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-Register Usage for Linux/PA-RISC
-
-[ an asterisk is used for planned usage which is currently unimplemented ]
-
- General Registers as specified by ABI
-
- Control Registers
-
-CR 0 (Recovery Counter) used for ptrace
-CR 1-CR 7(undefined) unused
-CR 8 (Protection ID) per-process value*
-CR 9, 12, 13 (PIDS) unused
-CR10 (CCR) lazy FPU saving*
-CR11 as specified by ABI (SAR)
-CR14 (interruption vector) initialized to fault_vector
-CR15 (EIEM) initialized to all ones*
-CR16 (Interval Timer) read for cycle count/write starts Interval Tmr
-CR17-CR22 interruption parameters
-CR19 Interrupt Instruction Register
-CR20 Interrupt Space Register
-CR21 Interrupt Offset Register
-CR22 Interrupt PSW
-CR23 (EIRR) read for pending interrupts/write clears bits
-CR24 (TR 0) Kernel Space Page Directory Pointer
-CR25 (TR 1) User Space Page Directory Pointer
-CR26 (TR 2) not used
-CR27 (TR 3) Thread descriptor pointer
-CR28 (TR 4) not used
-CR29 (TR 5) not used
-CR30 (TR 6) current / 0
-CR31 (TR 7) Temporary register, used in various places
-
- Space Registers (kernel mode)
-
-SR0 temporary space register
-SR4-SR7 set to 0
-SR1 temporary space register
-SR2 kernel should not clobber this
-SR3 used for userspace accesses (current process)
-
- Space Registers (user mode)
-
-SR0 temporary space register
-SR1 temporary space register
-SR2 holds space of linux gateway page
-SR3 holds user address space value while in kernel
-SR4-SR7 Defines short address space for user/kernel
-
-
- Processor Status Word
-
-W (64-bit addresses) 0
-E (Little-endian) 0
-S (Secure Interval Timer) 0
-T (Taken Branch Trap) 0
-H (Higher-privilege trap) 0
-L (Lower-privilege trap) 0
-N (Nullify next instruction) used by C code
-X (Data memory break disable) 0
-B (Taken Branch) used by C code
-C (code address translation) 1, 0 while executing real-mode code
-V (divide step correction) used by C code
-M (HPMC mask) 0, 1 while executing HPMC handler*
-C/B (carry/borrow bits) used by C code
-O (ordered references) 1*
-F (performance monitor) 0
-R (Recovery Counter trap) 0
-Q (collect interruption state) 1 (0 in code directly preceding an rfi)
-P (Protection Identifiers) 1*
-D (Data address translation) 1, 0 while executing real-mode code
-I (external interrupt mask) used by cli()/sti() macros
-
- "Invisible" Registers
-
-PSW default W value 0
-PSW default E value 0
-Shadow Registers used by interruption handler code
-TOC enable bit 1
-
-=========================================================================
-Register usage notes, originally from John Marvin, with some additional
-notes from Randolph Chung.
-
-For the general registers:
-
-r1,r2,r19-r26,r28,r29 & r31 can be used without saving them first. And of
-course, you need to save them if you care about them, before calling
-another procedure. Some of the above registers do have special meanings
-that you should be aware of:
-
- r1: The addil instruction is hardwired to place its result in r1,
- so if you use that instruction be aware of that.
-
- r2: This is the return pointer. In general you don't want to
- use this, since you need the pointer to get back to your
- caller. However, it is grouped with this set of registers
- since the caller can't rely on the value being the same
- when you return, i.e. you can copy r2 to another register
- and return through that register after trashing r2, and
- that should not cause a problem for the calling routine.
-
- r19-r22: these are generally regarded as temporary registers.
- Note that in 64 bit they are arg7-arg4.
-
- r23-r26: these are arg3-arg0, i.e. you can use them if you
- don't care about the values that were passed in anymore.
-
- r28,r29: are ret0 and ret1. They are what you pass return values
- in. r28 is the primary return. When returning small structures
- r29 may also be used to pass data back to the caller.
-
- r30: stack pointer
-
- r31: the ble instruction puts the return pointer in here.
-
-
-r3-r18,r27,r30 need to be saved and restored. r3-r18 are just
- general purpose registers. r27 is the data pointer, and is
- used to make references to global variables easier. r30 is
- the stack pointer.
-