第5章アルゴリズムのハードウェアによる実現：コンピューターシステム設計

By the end of this section, you will be able to:

• Define a computer system

• Explain how information is stored and transferred in a computer system

• Differentiate between high-level and machine-level programs

• Identify the elements of a typical computer system

At its core, a computer system is an electronic device that does computations. These computations appear to the outside world as executing programs. When you play a game, listen to a song, or browse the web, you are instructing your computer to do computations. You may wonder how does the computer function and how do the computations performed such as browsing the web or listening to a song relate to one another?

Let us start with the second part of that question: the relationship between computation and executing a program. A song that has been digitized for storage is actually a bunch of numbers that the computer reads; it produces sound based on those numbers. The computer must calculate (i.e., compute) the frequency and volume of the sound based on the numbers read. Another example is when you open a browser and type the address of a website. The computer reads what you typed on the keyboard and leverages network capabilities to translate it to a long number called an Internet Protocol (IP) address (Figure 5.2). The IP address consists of several digits, similar to your phone number, that allow the data to reach your computer over the Internet and for other computers over the Internet to recognize your computer. Your computer passes that IP address over the Internet to another big computer asking for the content of the required website, receives the content from that big computer, and executes browser software that translates it to content such as pictures, sounds, and animations on the screen. All these steps involve computations. But how does the computer do all this? To answer this question, we first need to know the components of a computer system.

A computer system consists of two major parts: hardware and software. The hardware includes the physical components of the computer system, such as hard drives, motherboards, and processor chips. The software consists of the programs you execute, such as media players or web browsers, and the data needed for these programs to function. For example, the media player is a program, and the songs that you listen to using the media player are data. So, the hardware executes programs that are fed with data. Computers come in different shapes, from the small ones in your smartwatch, for example, to big supercomputers. Different computers may have different components besides the components that exist in all computer shapes. Bigger computers, datacenters, and supercomputers need extra components for getting data from multiple sources, more sophisticated parts to connect to the Internet, and cooling equipment. Mainframes that were created a few decades ago are big computers used for a different purpose. You don’t need those in your smartphone.

All programs and their corresponding data are stored inside a computer or downloaded from the Internet. So how are these programs and data stored? And how does a computer understand and execute them? Computers do not understand English or other natural languages; they can only relate to strings of 1s and 0s and therefore all the programs and data must be stored inside the computer as a sequence of 1s and 0s. A 1 or a 0 is called a binary digit or bit. Every 8 bits unit is called a byte. This is pretty much how computers that depend on electricity work. Other types, still not in the mainstream market but in design and testing stages, such as quantum computers, do not use bits but use something else called quantum bits or qubits.

If we say that a text editor program takes 1 megabyte of storage, it means that the editor is stored inside the computer, more specifically in the disk of your computer, which is a storage mechanism for data, as a sequence of almost 1 million bytes (1 megabyte is a bit larger than 1 million bytes). We call a program stored inside a computer an executable. Since programs and data are stored as a sequence of bits or bytes, what is the difference between a program and the data?