Two types of disk exit. Floppy disks are thin like paper and can be slipped into and out of the computer； hard disks are fixed and are sealed inside a case. Disks are generally accessed by movable arms that， like a phonograph arm with a needle can go directly to a specific location without having to go through everything between the needle’s starting point and the desired location. This capability， called direct access， （historically called random access）， gives disks a considerable speed advantage over magnetic tape， which can take minutes to rewind or to search for a particular bit of information.
The recording surface of a disk has concentric circles called tracks， which are similar to the grooves in a record. Information is stored on a track in magnetized spots called bits. These bits are similar to the bits in internal memory and are situated on the track so that usually every eight of them make up one byte.
A stack of tracks is called a cylinder and it is accessed by all the recording heads acting at once. The recording capacity of a disk pack is measured in terms of a number of cylinders， the amount of data in each track.
Information on a disk is organized in terms of blocks， each having its own address， which consists of a cylinder number， a track number， and a record number. To access directly the necessary information， the recording heads first seek the required cylinder and the start point of the record， and then transfer the information to the memory of the computer or to another form of storage， all of which is done in a few milliseconds.
Scientists at IBM Corp. have developed a 3.5 inch magnetic disk that is capable of storing 50 times more information than those used in computers today. With a capacity of 10 billion bits， or room to store 620，000 double spaced pages of text， the magnetic disk could challenge the capacity of optical disks.
In ordinary non-flexible magnetic disks， information is encoded in 12 micron-wide magnetic regions， or cells， arranged concentrically in tracks on the disk’s surface （a micron equals 10-6 meter）. IBM researchers reduced this cell size to 0.5 micron and increased disk capacity by first coating the disk with a thin film of cobalt and then defining the tiny cells using an electron beam and advanced etching techniques.
【New words】
magnetic
磁力的，有磁性的
oxide
氧化物
retrieval
取回，恢复，修补
phonograph
留声机，电唱机
etching
蚀刻版画
，蚀刻术
milliseconds
毫
秒
Exercise
1. Multiple Choices
（1）Early computer solved _____ problems.
A. control
B. business applications
C. engineering
D. mathematical
（2）Storage hierarchy consists of which of the following three levels？
A. offline storage， random-access storage， online storage
B. online storage， near-online storage， offline storage
C. online storage， neat-online storage， optical storage
D. online storage， offline storage， read/write storage
（3）Cache is _____.
A. slow
B. fast
C. relatively small
D. high cost
（4）We can say a bus is simply _____.
A. a wire
B. a group of wires
C. a 8-bit bus
D. a 16-bit bus
（5）According to the physical size of computers we can classify the computers into _____.
A. microcomputer
B. minicomputer
C. mainframe
D. supercomputer
（6）Which of the following enable you to turn any floppy disk into a read-only disk？
A. sector
B. write-protect tab
C. head actuator
D. read/write head
（7）The basic hardware components of any computer include ______.
A. CPU
B. main memory
C. input devices
