Unit 7 Memory Devices

1）             RANDOM-ACCESS MEMORY

Random-access memory，or RAM，is the kind of memory we usually refer to when wespeak of computer memory．It is the most widely used type，and consists of rows of chips with locationsestablished in tables maintained by the control unit．

As the name suggests，itemsstored in RAM can be gotten（accessed）botheasily and in any order（randomly）ratherthan in some sequence．RAM relies on electric current for all itsoperations；moreover，if thepower is turned off or interrupted，RAMquickly empties itself of all your hard work．Thus，we say RAM is-volatile，ornonpermanent．

2）             READ-ONLY MEMORY

Read-only memory，or ROM，typically holds programs．Theseprograms are manufactured，or “hard-wired” in place on the ROM chips．For example，amicrocomputer has a built-in ROM chip（sometimescalled ROM BIOS，for ROM basic input/output system）that stores critical programs such as the one thatstarts up，or “boots”，thecomputer．ROM is “slower” than RAM memory，and as a result，items inROM are transferred to RAM when needed for fast processing．

Items held in ROM can be read，but they cannot be changed or erased by normal inputmethods．New items cannot be written into ROM．The only way to change items in most forms of ROM isto change the actual circuits．

3）             MAGNETIC DISKS

The magnetic disk is a circular platter with a smoothsurface and a coating that can be magnetized．Data isstored on it as magnetized spots．Thereading and recording device，the disk drive，spins the disk past read/write heads that detect orwrite the magnetized spots on the disk．

4）             CD-ROMS

Optical disks need thin beams of concentrated light tostore and read data．It is a form of laser storage，called CD-ROM．

There are two types of optical disks that can heuser-recorded：WORM and erasable optical．WORM stands for“write once，read many”：Data canbe written to this disk just one time，but thedata can be read many times．Erasable optical disks can be written to，read，and erased．

5）             MAGNETIC TAPE

A magnetic tape is a narrowplastic strip similar to the tape used in tape recorders．The tapes are read by tape drive moving the tape past a read／write head，which detects or writesmagnetized spots on the iron-oxide coating of the tape．Each pattern of spots matches the byte code for character being stored．