Storage and transmission

Computer systems consist of four basic parts: Input devices, output devices, the central processing unit, and secondary storage.

Input devices send data to the computer from the outside world. Common ones are the keyboard, the mouse, and the touchpad. Some systems have microphones. Other input devices, such as sensors, may be used on special purpose computers.

Output devices communicate results from the computer to the user. Most computers have a monitor, speakers, and a printer, for instance. Some systems may have controllers that operate motors or other mechanical devices.

The central processing unit (CPU) is the heart of the computer system. Physically small, the CPU is inside the case that contains the CD drives. On laptops, it is behind the keyboard. All actual computing takes place in the CPU. The CPU has several parts. Random access memory (RAM) temporarily stores data. Read only memory (ROM) holds data that is needed permanently by the system. Registers and the cache are high speed memory. Computations take place in the arithmetic logic unit (ALU). Information is moved around on groups of wires called busses.

Secondary storage devices, such as disk drives, CD and DVD drives, and thumb drives, hold data permanently. Some, such as thumb drives, are portable and can be used to move data between computers. Secondary storage memory is slower for the computer to access than is RAM, but it is permanent, and it can hold far more data than RAM.

The various parts of a computer system are not measured by their physical size, but by how much data they can hold or by the speed with which they operate. Here are some standard abbreviations.

Though the standard SI ("Système International", i.e. metric) prefixes are formally defined to indicate powers of ten, we often find it convenient in the computer world to use them to refer to a nearby power of two instead, so for instance a kilobyte (1 kB) usually refers to 1024 bytes. That said, manufacturers will often use whichever meaning will make their product sound more impressive!

Storage capacity is measured in bytes. A byte is exactly 8 bits. (This has been true by definition for decades, although in the early years there were some systems with differently-sized bytes.) Early microcomputers measured their memory in kilobytes (kB). For example, the first Apple II (1977) had 4KB of RAM. The first IBM PC (1981) had 64KB. The Atari ST (1985) was the first popular microcomputer to advertise its memory in megabytes (MB). It came with 1024 KB, or 1MB, of RAM. Today, even the smallest and cheapest computers come with memory measured in gigabytes (GB).

CPU speed is measured in Hertz (Hz). One Hertz is one clock cycle, which is the time the computer takes to do one basic machine operation. Computers work extremely quickly, doing millions, or even billions, of basic operations per second. The first Apple II had a 1 MHz CPU, which meant it could do around 1 million elementary operations per second. That is glacially slow by today's standards. If your CPU is still timed in MHz, you'd better replace it! Modern machines are rated at 1.6 GHz and up.

Secondary storage devices have changed radically over the years, both in type and in storage capacity. Early PCs made use of spinning magnetic media—"disks" or "diskettes"—that measured their capacity in kilobytes. "Zip" disks, mostly with a capacity of 100 MB, were popular for a while.

Few computers today come with diskette drives. Instead, they use CDs, which can hold 682 MB, and DVDs, which can store 4.7 GB. Dual layer DVDs can hold 8.5 GB. thumb drives, also called flash drives or jump drives, can hold from a few MB to hundreds of GB, and their capacities are steadily increasing.

Hard drives, which are internal, sealed disk drives, hold far more information than diskettes or CDs. Their capacity used to be measure in megabytes. Early PC hard drives, in the mid-late 1980s, held 20 MB of data. Today they are measured in gigabytes, with even moderately priced systems having hard drives in the terabyte range.

What does this mean in practice? Suppose you are writing a term paper, for instance. How large will it get? Would we have to worry about whether it would fit in storage?

We'll have to make some assumptions to answer this question. A standard 8 ½ by 11 inch piece of paper consists of 66 lines, each of which is 80 characters long. There will be margins on all sides, of course, and you'll probably double space your paper. Let's assume

• 33 lines per page (top and bottom margins, double space)
• 50 characters per line (left and right margins)
• 1 byte per character (in English, virtually everything would be ASCII)

Under these assumptions, there will be

Suppose the term paper is 15 pages long. Then

A typical cheap thumb drive holds 4 GB, or about 4,000,000,000 bytes. We can obviously fit our paper, which has only 24,750 bytes, onto such a thumb drive. How many such papers could we store on the drive? Notice that if we just naïvely write out our various sizes and numbers, the units don't line up as conveniently as before: