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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
- =========================================================================
- The PA-RISC architecture defines 7 registers as "shadow registers".
- Those are used in RETURN FROM INTERRUPTION AND RESTORE instruction to reduce
- the state save and restore time by eliminating the need for general register
- (GR) saves and restores in interruption handlers.
- Shadow registers are the GRs 1, 8, 9, 16, 17, 24, and 25.
- =========================================================================
- 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.
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