0195136047.pdf

312 DIGITAL BUILDING BLOCKS AND COMPUTER SYSTEMS

READ-AND-WRITEMEMORY(RAM)

Writing is the same as storing data into memory and reading is the same as retrieving the data later.

RAM is said to bevolatilebecause its contents are retained only as long as power is present. A

RAM device is a collection of 2naddressable storage locations, each of which containskbits. Its

block diagram is shown in Figure 6.2.16, in which each cell may be a flip-flop or a capacitor, and

naddress lines are decoded to selectkcells. AstaticRAM, in which each cell is a flip-flop, is the

read-and-write memory that retains its data so long as the power is applied, without any further

action needed from the computer. Static RAM is used in microprocessor-based systems requiring

small memory. Common static-RAM sizes are 2K×8, 8K×8, and 32K×8, where K stands for

210 =1024. AdynamicRAM, in which each cell is a capacitor (which leaks charges and therefore

requires continuous refreshing from the computer to maintain its value), is the read-and-write

memory that is used in large memory systems due to its lower cost and greater density. Common

dynamic-RAM packages are available in 16K-bit, 64K-bit, 256K-bit, and 1M-bit (where M stands

for mega= 220 ) sizes.

READ-ONLYMEMORY(ROM)

ROM isnonvolatile(because it maintains its contents even when its power is shut off) and is

used to store data and programs that do not change during the operation of the system. The

mask-programmedROMs are read-only devices that are programmed for data storage during the

manufacturing of the chip itself. These are generally less expensive devices for mass production.

Character-font memory for laser printers is a good example.Programmableread-only memory

(PROM) is a field-programmable memory that is fabricated by the manufacturer containing all 0s

and is programmed irreversibly by the user by electrically changing appropriate 0s to 1s. PROMs

are quite economical in small quantities.Erasableprogrammable read-only memory (EPROM)

is nonvolatile and widely used in microprocessor systems for program storage. It can be erased by

shining an ultraviolet light and reprogrammed if necessary. These are produced in low to moderate

volumes.

ELECTRICALLYERASABLEPROGRAMMABLEROM (EEPROM)

EEPROM is used for remote-area applications. The device is provided with special pins which,

when activated electrically, alter the rewriting of selected memory locations.

MAGNETICSTORAGEDEVICES

Magnetic disk memoryis nonvolatile and provides large storage capabilities with moderate access

times. The data are stored on one or more rigid aluminum circular disks coated with iron oxide. The

Enable

Read Write

k data lines

2 n × k matrix

of memory

n address lines cells

Figure 6.2.16Block diagram of 2n×k-bit RAM device.