On Mon, 2 Sep 2002 22:25:55 +0200 Jan Hudec <bulb@cimice.maxinet.cz> wrote: >On Mon, Sep 02, 2002 at 04:03:21PM +0200, Angelo Dell'Aera wrote: >> AFAIK Linux creates at boot time some segments. >> >> -two for kernel : kernel data segment and kernel code segment; >> -two for userland : user data segment and user code segment >> (this last ones are shared by all processes in User Mode); >> -in Linux 2.2 a task state segment (TSS) for each process, in >> Linux 2.4 simply one TSS per CPU; >> -a default Local Descriptor Table segment usually shared by all >> processes (but very rarely used). > >Segments are kind of superfluous for linux. They are superseeded by >paging, but ia32 insists on still using them. So there is as little of >them as possible. True. Infact Linux prefers paging since it guarantees portability to other archs which support segmentation in a very limited way. Btw in kernel sources (/usr/src/linux/include/asm-i386/sched.h) I found this...maybe it could be useful. Obviously it refers only to i386. * The layout of the GDT under Linux: * * 0 - null * 1 - not used * 2 - kernel code segment * 3 - kernel data segment * 4 - user code segment <-- new cacheline * 5 - user data segment * 6 - not used * 7 - not used * 8 - APM BIOS support <-- new cacheline * 9 - APM BIOS support * 10 - APM BIOS support * 11 - APM BIOS support * * The TSS+LDT descriptors are spread out a bit so that every CPU * has an exclusive cacheline for the per-CPU TSS and LDT: * * 12 - CPU#0 TSS <-- new cacheline * 13 - CPU#0 LDT * 14 - not used * 15 - not used * 16 - CPU#1 TSS <-- new cacheline * 17 - CPU#1 LDT * 18 - not used * 19 - not used * ... NR_CPUS per-CPU TSS+LDT's if on SMP Regards, Angelo Dell'Aera 'buffer' <buffer@users.sourceforge.net> -- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ FAQ: http://kernelnewbies.org/faq/
