Process region table & Global Descriptor able & virtual address

Question

I am going through linux notes from one of the training institute here. As per that when ever a process is created a region is allocated to it. Region contains all the segments for the process.

Also region is specified by region-table. Region table contains following entry ;--
virtual address to - Physical address pointer + Disk Block Discriptor

Disk block descriptor point to the swap or exe file on disk.

two douts i have :-----

1> Where does the Global & Local Descriptor role is here.

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2> Does each process have its own global descriptor table ? What i think is yes otherwise two processes vitual address will point towards same physical address .

Please suggest

Answer 1

1) The global descriptor table gives the base-address for the linear address. It is NEARLY always zero, and the "Limit" is set too "all ones" (that is, all addressable memory). In effect, the segment selectors are not actually used. The architecture requires them to be present and loaded, but the actual effect that they "can be used for" is not being used in Linux.

The Local descriptor table works exactly the same way, except there is a LDT per process. Typically it holds the stack segment of the task - it still has a base-address of zero. The process can modify the LDT, it can't modify the GDT.

To tell if the segment is GDT or LDT, look at bit 3 (the one worth 8) - for example, in my system ss has the value 0x2b, so it has bit 3 set. cs on the other hand is 33, so it's not got bit 3 set, and thus comes out of the GDT.

2) No. There is one GDT (per CPU core, to be precise) - that's why it's called "global" - there is one for everything. This is also why the stack segment is in the LDT, because there is one per process.