Question

Embedded System..

1. What is the page table?

2. How to get from Kernel to User and User to Kernel?

Answer 1

1) What is the page table?

A page table is the data structure used by a virtual memory system in a computer operating system to store the mapping between virtual addresses and physical addresses. Virtual addresses are used by the program executed by the accessing process, while physical addresses are used by the hardware, or more specifically, by the RAM subsystem. The page table is a key component of virtual address translation which is necessary to access data in memory.

In operating systems that use virtual memory, every process is given the impression that it is working with large, contiguous sections of memory. Physically, the memory of each process may be dispersed across different areas of physical memory, or may have been moved (paged out) to another storage, typically to a hard disk drive.

When a process requests access to data in its memory, it is the responsibility of the operating system to map the virtual address provided by the process to the physical address of the actual memory where that data is stored. The page table is where the operating system stores its mappings of virtual addresses to physical addresses, with each mapping also known as a page table entry (PTE)

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2) How to get from Kernel to User and User to Kernel?

User Mode

The system is in user mode when the operating system is running a user application such as handling a text editor. The transition from user mode to kernel mode occurs when the application requests the help of operating system or an interrupt or a system call occurs.

The mode bit is set to 1 in the user mode. It is changed from 1 to 0 when switching from user mode to kernel mode.

Kernel Mode

The system starts in kernel mode when it boots and after the operating system is loaded, it executes applications in user mode. There are some privileged instructions that can only be executed in kernel mode.

These are interrupt instructions, input output management etc. If the privileged instructions are executed in user mode, it is illegal and a trap is generated.

The mode bit is set to 0 in the kernel mode. It is changed from 0 to 1 when switching from kernel mode to user mode.

Necessity of Dual Mode (User Mode and Kernel Mode) in Operating System

The lack of a dual mode i.e user mode and kernel mode in an operating system can cause serious problems. Some of these are:

• A running user program can accidentaly wipe out the operating system by overwriting it with user data.
• Multiple processes can write in the same system at the same time, with disastrous results.

These problems could have occurred in the MS-DOS operating system which had no mode bit and so no dual mode.