Cache Memory

What is Cache Memory ?

cache memory, also called Cache,  a supplementary memory system that temporarily stores frequently used instructions and data for quicker processing by the central processor of acomputer. The cache augments, and is an extension of, a computer’s main memory. Both main memory and cache are internal, random-access memories (RAMs) that use semiconductor-based transistor circuits. Cache holds a copy of only the most frequently used information or program codes stored in the main memory; the smaller capacity of the cache reduces the time required to locate data within it and provide it to the computer for processing.

When a computer’s central processor accesses its internal memory, it first checks to see if the information it needs is stored in the cache. If it is, the cache returns the data to the processor. If the information is not in the cache, the processor retrieves it from the main memory. Disk cache memory operates similarly, but the cache is used to hold data that has been recently written on, or retrieved from, a magnetic disk or other external storage device.

Computer Main Memory And Its Types

Computer Memory

MEMORY IS THE INTERNAL STORAGE AREA OF THE COMPUTER. MEMORY IDENTIFIES THE DATA STORAGE.THE PHYSICAL MEMORY USUALLY REFERRED TO US MAIN MEMORY OR RAM.

TYPES OF MEMORY

• RAM

• ROM

RAM

RAM STANDS FOR RANDOM ACCESS MEMORY.IT WRITE DATA INTO RAM AND ASLO READ DATA FROM RAM. RAM IS A VOLATILE MEMORY.IT HOLDS DATA AS LONG AS THE COMPUTER IS SWITCHED ON.THERE ARE TWO TYPES OF RAM

• DRAM

• SRAM

• DRDRAM

DRAM

DRAM STANDS FOR DYNAMIC RANDOM ACCESS MEMORY. MOSTLY USED IN PERSONAL COMPUTERS.DRAM MUST HAVE ELECTRIC CURRENT TO MAINTAIN ELECTRICAL STATE(REFRESH).ITS ACCESS TIME IS 60 - 70 NAN0SEC0NDS.

SRAM

SRAM STANDS FOR STATIC RANDOM ACCESS MEMORY.ITS ACCESS TIME LESS THAN 60 NAN0SECONDS.

DRDRAM

DRDRAM STANDS FOR DIRECT RAMBUS DYNAMIC RANDOM ACCESS MEMORY.IT IS THE NEW TYPE OF RAM. IT ACCESS TIME 20 TIMES FASTER THAN DRAM.

ROM

ROM STANDS FOR READ ONLY MEMORY.IT IS A NON-VOLATILE MEMORY.IT READ DATA FROM MEMORY.

TYPES OF ROM

• PROM

• EPROM

PROM

PROM STANDS FOR PROGRAMMABLE READ ONLY MEMORY. IT ACTS LIKE A ROM. THERE ARE TWO TYPES OF PROMS.

• EPROM

• EEPROM

EPROM(ERASABLE)

EPROM STANDS FOR ERASABLE PROGRAMMABLE READ ONLY MEMORY. IT CAN BE ERASED AND REPROGRAMMED BY THE USER.

EEPROM(NON ERASABLE):

EEPROM STANDS FOR ELECTRICALLY ERASABLE PROGRAMMABLE READ ONLY MEMORY.NONERASABLE PROMS CANNOT BE CHANGED ONCE THEY ARE PROGRAMMED.

Operating system architecture

The computer architecture of a computing system defines its attributes as seen by the programs that are executed in that system, that is, the conceptual structure and functional behavior of the machine hardware. Then, the computer architect defines the functions to be executed in the hardware and the protocol to be used by the software in order to exploit such functions. Note that the architecture has nothing to do with the organization of the data flow, the logical design, the physical design, and the performance of any particular implementation in the hardware.

      Hence By Architecture we mean the order in which certain hardware Processes are carried out by the OS and has nothing to do with the logical software flow of the Computer.

Simple view

An Operating System is the layer between the hardware and software, as in
 
 
An Operating System is responsible for the following functions

• Device management using device drivers
• Process management using processes and threads
• Inter-process communication
• Memory management
• File systems

In addition, all operating systems come with a set of standard utilities. The utilities allow common tasks to be performed such as

• being able to start and stop processes
• being able to organise the set of available applications
• organise files into sets such as directories
• view files and sets of files
• edit files
• rename, copy, delete files
• communicate between processes

Kernel

The kernel of an operating system is the part responsible for all other operations. When a computer boots up, it goes through some initialisation functions, such as checking memory. It then loads the kernel and switches control to it. The kernel then starts up all the processes needed to communiate with the user and the rest of the environment (e.g. the LAN)
The kernel is always loaded into memory, and kernel functions always run, handling processes, memory, files and devices.
The traditional structure of a kernel is a layered system, such as Unix. In this, all layers are part of the kernel, and each layer can talk to only a few other layers. Application programs and utilities live above the kernel.
The Unix kernel looks like 
Most of the Operating Systems being built now use instead a micro kernel, which minimises the size of the kernel. Many traditional services are made into user level services. Communication being services is often by an explicit message passingmechanism.
The major micro-kernel Operating System is Mach. Many others use the concepts of Mach.
Some systems, such as Windows NT use a mixed approach