**************************************** * LUG meet on 12 Jan. 2003 @ VJTI ****************************************
On Wed, 8 Jan 2003 09:07:32 +0530 (IST) Nikhil Joshi wrote: >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 Ah! There are all the reasons why modern OSes use protected mode. [snip] >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. Uh no, the privilege levels are only used to control access to memory locations, CPU resources and certain instructions (like HLT). It doesn't cover file access/execution privileges. Those are handled by the uid/gid (real, effective, file, or saved uids/gids) associated with the process and the access permissions on the files the process is trying to access, as reported by the filesystem. This is done by the kernel and a single user OS like Windows 98 may choose to entirely skip this issue. >When reset, 386 points to a default location in PROM. ( the >location is FFFF0H note: for 8086 compatibility 16 bit address) You probably meant FFF0H. >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) For a bios does it really matter what the bootloader really is? Its just looking for a piece of code to run which has the correct magic number. The bootloader may either be the code that bootstraps an OS or it may be a proxy bootloader that calls the bootloading code of an OS later. -- Tahir Hashmi (VSE, NCST) http://staff.ncst.ernet.in/tahir tahir AT ncst DOT ernet DOT in We, the rest of humanity, wish GNU luck and Godspeed -- _______________________________________________ http://mm.ilug-bom.org.in/mailman/listinfo/linuxers
