Memories are one of the most important parts of a computer system. The term memory refers to any device that can be used for storage. There are two basic ways to store memory. One way is to store the memory within the actual circuitry of the computer, which allows for it to be quickly accessed by the central processing unit. These memory devices are generally used for temporary storage of data and programs that are currently used by the CPU. The second type of memory storage is to store the memory in external storage devices. They include hard drives (also known as hard disks), floppy disks, and CD-ROMs. Programs and information that you are not currently using can be permanently stored on these devices. The term "memory" usually pertains to the devices stored within the circuitry of the computer. They are temporary, meaning that when the computer is turned off, they will be deleted. These devices are connected directly to the CPU and are pretty fast! They will be discussed in the following section. On the other hand, the external storage devices are not considered part of a computer's memory, and these will be talked about in a later section of this website.
You might be wondering why the computer has so many different types of storage devices. Well, there is a simple answer to this. If computer systems did not have so many different types of storage, it would either be extremely slow, or extremely expensive. The fast memories that can be accessed by the CPU directly are extremely expensive, while cheaper storage devices are extremely slow. By using a combination of both, both the price and the performance of a computer system are optimized!
Scientists have developed a CPU hierarchy. This is a system describing the order in which memory devices are accessed by the CPU. First, there is the CPU itself. There are special registers in it that can temporarily store data and results of operations. These are fast, but are also very small because of their extremely high price. Second in the hierarchy are caches. Caches are used to temporarily store the information most frequently used by the CPU and by the user. They are also small, but can be extremely fast-almost as fast as the CPU itself! Next, there is the RAM, or random-access memory. RAM is the main memory of a computer. It is used to temporarily store all information currently in use by the CPU. However, sometimes the RAM is not big enough to store all of the programs in use at one time. This is when the virtual memory feature of a hard drive comes into play. Virtual memory is used to free up space in the RAM so that there will be more room for programs currently in use. It copies areas of the RAM that have not been recently used into the hard drive, thus freeing up space. As you can see, there is quite a complex system of memories in a computer!
Some Memory Terminology:
It is important to understand some key words in order to fully comprehend all of the information about memories. First, understand that data in a computer is stored in the form of binary digits, known as bits. There are two binary digits, 1 and 0 (for more information about this, see the section on the Binary System). 1 bit does not mean much by itself. However, if we combine bits together, then we'll be getting somewhere. If we have many bits together, then we can get thousands and millions of different possible combinations. For example, the following table represents the number of bits we have, and the possible combinations we can make with those bits:
|
Number of Bits
|
Number of Possible
Combinations
|
|---|---|
|
1
|
2
|
|
2
|
4
|
|
3
|
8
|
|
8
|
256
|
As you can see, the number of possible combinations you can make out of a certain number of bits doubles every time the number of bits increments by 1. It is important to know that 8 bits is equal to 1 byte. 1 byte is used to represent certain letters of the alphabets, numbers, punctuation marks, as well as certain special symbols. With 256 possible combinations of bits, there are more than enough possibilities to represent the entire alphabet, number digits, punctuation marks, and a variety of other symbols. Most memories are measured in kilobytes, megabytes, or gigabytes. One kilobyte (K) is equal to 1000 bytes, or 1000 characters (letters, numbers, etc). If you consider one byte equal to one letter, then one kilobyte would be equivalent to one page of text, double-spaced. A megabyte (MB) is equal to 1 million bytes. It would be equivalent to a novel, and one gigabyte (GB) is equal to 1 billion bytes! One gigabyte would be like 1,000 novels!
Another word often used with memory is megahertz (MHz). This is how we measure the CPU's processing speed. It measures, (in millions) how fast the CPU can process information every second. An 800 MHz Pentium III processor can process information 800 million times per second!
Two final important words to know when dealing with memory are "read" and "write." These are two basic functions that can be done to the data stored in the memory. The data can be read, meaning that you can open the file, take out the data, and read what it says. Read-only memories that contain files and programs that are permanently stored. They cannot be changed, altered, or erased. Meanwhile, the write function means that you can write over the current files in a memory and replace them with new data. Read-write memories are memories that can be read from and written into.
How Memories Generally Work:
It is important to get a basic idea of how memories, in general, work. First, memories are generally laid out in a spreadsheet form- they look like tables with many columns and rows. The cells of these tables are the memory cells. Each cell stores one bit of information. Furthermore, all of the data contained in a memory has an address, which is like your house address. The address pinpoint the exact location of the data, just like the address of your house pinpoints the exact location of where your house is. The instructions telling a computer what tasks to perform specify the address of the data it needs. The data can be easily gathered by using this address to locate the memory cell that it is in.
Types of Memories:
Random-access memory, or RAM, is a temporary storage place where your computer holds files and data that you are using at the present moment.
Read-only memory, or ROM, stores permanent information. The data stored here can not be altered or changed in any way.
Caches are extremely fast, temporary memories. Sometimes, they are even located right within the CPU for extra-quick data retrieval!
Virtual memory is a special memory that
lets your computer handle many programs and applications with only a small
amount of RAM!
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