topic Re: Question about Virtual Address Process Space in Operating System - HP-UX

Question about Virtual Address Process Space

Wang,MinJie — Thu, 06 Dec 2007 02:52:24 GMT

Hi all
As we all know
One process space is devided into four quadrants
on PA-Risc system.
The first quadrant is reserved to hold text for 32-bit process while it is reserved to hold Shared Objects for 64-bit.The order of using four quandrants is different.
Can you tell me why?
Thanks

Re: Question about Virtual Address Process Space

Wang,MinJie — Thu, 06 Dec 2007 04:44:42 GMT

Let me make it clear
32-bit process
Quadrant 1:text
Quadrant 2:data
Quadrant 3:Share
Quadrant 4:Share

64-bit process
Quadrant 1:Share
Quadrant 2:text
Quadrant 3:data
Quadrant 4:Share
I want to know why it's changed?

Re: Question about Virtual Address Process Space

Dennis Handly — Thu, 06 Dec 2007 07:45:55 GMT

It may be related to putting all of the 32 bit pointers into the first quadrant?
This may help the kernel when it is running a 32 bit application.

Figure 5-4 on the Tru64 Porting Guide indicates it is for 32 bit mmaps:
http://h30097.www3.hp.com/docs/porting/tru64-to-hpux/CHPLNKRX.HTM#fig-hpuxpa-64addrspace

(The program stack is really in quadrant 3 for that figure.)

>I want to know why it's changed?

Why does it matter to you?

Re: Question about Virtual Address Process Space

Don Morris_1 — Thu, 06 Dec 2007 12:17:16 GMT

Very simple -- because otherwise, how would you ever share anything between 32-bit and 64-bit processes if you don't?

Think about it -- PA doesn't allow aliasing, which is why HP-UX historically was a Global Address Space OS (unlike a lot of other Unix/Linux models which had Private Address spaces or Mostly Private). Sharing between processes is done using a single global shared address across all processes (i.e. if object A is in processes B, C and D -- they all use the same GVA (space.offset) for A). As such -- if you want to share anything in the 32-bit shared address space (q3/q4 in the 32-bit model) you _must_ use the 3 to 4Gb range of q1 of the 64-bit model. Since it would be rather silly to put Text there but sidestep the shared area, you just move Text to a different quadrant and make q1 shared for 64-bit in general.

QED.

Re: Question about Virtual Address Process Space

Wang,MinJie — Fri, 07 Dec 2007 05:26:12 GMT

Thank you
I think I get it