Primary Memory Types
Most of the system’s primary memory is located on the system board.
Primary memory typically exists in two or three forms on the system board:
Read-only memory (ROM) contains the computer’s permanent startup programs. ROM devices store information in a permanent fashion and are used to hold programs and data that do not change.
Random access memory (RAM)is quick enough to operate directly with the microprocessor and can be read from, and written to, as often as desired. RAM devices retain the information stored in them only as long as electrical power is applied to the IC. Any interruption of power causes the memory contents to vanish.
Cache memory is a fast RAM system designed to hold data recently accessed from the disk drive that the microprocessor may need again.
ROM (read-only) memory generally holds data that was programmed into it at the factory, and is not intended to be changed. There are several types of ROM, some of which can be erased and reprogrammed (but not during the normal operation of the computer).
Types of ROM
– Mask-Programmed ROM (MROM) — programmed at the factory.
– Programmable ROM (PROM) — can be custom-programmed by the user (once) using special circuitry.
– Erasable-Programmable ROM (EPROM) — can also be programmed and erased by the user using ultraviolet light and special circuitry external to the computer.
– Electrically Erasable PROM (EEPROM) — can be erased and reprogrammed by special circuitry within the computer.
The one thing all forms of ROM have in common is that they are all non-volatile. This means that the data contained in the memory is not lost when the computer is turned off or when electrical power is lost. This enables the computer to begin reading instructions and data from this type of memory as soon as it is powered up.
The term “Read-Only” truly applies to MROM and PROM memories, which are written once and then cannot be erased or rewritten. The other ROM classes are more appropriately referred to as Read Mostly Memories, where the ratio of Read operations to Write operations is very high. The generic term “Read-Only” is used with all non-volatile, semiconductor memories that cannot be written to during the normal operation of the computer.
Every system board contains one or two ROM ICs that hold the system’s Basic Input/Output System, or BIOS program. The BIOS program contains the basic instructions for communications between the microprocessor and the various input and output devices in the system.
The information in the BIOS represents all the intelligence that the computer has until it can load more information from another source, such as a hard or floppy disk. The situation in which programs (software) are stored in ROM chips (hardware) on a permanent basis is referred to as firmware. ROM chips can be located anywhere on the system board, but they are usually easy to recognize due to their relatively large size and immediate proximity to one another.
Advancements in EEPROM technology have produced Flash ROM devices that enable new BIOS information to be written (downloaded) into the ROM to update it. The download can come from an update disk or another computer.
Flash ROM Information
Unlike RAM ICs, the contents of the Flash ROM remain after the power has been removed from the chip.
NAND Flash vs NOR Flash
Random Access Memory
The other type of high-speed semiconductor memory used with computers and peripheral devices is IC random access memory or RAM. The term “random access” means that any address location in the memory can be accessed as quickly as any other location.
Because there are other types of RAM memory, IC devices used for primary memory are more appropriately referred to as Read/Write (R/W) memories. In the case of primary memory, the generic term “RAM” always refers to semiconductor R/W memory.
Working with Microprocessors
Semiconductor RAM memories are fast enough to work directly with the microprocessor without slowing it down. The computer uses the RAM portion of primary memory to hold programs and data currently being executed by the microprocessor.
During the execution of a program, the contents of many RAM address locations are changed as the microprocessor updates the program, by storing intermediate or final results of operations performed.
Types of RAM
Like semiconductor ROM, semiconductor RAM has more than one type. As a matter of fact, it has two general categories: Static RAM (SRAM) Dynamic RAM (DRAM) Although they both perform the same function, the methods they use are completely different
Static RAM (SRAM) stores binary bits in such a manner that the bits remain in RAM as long as power to the chip is not interrupted
Dynamic RAM (DRAM), on the other hand, requires that stored data be refreshed, or rewritten, periodically to keep it from fading away. As a matter of fact, each bit in the DRAM must be refreshed at least once every 2 milliseconds or the data dissipates
DRAM vs. SRAM
Although the extra circuitry and inconvenience associated with refreshing may initially make DRAM memory seem the obvious second choice behind SRAM, this is not the case.
The Economical Choice
Due to the simplicity of DRAM’s internal structure, the bit storage capacity of a DRAM chip is much greater than that of a similar SRAM chip. The DRAM chip also offers a much lower rate of power consumption. Both of these factors contribute to making DRAM memory the economical choice in certain RAM memory systems.
Using SRAM or DRAM
Generally, SRAM is used in smaller memory systems where the added cost of refresh circuitry would greatly add to the cost per bit of storage. DRAM is used in larger memory systems where the extra cost of refresh circuitry is distributed over a greater number of bits and is offset by the reduced operating cost associated with DRAM chips.
Both SRAM and DRAM have the disadvantage of being volatile. This means that any data stored in RAM is lost if power to the computer is disrupted for any reason. On the other hand, both types of RAM have the advantage of being fast: They can be written into and read from with equal ease.
In earlier PC designs — XT and AT — the system’s RAM memory was comprised of banks of discrete RAM ICs in Dual In-line Pin (DIP) sockets.
Intermediate clone designs placed groups of RAM ICs on small 30-pin daughterboards that plugged into the system board vertically. This mounting method required less horizontal board space. These RAM modules had pins along one side of the board and were referred to as single in-line pin (SIP) modules.
SIMMs and DIMMs
Further refinements of the RAM module produced snap-in, single in-line memory modules (SIMMs), and dual in-line memory modules (DIMMs). SIMM and DIMM units mount vertically on the system board. However, rather than using a pin and socket arrangement, both use special snap-in sockets that support the module firmly.
SIMM and DIMM Qualities
SIMMs and DIMMs are also keyed, so that they cannot be plugged in backwards.
SIMMs are available in 30- and 72-pin versions; DIMMs are larger 168-pin boards.
PCs are usually sold with less than their full RAM capacity. This enables a user to purchase a less expensive computer to fit the user’s individual needs, and yet retain the option to install more RAM if future applications call for it.