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.