1Register Usage for Linux/PA-RISC
3[ an asterisk is used for planned usage which is currently unimplemented ]
5    General Registers as specified by ABI
7    Control Registers
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
33    Space Registers (kernel mode)
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)
41    Space Registers (user mode)
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
50    Processor Status Word
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
73    "Invisible" Registers
75PSW default W value 0
76PSW default E value 0
77Shadow Registers used by interruption handler code
78TOC enable bit 1
81Register usage notes, originally from John Marvin, with some additional
82notes from Randolph Chung.
84For the general registers:
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:
91    r1: The addil instruction is hardwired to place its result in r1,
92    so if you use that instruction be aware of that.
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.
102    r19-r22: these are generally regarded as temporary registers.
103    Note that in 64 bit they are arg7-arg4.
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.
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.
112    r30: stack pointer
114    r31: the ble instruction puts the return pointer in here.
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.

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