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diff --git a/Documentation/parisc/registers.rst b/Documentation/parisc/registers.rst new file mode 100644 index 000000000..59c8ecf3e --- /dev/null +++ b/Documentation/parisc/registers.rst @@ -0,0 +1,154 @@ +================================ +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. |