Random-access memory

Chapter Random-access memory

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • Zip disks can hold about 100 MB of data; newer ones can hold 250 MB. Jaz disks holdabout 1 GB. There are several variants on the Zip/Jaz theme, produced by various man-ufacturers. All these disk types require special drives. Some new computers includebuilt-in Zip drives.A popular mass-storage medium is compact-disk, read-only memory (CD-ROM).You can buy CD-ROMs for various applications. They are commonly used for commer-cial software and also to store reference materials such as dictionaries and telephone di-rectories. The main asset of CD-ROM is its fairly large capacity and its long shelf life.The main drawback is that the medium cannot be erased and overwritten, unless youare willing to spend the money for a compact-disk, recordable (CD-R) drive.Tape mediaThe earliest computers used magnetic tape to store data. This is still done in somesystems. You can get a tape drive for making an emergency backup of the data onyour hard drive, or for archiving data you rarely need to use.Magnetic tape has very high storage capacity. There are microcassettes that canhold more than 1 GB of data; standard cassettes can hold many gigabytes. But tapes areextremely slow because, unlike their disk-shaped counterparts, they are a serial-ac-cess storage medium. This means that the data bits are written in a string, one after an-other, along the entire length of the tape. The drive might have to mechanically rewindor fast-forward through a football field’s length of tape to get to a particular data bit,whereas on a disk medium, the read/write head never has to travel further than the di-ameter of the disk to reach a given data bit.Random-access memoryIn a computer, the term random-access memory (RAM) refers to integrated cir-cuits (ICs) that store working data. The amount, and speed, of memory is a crucialfactor in determining what a computer can and cannot do.Data flowFigure 33-3 shows how data moves between a hard drive or diskette and the mem-ory, controlled by the CPU. When you open a file on your hard drive or on a diskette,the data goes immediately into the memory. The CPU, under direction of the micro-processor, manipulates the data in the memory as you work on the file. Thus, thedata in memory changes from moment to moment.When you hit a key to add a character, or drag the mouse to draw a line that showsup on your display, that character or line goes into memory at the same time. If you hitthe backspace key to delete a character, or drag the mouse to erase a line on the screen,it disappears from the memory. During this time the original file on the disk stays as itwas before you accessed it. No change is made to the disk data until you specifically in-struct the computer to overwrite the data on the disk.When you’re done working on a file, you tell the microprocessor to close it. Thenthe data leaves memory and goes back to the hard drive or diskette from which it came,or to some other place, as you might direct. If you tell the computer to overwrite the fileon the disk from which it came, many programs send the new data (containing theRandom-access memory629