Memories and Processors
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Memory Terminology
We will not discuss the topic of data storage technologies per se. We are mostly interested here in the question of how data storage can be e ectively organized. 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"). Here is a rundown on some terms:
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 intrisically 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" off 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 defined 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.
FLASH
Flash memory is non-volatile computer memory that can be electrically erased and reprogrammed.
