Although memory is technically any form of electronic storage, it is used most often to identify fast, temporary forms of storage. If a computer’s CPU had to constantly access the hard drive to retrieve every piece of data it needs, it would operate very slowly. When the information is kept in memory, the CPU can access it much more quickly. Most forms of memory are intended to store data temporarily.

The CPU accesses memory according to a distinct hierarchy. Whether it comes from permanent storage (the hard drive) or input (the keyboard), most data goes in random access memory (RAM) first. The CPU then stores pieces of data it will need to access, often in a cache, and maintains certain special instructions in the register.

All of the components in a computer, such as the CPU, the hard drive, and the operating system, work together as a team, and memory is one of the most essential parts of this team. From the moment you turn your computer on until the time you shut it down, your CPU is constantly using memory. The following is a typical scenario:

• Turn the computer on.

• The computer loads data from read-only memory (ROM) and performs a power-on self-test (POST) to make sure all the major components are functioning properly. The memory controller checks all of the memory addresses with a quick read/write operation to ensure that there are no errors in the memory chips.

• The computer loads the basic input/output system (BIOS) from ROM. The BIOS provides basic information about storage devices, boot sequence, security, and configuration.

• The computer loads the operating system (OS) from the hard drive into the system’s RAM. Usually, the critical parts of the operating system are maintained in RAM as long as the computer is on.

• When you open an application, it is loaded into RAM. To conserve RAM usage, many applications load only the essential parts of the program initially, then load other pieces as needed.

• After an application is loaded, any files that are opened for use in that application are loaded into RAM.

• When you save a file and close the application, the file is written to the specified storage device, then it and the application are purged from RAM.

• Every time something is loaded or opened, it is placed into RAM. This means that it has been put in the computer’s temporary storage area so that the CPU can access that information more easily. The CPU requests the data it needs from RAM, processes it and writes new data back to RAM in a continuous cycle.

  In most computers, this shuffling of data between the CPU and RAM happens millions of times every second. If the changed files are not saved to a permanent storage device before being purged, they are lost.

  Cache and registers

  It takes longer for data to get from the memory card to the CPU than it takes for the CPU to actually process the data. Caches are designed to alleviate this bottleneck by making the data used most often by the CPU instantly available. This is accomplished by building a small amount of memory, known as primary or level 1 cache, into the CPU. Level 1 cache is very small, normally ranging between 2 KB and 64 KB.

  The secondary, or level 2 cache, typically resides on a memory card located near the CPU. The level 2 cache has a direct connection to the CPU. A dedicated integrated circuit on the motherboard regulates the use of the level 2 cache by the CPU.

  Depending on the CPU, the size of the level 2 cache ranges from 256 KB to 2 MB. In most systems, data needed by the CPU is accessed from the cache approximately 95% of the time, greatly reducing the overhead needed when the CPU has to wait for data from the main memory.

  The final step in memory is the registers. These are memory cells built into the CPU that contain specific data needed by the CPU — particularly the arithmetic and logic unit (ALU).

  Types of Memory

  Memory can be split into two main categories: volatile and nonvolatile. Volatile memory loses data as soon as the system is turned off; it requires constant power to remain viable. Most types of RAM fall into this category.

  Nonvolatile memory does not lose its data when the system or device is turned off. A number of types of memory fall into this category. The most familiar is ROM, but flash memory storage devices are also forms of nonvolatile memory.

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