| 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. | |