**************************************** * LUG meet on 12 Jan. 2003 @ VJTI ****************************************
Hi Nikhil, Its almost the correct sequence. >It is the job of the OS to take 386 from Real to > Protected Mode ( maybe by setting the CR0 bit, > I'm trying to find this in the Kernel sources :) CR0 is a register not a bit. It contains system control flags. Switching from Real Mode to Protected mode happens when the PE (Protection Enable) bit in this register is set. I think protected mode is enabled in /usr/src/linux-2.4/arch/i386/boot/setup.S section "code32_start" and paging is enabled in /usr/src/linux-2.4/arch/i386/kernel/head.S section "startup_32". > Now it is expected that Track 0 contains >valid OS. (Some Magic Number AFAIK) >Now where does the Bootloader come into picture? ( I >dunno pls someone elucidate on this) BIOS looks for the content of sector 0 (bootsect.S in linux). If it has the magic number 'AA55' at offset 510 it is considered valid. In case a bootloader is used , part of the bootloader (lilo etc.) exists in the bootsector(Sector 0) and has the magic number AA55. Verify this in :- /usr/src/linux/arch/i386/boot/bootsect.S (note : this file is used when lilo not used) regards, Sabu --- Nikhil Joshi <[EMAIL PROTECTED]> wrote: > > 386DX has 3 modes: > > 1. Real Mode (for older apps, specifically for > 8086/88 support) > 2. Virtual Real Mode or Virtual 86 mode(for having > (some) protected mode > feat. in real mode) > 3. Protected Mode > > Protected mode has following advantages: > > 1. Full 32 bit addressing > 2. Better interrupt/fault handling (more no. of > interrupt conditions > possible) > 3. Multiuser, Multitasking environment possible > because of different > previlege levels, task switiching thru call gates > and interrupt > gates, etc. > 4. Data is secure at the hardware level. Non > previleged task will not be > able to access higher privelege data segment > 5. Better memory management > > and more ... > > However it is interesting to note that whenever 386 > is reset it will go > into Real Mode by *default* (obviously for DOS and > other real mode OSes > which dont know about protected mode.) > > It is the job of the OS to take 386 from Real to > Protected Mode ( maybe by > setting the CR0 bit, I'm trying to find this in the > Kernel sources :) ) > > After protected mode has been initialized, kernel > loads the IVT > (Interrupt Vector Table ), the previlege levels of > the data and code > segments ( *maybe* this facility is used for user > level file > acess/execution previleges ) and continues loading > the OS. > > P.S. > But how the Kernel is loaded in first place by the > BIOS? > Conjecture: > > When reset, 386 points to a default location in > PROM. ( the > location is FFFF0H note: for 8086 compatibility 16 > bit address) > > The default location then has a subroutine which > passes control to the > BIOS (a sort of PROM right ?) > > Now BIOS performs the IO checks. Afterwards it > searches for address (Track > 0) of a media (CDROM/HDD/FDD). Now it is expected > that Track 0 contains > valid OS. (Some Magic Number AFAIK) > > Now where does the Bootloader come into picture? ( I > dunno pls someone > elucidate on this) > > Ok lets assume that Bootloader "fools" BIOS in > believing that it actually > is an OS. > > Now control is transferred to the bootloader which > according to the option > selected loads the kernel of the selected OS. > The kernel now continues with its operation.... > > whew... > Can someone pls tell whether this is the right > sequence? > > -- > Nikhil > Earth,19.12N 73.02E > > -- > _______________________________________________ > > http://mm.ilug-bom.org.in/mailman/listinfo/linuxers > > > __________________________________________________ Do you Yahoo!? Yahoo! Mail Plus - Powerful. Affordable. Sign up now. http://mailplus.yahoo.com -- _______________________________________________ http://mm.ilug-bom.org.in/mailman/listinfo/linuxers
