Have you ever wondered what goes on in your computer from the moment you switch it on? Here’s how computer memory works.

Memory is one of the most crucial components of your computer—along with the CPU (central processing unit), the hard drive and the operating system. From the moment that a computer is switched on until it is shut down, the CPU is continuously utilizing memory. It accesses memory in a particular order—beginning with random access memory (RAM). That means that most data goes into RAM first, whether originating from permanent storage (the hard drive) or input (the keyboard). After RAM, the next thing the CPU accesses is cache, where it stores pieces of data it will need to use. Third in the hierarchy is the register, where the CPU puts certain special instructions.

The term “memory” most commonly refers to fast, temporary forms of storage, even though by definition, it is any form of electronic storage. Thus, the hard drive is usually not considered memory. By keeping data in memory, computers can run much faster. If the computer’s CPU had to access the hard drive to get every bit of information it requires, the computer would have an extremely slow operating speed. Most forms of memory are designed for temporary data storage.

Computers typically go through a certain sequence of operations. When a computer is turned on, it loads data from read-only memory (ROM) and conducts a power-on self-test (POST) to ensure that all main components are functioning. The memory controller checks the memory chips for errors. Then from ROM, the computer loads the basic input/output system (BIOS) which gives basic information about storage devices, boot sequence, security and configuration. The computer then loads the operating system (OS) from the hard drive into the RAM, where its critical parts typically remain until the computer is switched off.

Opening an application will cause it to be loaded into RAM. Many applications initially load only the crucial parts of the program so that not too much RAM space is taken up. They then load other parts as required. Any files that you open in the current application will be placed into RAM as well. In fact, every time you open or load something, it goes into RAM, where the CPU can easily access it. The CPU gets the data it needs from RAM, processes it, and then sends new data back to RAM in a constant cycle.

In most computers, this constant movement of data between CPU and RAM occurs millions of times per second. When you save a file, it is written to the specified storage device. Then when you close it and the application, they are both removed from RAM. If you neglect to save modified files to a permanent storage device before closing them and thus purging them from RAM, the information will be lost.

Following RAM in the hierarchy is cache. Caches, like RAM, make information access easier for the CPU. It actually takes more time for data to travel from the memory card to the CPU than for the CPU to process that information. That’s where cache comes in—making data utilized most often by the CPU immediately available. Primary or level 1 cache is built into the CPU itself. Level 1 cache is very tiny—usually between 2 KB and 64 KB.

The secondary or level 2 cache can usually be found on a memory card located near the CPU. Secondary cache is directly connected to the CPU. A dedicated integrated circuit on the motherboard controls the CPU’s use of the level 2 cache. Secondary cache is much bigger in size than primary cache, ranging from 256 KB to 2 MB. In most systems, when the CPU needs data, it accesses it from the cache roughly 95% of the time. This means that the vast majority of the time, the CPU does not need to wait for data from the main memory—a boon for operating speed.

The third and final part of the hierarchy is the registers. These memory cells reside in the CPU and store specific data utilized by the CPU—specifically the arithmetic and logic unit (ALU).

There are two main categories of memory—volatile and nonvolatile. Volatile memory cannot hang on to data when the system is shut off. It needs continuous power to maintain data. This category includes most types of RAM. Nonvolatile memory, in contrast, retains data even when the system or device is switched off. Several types of memory belong in this category. The most known is ROM. Flash memory storage devices also fall into this group.

Source: How Computer Memory Works
Marshall Brain
Plant Engineering, Aug. 1, 2002
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