Classification of memories

MEMORY DEVICES

Introduction:

The important common element of the memories we will study is that they are random access memories, or RAM. This means that each bit of information can be individually stored or retrieved | with a valid input address. This is to be contrasted with sequential memories in which bits must be stored or retrieved in a particular sequence, for example with data storage on magnetic tape. Unfortunately the term RAM has come to have a more specific meaning: A memory for which bits can both be easily stored or retrieved (\written to" or \read from").

Classification of memories

RAM.

In general, refers to random access memory. All of the devices we are considering to be \memories" (RAM, ROM, etc.) are random access. The term RAM has also come to mean memory which can be both easily written to and read from. 

There are two main technologies used for RAM:

1.) Static RAM.

These essentially are arrays of flip-flops. They can be fabricated in ICs as large arrays of tint flip-flops.) \SRAM" is intrinsically somewhat faster than dynamic RAM.

2.) Dynamic RAM.

Uses capacitor arrays. Charge put on a capacitor will produce a HIGH bit if its voltage V = Q=C exceeds the threshold for the logic standard in use. Since the charge will \leak" o_ through the resistance of the connections in times of order _ 1 msec, the stored information must be continuously refreshed (hence the term \dynamic"). Dynamic RAM can be fabricated with more bits per unit area in an IC than static RAM. Hence, it is usually the technology of choice for most large-scale IC memories.

ROM. Read-only memory.

Information cannot be easily stored. The idea is that bits are initially de_ned and are never changed thereafter. As an example, it is generally prudent for the instructions used to initialize a computer upon initial power-up to be stored in ROM. The following terms refer to versions of ROM for which the stored bits can be over-written, but not easily.

PROM Programmable ROM.

Bits can be set on a programming bench by burning fusible links, or equivalent. This technology is also used for programmable array logic (PALs), which we will briefly discuss in class.

EPROM.

ROM which can be erased using ultraviolet light.

EEPROM.

ROM which can be erased electronically.

A few other points of terminology:

As you know, a bit is a binary digit. It represents the smallest element of information.

A byte is 8 bits.

A K of memory is 210 = 1024 bits (sometimes written KB). And a megabit (MB) is

1K _ 1K bits.

RAM is organized into many data \words" of some prescribed length. For example, a RAM which has 8K = 8192 memory locations, with each location storing a data word of \width" 16 bits, would be referred to as a RAM of size 8K _ 16. The total storage capacity of this memory would therefore be 128KB, or simply a \128K" memory. (With modern very large scale integration (VLSI) technology, a typical RAM IC might be 16 MB. Besides the memory \size," the other important specification for memory is the access time. This is the time delay between when a valid request for stored data is sent to a memory and when the corresponding bit of data appears at the output. A typical access time, depending upon the technology of the memory, might be _ 10 ns.