 0 while executing real-mode code
 62V (divide step correction)	used by C code
 63M (HPMC mask)			0, 1 while executing HPMC handler*
 64C/B (carry/borrow bits)		used by C code
 65O (ordered references)		1*
 66F (performance monitor)		0
 67R (Recovery Counter trap)	0
 68Q (collect interruption state)	1 (0 in code directly preceding an rfi)
 69P (Protection Identifiers)	1*
 70D (Data address translation)	1, 0 while executing real-mode code
 71I (external interrupt mask)	used by cli()/sti() macros
 72
 73	"Invisible" Registers
 74
 75PSW default W value		0
 76PSW default E value		0
 77Shadow Registers		used by interruption handler code
 78TOC enable bit			1
 79
 80=========================================================================
 81Register usage notes, originally from John Marvin, with some additional
 82notes from Randolph Chung.
 83
 84For the general registers:
 85
 86r1,r2,r19-r26,r28,r29 & r31 can be used without saving them first. And of
 87course, you need to save them if you care about them, before calling
 88another procedure. Some of the above registers do have special meanings
 89that you should be aware of:
 90
 91    r1: The addil instruction is hardwired to place its result in r1,
 92	so if you use that instruction be aware of that.
 93
 94    r2: This is the return pointer. In general you don't want to
 95	use this, since you need the pointer to get back to your
 96	caller. However, it is grouped with this set of registers
 97	since the caller can't rely on the value being the same
 98	when you return, i.e. you can copy r2 to another register
 99	and return through that register after trashing r2, and
100	that should not cause a problem for the calling routine.
101
102    r19-r22: these are generally regarded as temporary registers.
103	Note that in 64 bit they are arg7-arg4.
104
105    r23-r26: these are arg3-arg0, i.e. you can use them if you
106	don't care about the values that were passed in anymore.
107
108    r28,r29: are ret0 and ret1. They are what you pass return values
109	in. r28 is the primary return. When returning small structures
110	r29 may also be used to pass data back to the caller.
111
112    r30: stack pointer
113
114    r31: the ble instruction puts the return pointer in here.
115
116
117r3-r18,r27,r30 need to be saved and restored. r3-r18 are just
118    general purpose registers. r27 is the data pointer, and is
119    used to make references to global variables easier. r30 is
120    the stack pointer.
121