SOFTWARE OPTIMIZATION TECHNIQUES - Software Design for Six-Sigma: A Roadmap for Excellence - page 449

its contents never can be changed. An erasable-and-programmable ROM (EPROM)
is programmed in exactly the same manner as a PROM but can be erased and
reprogrammed repeatedly. To erase an EPROM, the device should be exposed to a
strong source of ultraviolet light.
Nowadays, several types of memory combine features of both RAM and ROM.
These devices do not belong to either group and can be referred to collectively as
hybrid memory devices, which include EEPROM and flash, and nonvolatile random
access memory (NVRAM). EEPROMs are electrically erasable and programmable,
but the erase operation is accomplished electrically, rather than by exposure to
ultraviolet light, like EPROM. Any byte within an EEPROM may be erased and
rewritten.
Flash memory originally was created as a replacement for mass storage media
such as floppy and hard disks and is designed for maximum capacity and density,
minimum power consumption, and a high number of write cycles. 21 However, it
should be noted that all nonvolatile solid-state memory can endure a limited number
of write cycles. Information stored in flash memory usually is written in blocks rather
than one byte or word at a time. Despite this, flash memory is still more preferred
than EEPROM and is rapidly displacing many of the ROM devices as well. 22
There are generally two main types of flash memory—linear flash and advanced
technology attachment (ATA) flash. 23 Linear flash is laid out and addressed linearly,
in blocks, where the same address always maps to the same physical block of memory,
and the chips and modules contain only memory with address decoding and buffer
circuits. This makes linear memory relatively simple and energy-efficient. This type
of memory typically is used for nonvolatile memory that is permanently part of
an embedded system. The ATA flash memory module interfaces with the rest of
the system using the AT Attachment standard in which the memory seems as if it
were sectors on a hard disk. The main advantages of an ATA flash are flexibility
and interchangeability with hard disks, as linear flash modules are not completely
interchangeable between devices that accept removable memory modules.
The third member of the hybrid memory class is NVRAM (nonvolatile RAM). An
NVRAM is basically an SRAM with battery backup, and when power is supplied, the
NVRAM operates just like SRAM. When the power is turned off, the NVRAM draws
just enough power from the battery to retain its data. NVRAM is fairly common in
embedded systems but is even more expensive than SRAM because of the battery. 24
Figure 17.4 is a useful illustration of the different classifications of memory that
typically are used in embedded systems.
Table 17.2 (LaPlante, 2005) is a summary of the memory discussed; however,
different memory types serve different purposes, and each type of memory has its
own strengths and weaknesses.
21 http://www.embedded.com/98/9801spec.htm
22 http://www.netrino.com/Embedded-Systems/How-To/Memory-Types-RAM-ROM-Flas
23 http://www.embedded.com/98/9801spec.htm
24 http://www.netrino.com/Embedded-Systems/How-To/Memory-Types-RAM-ROM-Flas