Question

    2.1) What is the kernel mode?

    2.2) What is the user mode?

    2.3) Which mode has more different instructions?

    2.4) Why do we need these two modes in designing an operating system?

Answer 1

`Hey,

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In Kernel mode, the executing code has complete and unrestricted access to the underlying hardware. It can execute any CPU instruction and reference any memory address. Kernel mode is generally reserved for the lowest-level, most trusted functions of the operating system. Crashes in kernel mode are catastrophic; they will halt the entire PC.

In User mode, the executing code has no ability to directly access hardware or reference memory. Code running in user mode must delegate to system APIs to access hardware or memory. Due to the protection afforded by this sort of isolation, crashes in user mode are always recoverable. Most of the code running on your computer will execute in user mode.

These two modes aren't mere labels; they're enforced by the CPU hardware. If code executing in User mode attempts to do something outside its purview-- like, say, accessing a privileged CPU instruction or modifying memory that it has no access to -- a trappable exception is thrown. Instead of your entire system crashing, only that particular application crashes. That's the value of User mode.

x86 CPU hardware actually provides four protection rings: 0, 1, 2, and 3. Only rings 0 (Kernel) and 3 (User) are typically used.

If we're only using two isolation rings, it's a bit unclear where device drivers should go-- the code that allows us to use our video cards, keyboards, mice, printers, and so forth. Do these drivers run in Kernel mode, for maximum performance, or do they run in User mode, for maximum stability? In Windows, at least, the answer is it depends. Device drivers can run in either user or kernel mode. Most drivers are shunted to the User side of the fence these days, with the notable exception of video card drivers, which need bare-knuckle Kernel mode performance. But even that is changing; in Windows Vista, video drivers are segmented into User and Kernel sections. Perhaps that's why gamers complain that Vista performs about 10 percent slower in games.

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