Slides from First Lecture of CMPSCI 105

Spreadsheets constitute a major portion of the material covered; it is critical that you know how to use them. I spoke to a person while on my summer vacation in 1996 who told me that to get the most basic scut-work job these days a person can't get by just knowing how to use a word processor; they must also know how to use a spreadsheet. No spreadsheets, no job.

I may be able to do live, on-line demonstrations of some of these tools in the classroom.

Do NOT get a VMS account. If you already have a VMS account, you must also get a UNIX account (it won't cost you anything extra).

Similarly, you may bring older disks to the lab as long as you don't mind erasing them. The 3.5" double density diskettes will also work in our computers, but I don't recommend that you use that type.

The lab now has mouse pads, but you might still want to get one for your own use. Also, a wrist rest is a nice item to have if you do a lot of typing. Both items are available at the University Store.

A computer is a tool. A hammer is a tool. I can kill a man with a hammer as easily as I can drive a nail: does this mean the hammer is evil? Evil only applies to the hand that wields it. The same goes for computers. They have great power for error, and misuse, but they also have great power for good as well.

Are computers perfect? Absolutely not. Computers are machines. Machines fail. Sometimes it is the hardware; more often it is the software. On that horrible day that we lost the Challenger, there was a brief time between the explosion and the explanation where no one knew what had caused the problem. During that time, many technical people, all around the world, were being very introspective, hoping that it was not their specialties which contributed to the loss of a billion dollar machine and seven lives. One of my colleagues put it best for computer science, when he said: "I hope it wasn't divide-by-zero. That is a lousy way to die." His statement was an acknowledgement that computer software has a long way to go, and that something as simple as attempting to divide a number by zero could have triggered a chain of events ultimately leading to the loss of the vehicle. As it happened, it was not a computer problem, but it could have as easily been one.

Computing has similar structures: there are things you know that apply directly (typing on a standard keyboard, for example), things you think you know but have a different meaning in the world of computers, things that on the surface look strange until I (or someone) explains that they really are familiar but have a slight twist, and things that look unfamiliar which you must struggle with to learn. I will point these things out as the semester progresses.

An analog computer computes by analogy-- the mechanism for computing is an analog of a physical process. Typically, analog computers model natural phenomina very well, but not very precisely.

A digital computer is one in which all computations are performed upon numbers stored as individual digits (usually in binary).

Another analog device is the odometer in most automobiles. It calculates the distance travelled by analogy to the rotations of the wheels. Since odometers are often groups of parallel wheels, the rightmost digit wheel may be between two values. This is a sure sign that it is an analog device.

For other analog calculating machines, I refer you to an ancient astronomical analog computer called the Antikythera machine (First Second Third) or, of course, everybody's favorite pocket calculator: Stonehenge (First Second Third).

The little plastic counters that some people take to the supermarket are also digital. To enter $1.49, the shopper presses the hundreds button once, the tens button four times, and the ones button nine times. The number is added to what was already present, and any carries to higher digits are handled automatically. These are direct descendants of a machine built by philosopher/mathematician Blaise Pascal in the 1600's (a modern recreation of which is on display at the Deutches Museum in Munich).

In both cases, numbers have fixed, discrete values. No possibility of intermediate values are allowed.

A "BIT" (short for binary digit) is the smallest unit of information possible. It can contain the answer to a single yes-or-no question. It has two possible values only: 0 and 1. You can encode 1 as "Yes" and 0 as "No", but you may also encode any two-value system as 1 or 0: Yes/No, True/False, Up/Down, On/Off, Male/Female, Open/Closed, Black/White, etc.

One bit by itself isn't very useful, so we group a bunch of them together and treat the whole packet as a single object. Eight bits in a packet is the most common form, and is called a "BYTE". With eight bits, you have 2 to the 8th power, or 256, different patterns possible (00000000, 00000001, 00000010, 00000011, ... , 11111110, 11111111). This is enough distinct patterns that we can assign one to each letter of the alphabet (both upper case and lower case), the numbers, and a bunch of special characters, with lots of patterns left over. The most common assignment of patterns to characters is called "ASCII" (American Standard Code for Information Interchange). For this reason, one byte can be said to be the same as one character.

In the same way, "mega" means million, but 1024x1024 (2 to the 20th power), not 1000x1000. One megabyte is 1,048,576 bytes, not 1,000,000.

RAM is generally volatile, which means that it will lose its values when the power is turned off. This may not be true in a few years, but it is now (I've been saying this for years already).

RAM comes in units of kilobytes, or more recently megabytes. It is not unusual for a computer today to have 4, 8, 16, 32, or 64 megabytes of RAM.

So what good is it? ROM holds information placed there by the manufacturer so that the computer knows how to start itself up. Three things are stored in ROM: the POST (Power On Self Test, which checks the health of the hardware), the BIOS (Basic Input Output System, which contains code for all programs to be able to get characters from the keyboard, display them on screen, save them onto the disk, etc.), and the Bootstrap Loader (so that the computer can find the MS-DOS program when it starts up).

As another example of where ROM is used, consider those handheld language translators, that allow you to type in a word in one language and see it in one of several other languages. Do I, as a user of this device, have any reason to change the translator's dictionaries? Generally, no. All of the words in the dictionaries, and the links between them, are stored in ROM. Any time I power up the translator, those words are available for use.

Floppy disks are removable disks that you can use to archive files, or move files between two computers. The 3.5" HD diskette can hold 1.44 megabytes of information, or just under a million and a half bytes. Tiny, by today's standards.

Hard disks are typically not removable (they stay fixed inside the computer), but they can hold many millions, or even billions, of bytes of information. A common disk size today is 1.6 gigabytes (1.6x1024x1024x1024 bytes).

I went on a trip with a friend one time who purchased one of these receivers. He had entered the position of several expected way-points along our trip, and told the device to record position every thirty seconds for the duration of the drive. As we drove along, we could see our track, and as way-points approached they would appear on the GPS receiver's small screen. It became a wonderful tool when we had to detour off of the main road and use unfamiliar roads: we could tell by the way points when we were near to getting back onto the right track. Once at our destination, I connected his GPS receiver to my PC laptop, and had a complete record of where we went, and what time we reached each point.