1 Register 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 FPU Registers must not be used in kernel mode
8
9 Control Registers
10
11 CR 0 (Recovery Counter) used for ptrace
12 CR 1-CR 7(undefined) unused
13 CR 8 (Protection ID) per-process value*
14 CR 9, 12, 13 (PIDS) unused
15 CR10 (CCR) lazy FPU saving*
16 CR11 as specified by ABI
17 CR14 (interruption vector) initialized to fault_vector
18 CR15 (EIEM) initialized to all ones*
19 CR16 (Interval Timer) timer interrupt
20 CR17-CR22 interruption parameters
21 CR23 (EIRR) read for pending interrupts
22 CR24 (TR 0) Kernel Space Page Directory Pointer
23 CR25 (TR 1) User Space Page Directory Pointer
24 CR26 (TR 2)
25 CR27 (TR 3)
26 CR28 (TR 4) used by interruption handlers
27 CR29 (TR 5) used by interruption handlers
28 CR30 (TR 6) current / 0
29 CR31 (TR 7) used by interruption handlers
30
31 Space Registers (kernel mode)
32
33 SR0 temporary space register
34 SR4-SR7 set to 0
35 SR1 temporary space register
36 SR2 unused
37 SR3 used for userspace accesses (current process)*
38
39 Space Registers (user mode)
40
41 SR0 temporary space register
42 SR1 temporary space register
43 SR2 holds space of linux gateway page
44 SR3 holds user address space value while in kernel
45 SR4-SR7 Defines short address space for user/kernel
46
47
48 Processor Status Word
49
50 W (64-bit addresses) 0
51 E (Little-endian) 0
52 S (Secure Interval Timer) 0
53 T (Taken Branch Trap) 0
54 H (Higher-privilege trap) 0
55 L (Lower-privilege trap) 0
56 N (Nullify next instruction) used by C code
57 X (Data memory break disable) 0
58 B (Taken Branch) used by C code
59 C (code address translation) 1, 0 while executing real-mode code
60 V (divide step correction) used by C code
61 M (HPMC mask) 0, 1 while executing HPMC handler*
62 C/B (carry/borrow bits) used by C code
63 O (ordered references) 1*
64 F (performance monitor) 0
65 R (Recovery Counter trap) 0
66 Q (collect interruption state) 1 (0 in code directly preceding an rfi)
67 P (Protection Identifiers) 1*
68 D (Data address translation) 1, 0 while executing real-mode code
69 I (external interrupt mask) used by cli()/sti() macros
70
71 "Invisible" Registers
72
73 PSW default W value 0
74 PSW default E value 0
75 Shadow Registers used by interruption handler code
76 TOC enable bit 1
77
78 =========================================================================
79 Info from John Marvin:
80
81 From: "John Marvin" <jsm@udlkern.fc.hp.com>
82 To: randolf@tausq.org
83 Subject: Re: parisc asm questions
84
85 [...]
86
87 For the general registers:
88
89 r1,r2,r19-r26,r28,r29 & r31 can be used without saving them first. And of
90 course, you need to save them if you care about them, before calling
91 another procedure. Some of the above registers do have special meanings
92 that you should be aware of:
93
94 r1: The addil instruction is hardwired to place its result in r1,
95 so if you use that instruction be aware of that.
96
97 r2: This is the return pointer. In general you don't want to
98 use this, since you need the pointer to get back to your
99 caller. However, it is grouped with this set of registers
100 since the caller can't rely on the value being the same
101 when you return, i.e. you can copy r2 to another register
102 and return through that register after trashing r2, and
103 that should not cause a problem for the calling routine.
104
105 r19-r22: these are generally regarded as temporary registers.
106 Note that in 64 bit they are arg7-arg4.
107
108 r23-r26: these are arg3-arg0, i.e. you can use them if you
109 don't care about the values that were passed in anymore.
110
111 r28,r29: are ret0 and ret1. They are what you pass return values
112 in. r28 is the primary return. I'm not sure I remember
113 under what circumstances stuff is returned in r29 (millicode
114 perhaps).
115
116 r31: the ble instruction puts the return pointer in here.
117
118
119 r3-r18,r27,r30 need to be saved and restored. r3-r18 are just
120 general purpose registers. r27 is the data pointer, and is
121 used to make references to global variables easier. r30 is
122 the stack pointer.
123
124 John
125
126
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