PA-RISC Processor Registers

Register usage for Linux/PA-RISC is documented in the Linux kernel source tree in the file Documentation/parisc/registers: https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/plain/Documentation/parisc/registers

Note

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

(SAR)

as specified by ABI (5 or 6 bits depending on 32/64bit)

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

(IIR)

Interrupt Instruction Register

CR20

(ISR)

Interrupt Space Register

CR21

(IOR)

Interrupt Offset Register

CR22

(IPSW)

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

SR1

temporary space register

SR2

kernel should not clobber this

SR3

used for userspace accesses (current process)

SR4-SR7

set to 0

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

See also

More about Space Registers.

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

Notes

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.