MadSci Network: Engineering
Query:

Re: HOW DOES THE HARD DRIVE AND C D ROM DRIVE IN A COMPUTER WORK?

Area: Engineering
Posted By: Michael Freed, Aerospace Human Factors, NASA Ames Research Center
Date: Mon Feb 3 16:44:19 1997
Message:

As you say, the CPU is involved in all these activities. Each device (hard-drive, CD-ROM, screen, printer) works differently, but there are some commonalities. The main thing to understand is the role of the CPU in "reading" and "writing."

Reading means taking information out of storage and bringing it into the CPU where something can be done with it. Info can be stored in RAM or any of a number of long-term ("secondary") storage devices. Devices can differ quite a bit in how they store data. Hard disks, for example, store data using billions of tiny iron rectangles (or some other material that has magnetic properties). Each piece can be oriented either on the disks radius (|) or perpendicular to that (-). The disk has a small movable electromagnet called a "read head." When the head passes over a piece of iron, the magnetic field and thus the current passing through the magnet changes in a way that depends on how the iron fragment is oriented.

Information in computers is all in the form of "bits" which can have the value of 1 or 0. A sequence of 8 bits (a byte) is a meaningful-sized piece of information equal to one letter or number. For example, the sequence 1000101 corresponds to the letter "E." The read-head on a hard-drive treats a sequence of oriented iron fragments as bits. Thus the sequence |---|-| would be translated into 1000101 and sent to the CPU to be treated as an E.

Different storage device have different ways of notating bits, and different kinds of read heads to interpret the notation. A CD-ROM uses uses microscopic depressions in the surface of the CD. A laser and light-sensor function as a read head. When the laser hits a depression, the light is not reflected back so the light sensor interprets this as a 0. When there is no depression, the light is reflected back; this gets treated as a 1. A laser is needed rather than a regular light source because bits of information (depression or no depression) are stored very near each other. The laser allows the read mechanism to focus on one at a time.

Writing to these devices involves doing whatever it takes to set up a future read. For hard disks, this means generating a magnetic field strong enough to set the orientation of a particular iron fragment. For CD-ROM, this means creating or erasing a depression on the CD surface.

Sending output to the screen (monitor) is like writing. In the same way that the CPU sends a signal to the hard disk saying that iron fragment #318877 should be set to the | orientation, it can say that screen location #318877 should be set to bright green. It gets a little more complicated with monitors for a few reasons. Whereas the information on a disk can be written once, the monitor requires that each screen location has to be written to over and over (refreshed) with the same information. This typically has to happen about 70 times each second. Specialized circuitry takes care of this (not the CPU). Another difference is that the monitor is not a storage device; it can be written to but not read from.

One thing to know about the CPU is that it doesn't know the difference between one device and another. All it knows is that, for example, it wants to write a 0 to location #2921174 of device 6. It doesn't know whether device 6 is a hard drive, a monitor or something else. The device itself receives the information that a particular location whould be read from or written to and then goes about doing this in its own specialized way.


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