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 Control Registers
8
9 CR 0 (Recovery Counter) used for ptrace
10 CR 1-CR 7(undefined) unused
11 CR 8 (Protection ID) per-process value*
12 CR 9, 12, 13 (PIDS) unused
13 CR10 (CCR) lazy FPU saving*
14 CR11 as specified by ABI (SAR)
15 CR14 (interruption vector) initialized to fault_vector
16 CR15 (EIEM) initialized to all ones*
17 CR16 (Interval Timer) read for cycle count/write starts Interval Tmr
18 CR17-CR22 interruption parameters
19 CR19 Interrupt Instruction Register
20 CR20 Interrupt Space Register
21 CR21 Interrupt Offset Register
22 CR22 Interrupt PSW
23 CR23 (EIRR) read for pending interrupts/write clears bits
24 CR24 (TR 0) Kernel Space Page Directory Pointer
25 CR25 (TR 1) User Space Page Directory Pointer
26 CR26 (TR 2) not used
27 CR27 (TR 3) Thread descriptor pointer
28 CR28 (TR 4) not used
29 CR29 (TR 5) not used
30 CR30 (TR 6) current / 0
31 CR31 (TR 7) Temporary register, used in various places
32
33 Space Registers (kernel mode)
34
35 SR0 temporary space register
36 SR4-SR7 set to 0
37 SR1 temporary space register
38 SR2 kernel should not clobber this
39 SR3 used for userspace accesses (current process)
40
41 Space Registers (user mode)
42
43 SR0 temporary space register
44 SR1 temporary space register
45 SR2 holds space of linux gateway page
46 SR3 holds user address space value while in kernel
47 SR4-SR7 Defines short address space for user/kernel
48
49
50 Processor Status Word
51
52 W (64-bit addresses) 0
53 E (Little-endian) 0
54 S (Secure Interval Timer) 0
55 T (Taken Branch Trap) 0
56 H (Higher-privilege trap) 0
57 L (Lower-privilege trap) 0
58 N (Nullify next instruction) used by C code
59 X (Data memory break disable) 0
60 B (Taken Branch) used by C code
61 C (code address translation) 1, 0 while executing real-mode code
62 V (divide step correction) used by C code
63 M (HPMC mask) 0, 1 while executing HPMC handler*
64 C/B (carry/borrow bits) used by C code
65 O (ordered references) 1*
66 F (performance monitor) 0
67 R (Recovery Counter trap) 0
68 Q (collect interruption state) 1 (0 in code directly preceding an rfi)
69 P (Protection Identifiers) 1*
70 D (Data address translation) 1, 0 while executing real-mode code
71 I (external interrupt mask) used by cli()/sti() macros
72
73 "Invisible" Registers
74
75 PSW default W value 0
76 PSW default E value 0
77 Shadow Registers used by interruption handler code
78 TOC enable bit 1
79
80 =========================================================================
81 Register usage notes, originally from John Marvin, with some additional
82 notes from Randolph Chung.
83
84 For the general registers:
85
86 r1,r2,r19-r26,r28,r29 & r31 can be used without saving them first. And of
87 course, you need to save them if you care about them, before calling
88 another procedure. Some of the above registers do have special meanings
89 that 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
117 r3-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
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