Computer Terms

Access Time
The time interval between the moment that specific information is requested from a memory device, and when it actually becomes available. This time is measured in nanoseconds (ns).

Asynchronous Memory Asynchronous Memory (See Illustration)
Asynchronous devices are not synchronized with the system clock. Asynchronous also refers to an operating mode in which a memory device responds to input signals whenever they occur; this as opposed to synchronous operation, in which the input signals must be present at specified times in the device’s clock cycle. FPM DRAM and EDO DRAM are examples of Asynchronous DRAM.

BEDO (Burst EDO DRAM)
An EDO DRAM with features such as a four-bit burst length that can be delivered in sequential or interleaved fashion.

Binary
A numbering system that uses 2 as a base and requires only two digits: 0 and 1.

Bit (Binary Digit)
The smallest piece of data (a 1 or 0) that a computer recognizes. Combinations of 1s and 0s are used to represent characters and numbers.

Buffered
Buffered means adding logic, particularly drivers, to a SIMM or DIMM to increase the output current. Buffering is used to overcome signal attenuation due to capacitive loading. Modules that are “buffered” usually have small buffer chips mounted on them.

Byte
A byte equals 8 bits. Computer memory capacities are commonly measured in multiples of Megabytes (millions of bytes). A byte is also the number of binary bits typically used to represent one numeric or alphabetic character.

“C” (Composite)
These parts are made with low density IC’s. This type of memory requires buffers for address and control signals. Composite SIMMs are denoted with a “C” at the end of the description. An example would be 16×36-60 (36) C.

Cache (pronounced “cash”)
A high-speed memory array which acts as a buffer between the CPU and main memory. Cache consists of a controller, directory, and data memory. Cache controllers try to anticipate which data the CPU will need to perform upcoming operations. That data is then loaded into the memory section of the Cache just before it is requested by the CPU. The controller will also try to store frequently accessed data in Cache memory, which allows faster subsequent access to the same data.

CAS (Column Address Strobe)
A control pin on a DRAM used to latch and activate a column address. The column selected on a DRAM is determined by the data present at the address pins when CAS becomes active.

Check bits
Extra data bits provided by a DRAM module to support the ECC function. For a 4-byte bus, 7 or 8 check bits are needed to implement ECC, resulting in a total bus width of 39 or 40 bits. On an 8-byte bus, 8 additional bits are required, resulting in a bus width of 72 bits.

Clock
An electrical current that alternates between high and low voltages. The speed of the clock is measured in Megahertz (MHz).

Clock speed
The rate at which a computer’s internal system clock operates. The clock is used to synchronize operations between the components within the computer.

CMOS (Complementary Metal Oxide Semiconductor)
A MOS device containing both N-channel and P-channel MOS active elements. One of two basic processes (MOS and Bipolar) used to fabricate integrated circuits.

CPU (Pentium III Flip Chip) CPU (Central Processing Unit – See Illustration)
A chip or circuit that interprets and executes programs by processing a list of machine instructions which perform binary operations on data stored in memory.

DDR SDRAM (Double Data Rate SDRAM)
A variant of SDRAM technology that may allow for speeds of up to 250 MHz. DDR SDRAM is still in the development stage.

DIMM (Dual Inline Memory Module)
A configuration for RAM modules that features memory chips mounted on a printed circuit board that has two independent rows of I/O (Input/Output) contacts. DIMMs have pins on opposite sides of the circuit board that remain electrically isolated from each other, which allows for additional data paths and higher throughput. DIMMs have 168 pins and can transfer 64, 72, and 80 bits at a time.

DRAM (Dynamic Random Access Memory)
The most common form of computer memory. DRAM, along with the more advanced forms of memory (such as EDO and SDRAM), hold programs and data while your computer is being used. The advantages to having faster memory (and more of it) include faster program execution and less lag time when switching between programs. DRAM must be refreshed (a re-energizing of the capacitors) every 15ms or so.

ECC (Error Checking and Correcting)
ECC is logic designed to correct memory errors. The number of errors that can be corrected depends upon the algorithms being used, and the number of error correction bits (non-data bits) present. This logic may be included on a DIMM, or it may be found on the computer’s circuit board.

EDO (Extended Data Output)
A type of memory that provides faster access to data (up to 25% faster) than standard FPM and reduces the need for second level Cache. It is generally not compatible with 386′s, 486′s, and older Pentium machines.

EMM (Expanded Memory Manager)
This is a driver that provides a software interface to expanded memory. EMM386.EXE is an example of an EMM that comes with DOS.

EMS (Expanded Memory Specification)
Some older programs can use this type of memory. Unlike conventional or extended memory, expanded memory is not directly addressable by the processor. Instead it can only be accessed through a small 64K window established in the upper memory area (UMA). It provides a way for computers running under DOS to access additional memory.

