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1. Introduction

The aim for this report is to describe how an operating system is managing securely system main memory. We chose to focus on main memory because every piece of information must be in memory before touch the storage place.

2. Objectives

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3. Operating System general assets description

Modern operating systems are multiuser and multiprogramming meant that several aspects of a computing system required protection:
• memory
• sharable I/O devices; example storage
• serially reusable I/O devices, such as printers
• sharable programs and sub procedures
• networks
• sharable data
As it assumed responsibility for controlled sharing, the operating system had to protect these objects. In the following sections, we look at some of the mechanisms with which operating systems have enforced these objects' protection. Many operating system protection mechanisms have been supported by hardware.
Memory protection is a way to control memory access rights on a computer. The purpose of memory protection is to prevent a process from accessing memory that has not been allocated to it. This prevents a bug within a process from affecting other processes, or the operating system itself. Computer security includes additional techniques such as address space layout randomization and executable space protection.
Separation in an operating system can be one of following or a combination:
 physical separation, in which different processes use different physical objects, such as separate printers for output requiring different levels of security
 temporal separation, in which processes having different security requirements are executed at different times
 logical separation, in which users operate under the illusion that no other proce...

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...n would be useless or harmless if the subject were to have access, the subject should not have that additional access. For example, a program should not have access to the absolute memory address to which a page number reference translates, even though the program could not use that address in any effective way. Not allowing access to unnecessary objects guards against security weaknesses if a part of the protection mechanism should fail.
Verify acceptable usage. Ability to access is a yes-or-no decision. But it is equally important to check that the activity to be performed on an object is appropriate. For example, a data structure such as a stack has certain acceptable operations, including push, pop, clear, and so on. We may want not only to control who or what has access to a stack but also to be assured that the accesses performed are legitimate stack accesses.