What a Computer Does
The instructions in the program direct the computer to input, process and output as follows:
Input/Output The computer can selectively retrieve data into its main memory (RAM) from any peripheral device (terminal, disk, tape, etc.) connected to it. After processing the data internally, the computer can send a copy of the results from its memory out to any peripheral device. The more memory it has, the more programs and data it can work with at the same time.
Storage By outputting data onto a magnetic disk or tape, the computer is able to store data permanently and retrieve it when required. A system’s size is based on how much disk storage it has. The more disk, the more data are immediately available.
Processing (The 3 C’s)
The computer performs all processing by “calculating,” “comparing” and “copying” the data stored in its memory (RAM).
Calculate The computer can perform any mathematical operation on data by adding, subtracting, multiplying and dividing one set with another.
Compare The computer can analyze and evaluate data by matching it with sets of known data that are included in the program or called in from storage.
Copy The computer can move data around to create any kind of report or listing in any order.
Calculate, Compare and Copy By calculating, comparing and copying, the computer accomplishes all forms of data processing. For example, records are sorted into a new order by comparing two records at a time and copying the record with the lower value in front of the one with the higher value. The computer finds one customer out of thousands by comparing the requested account number to each record in the file. The query statement: SUM SALARY FOR TITLE = “NURSE” causes the computer to compare the title field in each record for NURSE and then add (calculate) the salary field for each match. In word processing, inserting and deleting text is accomplished by copying characters from one place to another.
Remember The 3 C’s If you wonder whether the computer can solve a problem, identify your data on paper. If it can be calculated, compared and copied on paper, it can be processed in the computer.
The Stored Program Concept
The computer’s ability to call in instructions and follow them is known as the “stored program concept.” Instructions are copied into memory from a disk, tape or other source before any data can be processed. The computer is directed to start with the first instruction in the program. It copies the instruction from memory into its control unit circuit and matches it against its built-in set of instructions. If the instruction is valid, the processor carries it out. If not, the computer comes to an abnormal end. The computer executes instructions sequentially until it finds a GOTO instruction that tells it to go to a different place in the program. It can execute billions of instructions per second, using the same program logic on each new set of data brought in.
Operations Overlap Input/output and processing are made to overlap. While one program is waiting for input from one user, the operating system (master control program) directs the computer to process data in another program. Large computers allow many input/output operations to occur simultaneously with processing. It can take hundreds of thousands of discrete machine steps to perform very routine tasks. Your computer could easily execute several million instructions to put a requested record on screen for you.
First-generation computers, starting with the UNIVAC I in 1951, used vacuum tubes, and their memories were made of thin tubes of liquid mercury and magnetic drums. Second-generation systems in the late 1950s replaced tubes with transistors and used magnetic cores for memories (IBM 1401, Honeywell 800). Size was reduced and reliability was significantly improved. Third-generation computers, beginning in the mid-1960s, used the first integrated circuits (IBM 360, CDC 6400) and the first operating systems and DBMSs. Online systems were widely developed, although most processing was still batch oriented using punch cards and magnetic tapes. Starting in the mid-1970s, the fourth generation brought us computers made entirely of chips. It spawned the microprocessor and personal computer. It introduced distributed processing and office automation. Query languages, report writers and spreadsheets put large numbers of people in touch with the computer for the first time. Even with the hundreds of millions of people using computers every day, we are still in the fourth generation. Some skill is still required to use the computer even if only to surf the Web and send e-mail. The fifth generation implies faster hardware and more sophisticated software that uses artificial intelligence (AI) routinely. Natural language recognition is a major component of the fifth generation. When you can have a reasonably intelligent conversation with the average computer, you will be in the fifth generation, perhaps in the 2015-2020 time frame.