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=========================================================================
81
82The PA-RISC architecture defines 7 registers as "shadow registers".
83Those are used in RETURN FROM INTERRUPTION AND RESTORE instruction to reduce
84the state save and restore time by eliminating the need for general register
85(GR) saves and restores in interruption handlers.
86Shadow registers are the GRs 1, 8, 9, 16, 17, 24, and 25.
87
88=========================================================================
89Register usage notes, originally from John Marvin, with some additional
90notes from Randolph Chung.
91
92For the general registers:
93
94r1,r2,r19-r26,r28,r29 & r31 can be used without saving them first. And of
95course, you need to save them if you care about them, before calling
96another procedure. Some of the above registers do have special meanings
97that you should be aware of:
98
99    r1: The addil instruction is hardwired to place its result in r1,
100	so if you use that instruction be aware of that.
101
102    r2: This is the return pointer. In general you don't want to
103	use this, since you need the pointer to get back to your
104	caller. However, it is grouped with this set of registers
105	since the caller can't rely on the value being the same
106	when you return, i.e. you can copy r2 to another register
107	and return through that register after trashing r2, and
108	that should not cause a problem for the calling routine.
109
110    r19-r22: these are generally regarded as temporary registers.
111	Note that in 64 bit they are arg7-arg4.
112
113    r23-r26: these are arg3-arg0, i.e. you can use them if you
114	don't care about the values that were passed in anymore.
115
116    r28,r29: are ret0 and ret1. They are what you pass return values
117	in. r28 is the primary return. When returning small structures
118	r29 may also be used to pass data back to the caller.
119
120    r30: stack pointer
121
122    r31: the ble instruction puts the return pointer in here.
123
124
125r3-r18,r27,r30 need to be saved and restored. r3-r18 are just
126    general purpose registers. r27 is the data pointer, and is
127    used to make references to global variables easier. r30 is
128    the stack pointer.
129
130