Introducing to Operating SystemSee also :
Description of the memory
The physical memory on a system falls into two categories:
- the random access memory: made up of integrated circuits, therefore very fast
- mass storage: made up of magnetic supports (hard drive, magnetic tapes, etc.), much slower
The physical memory is a temporary storage area for the programmes and data that you use. Generally speaking, the larger the memory space, the more applications can be launched at the same time. In addition, the faster the memory, the faster your system will react. It (
the operating system) thus needs to be organized as efficiently as possible to achieve the best performance.
Memory management
Memory management is a tricky compromise between performance (access time) and quantity (available space). We always seek the maximum available memory space but we are rarely prepared to compromise on performance.
Memory management must also perform the following functions:
- allow memory sharing (for a multi-threaded system);
- allocate blocks of memory space for different tasks;
- protect the memory spaces used (e.g. prevent a user from changing a task performed by another user);
- optimise the quantity of available memory, specifically via memory expansion systems.
Memory expansion
The memory can be expanded in two ways:
- By dividing a program into a portion that remains in the random access memory and a portion which is only loaded into the memory when access to the data is needed.
- By using a virtual memory mechanism, which involves using the hard drive as the main memory and only storing only the instructions and data used by the processor in the RAM. The operating system performs this operation by creating a temporary file (known as SWAP file, or "exchange file") in which information is stored when RAM is no longer sufficient. This operation leads to a significant decrease in performance, given that the hard drive can be accessed a great deal faster than the RAM. When using the virtual memory, it is common for the hard drive LED to remain almost constantly illuminated and in the case of the Microsoft Windows system, it is common for a "win386.swp file - large in proportion to the random access memory needs - to appear.
The memory breakdown mechanisms
The central memory can be broken down in three ways:
- segmenting: the programmes are broken down into parcels of varying lengths known as "segments".
- la paging: this involves dividing the memory into blocks and programmes into pages of fixed length.
- a combination of segmenting and paging: certain parts of the memory are segmented and others are paged.
Last update on Thursday October 16, 2008 02:43:19 PM.