Memory consists of electronic components that store instructions waiting to be executed by the processor, data needed by those instructions, and the results of processed data (information). Memory usually consists of one or more chips on the motherboard or some other circuit board in the computer.
Memory stores theree basic categories of items:
- the operating system and other system software that controls or maintain the comptuer and its devices;
- application programs that carry out a specific task such as word processing; and
- the data being processed by the application programs and resulting information.
The role of memory to store both data and programs is known as the stored program concept.
Bytes & Addressable Memory
A byte (character) is the basic storage unit in memory. When application program instructions and data are transferred to memory from storage devices, the instructions and data exist as bytes. Each byte resides temporarily in a location in memory that has an address. An address simply is a unique number that identifies the location of a byte in memory. Memory address arrangement can be compared to seating in a stadium:
- a seat, which is identified by the unique seat number, holds one person at a time, and a location in memory, which is identified by a unique address, holds a single byte; and,
- both a seat, identified by a seat number, and a byte, identified by an address, can be empty.
To access data or instructions in memory, the computer references the addresses that contain bytes of data.
Types of Memory
The system unit contaisn two types of memory: volatile and non-volatile. When the computer’s power is turned off, volatile memory loses its contents. Non-volatile memory, by contrast, does not lose its contents when power is removed from the computer. Thus, volatile memory is temporary and non-volatile memory is permanant. RAM is the most common type of volatile memory. Example of non-volatile memory include ROM, flash memory, and CMOS.
Users typically are referring to RAM when discussing computer memory. RAM (Random Access Memory), also called main memory, consists of memory chips that can be read from and written to by the processor and other devices. When you turn on power to a computer, certain operating system files (such as the files that determine how the Windows Vista desktop appears) loads into RAM from a storage device such as a hard disk. These files remain in RAM as long as the computer has continuous power. As additional programs and data are requested, they also load into RAM from storage.
The processor interprets and executes a program’s instructions while the program is in RAM. During this time, the contents of RAM may change. RAM can accomodate multiple program simultaneously.
Most RAM is volatile, which means it loses its contents when the power is removed from thec omputer. For this reason, you must save any data, instructions, and information you may need in the future. Saving is the process of copying data, instructions, and information from RAM to a storage device such as a hard disk. Three basic types of RAM chips exist: dynamic RAM, static RAM, and
Cache Memory is RAM (Random Access Memory) that a computer microprocessor can access more quickly than it can access regular RAM. As the microprocessor processes data, it looks first in the cache memory and if it finds the data there (from a previous reading of data), it does not have to do the more time-consuming reading of data from larger memory.
Cache also exists within Processors as such there is more information on Cache under CPU Architecture.
Read-only Memory (ROM) refers to memory chips storing permanent data and instructions. The data on most ROM chips cannot be modified – hence, the name read-only. ROM is non-volatile, which means its contents are not lost when power is removed from the computer. In addition to computers, many devices contain ROM chips. For example, ROM chips in printers contain data for fonts.
Flash Memory is a type of non-volatile memory that can be erased electornically and rewritten. Most computer use flash memory to hold their startup instructions because it allows the computer easilty to update its contents. For example, when the computer changes from standard time to day-light savings time, the contents of a flash memory chip (and real-time clock chip) change to reflect the new time.
Flash memory chips also store data and programs on many mobile computers and devices, such as smart phones, portable media players, PDAs, printers, digital cameras, automotive devices, digital voice recorders,a dn pagers.
Some RAM chips, flash memory chips, and other types of memory chips use complementary metal-oxide semiconductor (CMOS) technology because it provides high speeds and comsumes little power. CMOS technology uses battery power to retrain information even whent he power to the computer is off. Battery-backed CMOS memory chips, for example, can keep the calendar, data, and time current even when the computer is off. The flash memory chips that store a computer’s startup information often use CMOS technology.
Memory Access Times
Access time is the amount of time it takes the processor to read data, instructions, and information from memory. A computer’s access time directly affects how fast the computer processes data. Accessing data in memory can be more than 200,000 times faster than accessing data on a hard disk.
Today’s manufacturers use a variety of terminology to state access times. Some use fractions of a second, which for memory occurs in nanoseconds. A nanosecond is one billionth of a second, for example electricity levels travel about one foot in a nanosecond.