Extended Memory
Extended Memory is basically all memory above the first megabyte in a PC computer.

FPM (Fast Page Mode)
A standard form of memory usually found in 386′s, 486′s, and older Pentium systems.

Groups
This number refers to how many modules must be installed at any one time. Many older machines require installation of modules in sets of two or even four at a time. Most of the newer (x32, x36, and x64) machines allow upgrades in sets of one or two modules.

Integrated Circuit IC (Integrated Circuit – See Illustration)
The etched silicon wafer containing the individual circuits that make up CPUs, memory modules and other components. These wafers are placed into the small black chips found on SIMMs, DIMMs, or into CPUs and other individual chips.

Interleaving
The process of taking data bits (singly or in bursts) alternately from two or more memory pages (on an SDRAM) or devices (on a memory card or subsystem).

JEDEC (Joint Electron Devices Engineering Council)
An organization that establishes standards for memory operation, features, and packaging.

Parity
A bit used for error checking in the transmission of data.

PCMCIA (Personal Computer Memory Card International Association)
A standard created to allow for interchangeability between various computer components such as memory, FAX/modem, SCSI, and networking products all on the same connector.

PD (Presence Detect)
Indicator pins on SIMMs and DIMMs that provide speed and density information to the system using the memory module.

Proprietary Memory
Memory that is designed specifically for a certain computer or printer.

RAM (Random Access Memory)
Computer storage that resides on silicon chips; chips that can store and retrieve data quickly; code and data that changes while the computer is running.

RAM sizes: The size of a module listed in megabytes (MB), or kilobytes (K) or (KB).
Maximum RAM: The maximum amount of RAM recognized by a particular machine.
Base RAM: The amount of RAM on the motherboard itself (permanent RAM). Also, the first 640kb on a PC.

RAMDAC (Random Access Memory Digital to Analog Converter)
This microchip sits on a VGA card or other graphics display board and translates the digital representation of a screenful of information into an analog signal that the monitor can display. The faster the RAMDAC (measured in MHz), the higher the screen refresh rates that the card will support at each given resolution.

Refresh
The process used to restore the charge in DRAM memory cells at specified intervals. The required refresh interval is a function of the memory cell design and the semiconductor technology used to manufacture the memory device. Refreshing is required to maintain the data stored in DRAM.

SDRAM (Synchronous Dynamic Random Access Memory)
This is standard DRAM with the addition of synchronous control logic. By synchronizing all address data and control signals with a single system clock, SDRAM technology enhances performance, simplifies design, and provides faster data transfer.

SGRAM (Synchronous Graphics Random Access Memory)
A newer type of high speed memory used exclusively for graphics.

SIMM (Single Inline Memory Module)
A particular configuration for RAM. An array of memory chips on a small PC board with a single row of I/O contacts.

SO DIMM (144 Pin Memory Module) SO DIMM (Small Outline Dual Inline Memory Module – See Illustration)
These modules are much smaller than regular DIMMs and are usually used in laptop computers. They have a 32-bit transfer rate.

EDO SO DIMM: Extended Data Output Small Outline Dual Inline Memory Module
SDRAM SO DIMM: Synchronous Dynamic Random Access Memory Small Outline Dual Inline Memory Module.

SOJ (Small Outline J-Lead package)
This plastic package, designed for memory chips, is a surface mount package with turned under leads that look like the letter J.

Speed
Given in nanoseconds (1ns = 1 billionth of a second), the access time of a memory module is the amount of time needed by the module to react to requests from the CPU. Smaller access times allow the CPU to store and retrieve information more quickly.

Therefore, smaller numbers mean higher potential performance (i.e. – 60ns is faster than 70ns). When meeting the speed requirements for a particular machine, remember that the idea is to either match the speed exactly, or to get memory modules that have a faster speed.

SRAM (Static Random Access Memory)
A form of high speed memory. SRAM chips function at faster speeds and do not require a refresh cycle like standard DRAM.

“T” (True)
Parts that are made using the “true” IC configuration (i.e., it is a non-composite part).

TSOP (Thin Small Outline Package)
This plastic package, designed for memory chips, has approximately half the height of a standard surface mounted plastic package (SOJ).

Type
This number (x8, x9, x32, x36, x64, x72, x80) refers to the bit depth of a module, or to the size of the data path used to access the memory.

Video Card Memory VRAM (Video Random Access Memory – See Illustration)
A type of memory in a video display adapter used to create the image appearing on the CRT screen. It uses dual-ported memory allowing simultaneous reads and writes and is faster than dynamic RAM.

XMS memory (Extended Memory Specification)
This specification was developed to define how programs would use extended memory. The XMS specification functions on systems based on 80286 or higher processors and allows real mode programs (those designed to run in DOS) to use extended memory.