Memories - Memory Types - Part 2

Semiconductor memories are characterized according to the following properties:
Serial or random access, Volatile or nonvolatile, Static or dynamic, Destructive or nondestructive readout.

Serial access memories are those in which data are accessed in a fixed, predetermined sequence. Magnetic tape units are an example of serial access. To read a record it is necessary to read the entire tape up to the point where the desired data exists.

By way of contrast, a random access memory (RAM) permits reading of data at any specific location without first reading other data. When performing a read of a FIFO (first-in, first-out) memory, the first location stored is the first to be read out.

These memories act as buffers when transferring data between functional units with different data rates. A stack in a computer, often used to save data and return addresses, is an example of a LIFO (last-in, first-out) memory. The last data pushed onto the stack is the first data to become available when the stack contents are popped from the stack.

Memories are categorized according to whether or not they can retain information when power is removed.

A nonvolatile memory can retain information when power is removed. Examples of nonvolatile memories include magnetic cores, magnetic tapes, disks, MROMs, EPROMS, EEPROMS, and flash memories.

Volatile memory devices lose information when power is removed. Volatile memories can be further broken down into static and dynamic memories.

A static memory retains information as long as power is applied, while a dynamic memory can lose information even when power is continuously applied. Static RAMs (SRAMs) are flip-flops that, with their two stable states, can remain in a given state indefinitely, without need for refresh, as long as power is applied; that is, they are static but volatile.

The dynamic RAM (DRAM), illustrated in Figure, is an example of a dynamic memory. The cell is chosen if decoding the memory address causes its wordline to be selected. It is basically a capacitor that can either be discharged onto the bitline or that can be recharged from the bit-line. Since it is a capacitor, the charge can leak away over time. The memory system must employ refresh circuitry that periodically reads the cells and writes back a suitably amplified version of the signal. If the contents of a memory device are destroyed by a read operation, it is classified as a destructive readout (DRO); otherwise it is a nondestructive readout (NDRO) device.

DRAMs must be refreshed when their contents are read out, since a read causes the capacitor to discharge. Programmable read-only memories (PROMs) are slightly more complicated to characterize. They are static and nonvolatile. Mask programmable ROMs and fuse programmable ROMs are programmed once and thereafter can only be read.

EPROMs (erasable PROMs) can be erased by means of ultraviolet light, which involves physically removing them from the system in which they are installed. For all practical purposes, they are programmed only once because it is quite inconvenient to erase and reprogram them, unless they are being used to emulate a new design for the purposes of debugging that design.

EEPROMs (electrically erasable PROMs) can be reprogrammed after being installed in a system, but their response time is slower than DRAMs or SRAMs; hence they are confined to applications where nonvolatility is required.

Flash memories are structurally almost identical to EPROMs, but they can be reprogrammed in a system and are more dense than EEPROMs. However, EEPROMs can be programmed a bit at a time, whereas flash memories are erased a block at a time before being reprogrammed. The Venn diagram in Figure illustrates this distribution of properties among the various kinds of semiconductor memories.

To be continued...