Control Registers¶

Space Registers (kernel mode)¶

Space Registers (user mode)¶

Processor Status Word¶

“Invisible” Registers¶

The PA-RISC architecture defines 7 registers as “shadow registers”.Those are used in RETURN FROM INTERRUPTION AND RESTORE instruction to reducethe 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 additionalnotes from Randolph Chung.

For the general registers:

r1,r2,r19-r26,r28,r29 & r31 can be used without saving them first. And ofcourse, you need to save them if you care about them, before callinganother procedure. Some of the above registers do have special meaningsthat 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 touse this, since you need the pointer to get back to yourcaller. However, it is grouped with this set of registerssince the caller can’t rely on the value being the samewhen you return, i.e. you can copy r2 to another registerand return through that register after trashing r2, andthat 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 youdon’t care about the values that were passed in anymore.

r28,r29:

are ret0 and ret1. They are what you pass return valuesin. r28 is the primary return. When returning small structuresr29 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 justgeneral purpose registers. r27 is the data pointer, and isused to make references to global variables easier. r30 isthe stack pointer.