| Register Usage for Linux/PA-RISC |
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| [ an asterisk is used for planned usage which is currently unimplemented ] |
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| General Registers as specified by ABI |
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| Control Registers |
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| 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 |
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| Space Registers (kernel mode) |
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| 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) |
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| Space Registers (user mode) |
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| 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 |
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| Processor Status Word |
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| 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 |
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| "Invisible" Registers |
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| PSW default W value 0 |
| PSW default E value 0 |
| Shadow Registers used by interruption handler code |
| TOC enable bit 1 |
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| ========================================================================= |
| Register usage notes, originally from John Marvin, with some additional |
| notes from Randolph Chung. |
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| For the general registers: |
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| 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: |
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| 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. |
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| r19-r22: these are generally regarded as temporary registers. |
| Note that in 64 bit they are arg7-arg4. |
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| 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. |
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| 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. |
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| r30: stack pointer |
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| r31: the ble instruction puts the return pointer in here. |
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| 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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