Question about preserving caller register values

[Dany]

Hi, as you probably have seen I am busy with the CRTOS library for PIC32 (http://www.mikroe.com/forum/viewtopic.php?f=172&t=64439).

Q1:

I think that the task context I do save is too big, after all the context switching in CRTOS looks to the caller (a task) as a simple function call (the "yield" to the scheduler).

So my question is: which register values do I have to preserve inside a called procedure (e.g. the yield procedure) (should be the same as any other arbitrary called procedure), and which register values can be changed inside a called procedure without the need to restore them afterwards.

I have seen that the compiler, in a function, saves RA (which is normal, if deeper calls happen), but also e.g. R25, R26 etc if it uses them, but not R30. Can you please provide such a list?

I also experienced that, when I try to "Restore" R1 (used as GP by the compiler), the CRTOS library does not work any more. I do not know why, my knowledge of the MIPS is very poor at the moment. Anyone knows?


Q2:

An extra question: I see that the compiler initializes GP outside the actual ram addresses: in the mini32 system with usb bootloader (P32MX534F064H with 16 Kbytes of Ram), the GP is initialized at $A0008000, while ram resides from $A0000000 to $A0003FFF.
This also means that in instructions as "SB R2, Offset(_Task0_ID+0)(GP)", the ofset of the variable is negative (or am I mistaking?)
I was actually expecting that the GP would point to the ram base address... so why is above done the way it is done?

[Dany]

No reply? I know I am asking about the inner space of the compiler, but still...