Recall from the discussion of number systems that each storage unit, called a bit, is capable of holding a 1 or a 0; these bits are grouped together into bytes (8 bits), and these bytes are in turn grouped together into words. Memory is a collection of cells, each with a unique physical address. We use the generic word cell here rather than byte or word, because the number of bits in each addressable location, called the memory’s addressability, varies from one machine to another. Today, most computers are byte addressable.

The ad in the previous section describes a memory of 4 × 2³⁰ bytes. This means that each of the 4 GB is uniquely addressable. Therefore, the addressability of the machine is 8 bits. The cells in memory are numbered consecutively beginning with 0. For example, if the addressability is 8, and there are 256 cells of memory, the cells would be addressed as follows:

What are the contents of address 11111110? The bit pattern stored at that location is 10101010. What does it mean? We can’t answer that question in the abstract. Does location 11111110 contain an instruction? An integer with a sign? A two’s complement value? Part of an image? Without knowing what the contents represent, we cannot determine what it means: It is just a bit pattern. We must apply an interpretation on any bit pattern to determine the information it represents.

When referring to the bits in a byte or word, the bits are numbered from right to left beginning with zero. The bits in address 11111110 are numbered as follows:

The arithmetic/logic unit (ALU) is capable of performing basic arithmetic operations such as adding, subtracting, multiplying, and dividing two numbers. This unit is also capable of performing logical operations such as AND, OR, and NOT. The ALU operates on words, a natural unit of data associated with a particular computer design. Historically, the word length of a computer has been the number of bits processed at once by the ALU. However, the current Intel line of processors has blurred this definition by defining the word length to be 16 bits. The processor can work on words (16 bits), double words (32 bits), and quadwords (64 bits). In the rest of this discussion, we continue to use word in its historical sense.

Most modern ALUs have a small number of special storage units called registers. These registers contain one word and are used to store information that is needed again immediately. For example, in the calculation of

One * (Two + Three)

Two is first added to Three and the result is then multiplied by One. Rather than storing the result of adding Two and Three in memory and then retrieving it to multiply it by One, the result is left in a register and the contents of the register are multiplied by One. Access to registers is much faster than access to memory locations.

All of the computing power in the world wouldn’t be useful if we couldn’t input values into the calculations from outside or report to the outside the results of those calculations. Input and output units are the channels through which the computer communicates with the outside world.

An input unit is a device through which data and programs from the outside world are entered into the computer. The first input units interpreted holes punched on paper tape or cards. Modern-day input devices include the keyboard, the mouse, and the scanning devices used at supermarkets.

An output unit is a device through which results stored in the computer memory are made available to the outside world. The most common output devices are printers and displays.

The control unit is the organizing force in the computer, for it is in charge of the fetch–execute cycle, discussed in the next section. There are two special registers in the control unit. The instruction register (IR) contains the instruction that is being executed, and the program counter (PC) contains the address of the next instruction to be executed. Because the ALU and the control unit work so closely together, they are often thought of as one unit called the central processing unit, or CPU.

FIGURE 5.2 shows a simplified view of the flow of information through the parts of a von Neumann machine. The parts are connected to one another by a collection of wires called a bus, through which data travels in the computer. Each bus carries three kinds of information: address, data, and control. An address is used to select the memory location or device to which data will go or from which it will be taken. Data then flows over the bus between the CPU, memory, and I/O devices. The control information is used to manage the flow of addresses and data. For example, a control signal will typically be used to determine the direction in which the data is flowing, either to or from the CPU. The bus width is the number of bits that it can transfer simultaneously. The wider the bus, the more address or data bits it can move at once.

Because memory accesses are very time consuming relative to the speed of the processor, many architectures provide cache memory. Cache memory is a small amount of fast-access memory into which copies of frequently used data are stored. Before a main memory access is made, the CPU checks whether the data is stored in the cache memory. Pipelining is another technique used to speed up the fetch–execute cycle. This technique splits an instruction into smaller steps that can be overlapped.

In a personal computer, the components in a von Neumann machine reside physically in a printed circuit board called the motherboard. The motherboard also has connections for attaching other devices to the bus, such as a mouse, a keyboard, or additional storage devices. (See the section on secondary storage devices later in this chapter.)

So just what does it mean to say that a machine is an n-bit processor? The variable n usually refers to the number of bits in the CPU general registers: Two n-bit numbers can be added with a single instruction. It also can refer to the width of the address bus, which is the size of the addressable memory—but not always. In addition, n can refer to the width of the data bus—but not always.

The program counter (PC) contains the address of the next instruction to be executed, so the control unit goes to the address in memory specified in the PC, makes a copy of the contents, and places the copy in the instruction register. At this point the IR contains the instruction to be executed. Before going on to the next step in the cycle, the PC must be updated to hold the address of the next instruction to be executed when the current instruction has been completed. Because the instructions are stored contiguously in memory, adding the number of bytes in the current instruction to the program counter should put the address of the next instruction into the PC. Thus the control unit increments the PC. It is possible that the PC may be changed later by the instruction being executed.

In the case of an instruction that must get additional data from memory, the ALU sends an address to the memory bus, and the memory responds by returning the value at that location. In some computers, data retrieved from memory may immediately participate in an arithmetic or logical operation. Other computers simply save the data returned by the memory into a register for processing by a subsequent instruction. At the end of execution, any result from the instruction may be saved either in registers or in memory.

To execute the instruction in the instruction register, the control unit has to determine what instruction it is. It might be an instruction to access data from an input device, to send data to an output device, or to perform some operation on a data value. At this phase, the instruction is decoded into control signals. That is, the logic of the circuitry in the CPU determines which operation is to be executed. This step shows why a computer can execute only instructions that are expressed in its own machine language. The instructions themselves are literally built into the circuits.

The instruction to be executed may potentially require additional memory accesses to complete its task. For example, if the instruction says to add the contents of a memory location to a register, the control unit must get the contents of the memory location.

Once an instruction has been decoded and any operands (data) fetched, the control unit is ready to execute the instruction. Execution involves sending signals to the arithmetic/logic unit to carry out the processing. In the case of adding a number to a register, the operand is sent to the ALU and added to the contents of the register.

When the execution is complete, the cycle begins again. If the last instruction was to add a value to the contents of a register, the next instruction probably says to store the results into a place in memory. However, the next instruction might be a control instruction—that is, an instruction that asks a question about the result of the last instruction and perhaps changes the contents of the program counter.

FIGURE 5.3 summarizes the fetch–execute cycle.

Hardware has changed dramatically in the last half-century, yet the von Neumann machine remains the basis of most computers today. As Alan Perlis, a well-known computer scientist, said in 1981, “Some-times I think the only universal in the computing field is the fetch–execute cycle.”³ This statement is still true today, more than three decades later.

Card readers and card punches were among the first input/output devices. Paper tape readers and punches were the next input/output devices. Although paper tapes, like cards, are permanent, they cannot hold much data. The first truly mass auxiliary storage device was the magnetic tape drive. A magnetic tape drive is like a tape recorder and is most often used to back up (make a copy of) the data on a disk in case the disk is later damaged. Tapes come in several varieties, from small streaming-tape cartridges to large reel-to-reel models.

Tape drives have one serious drawback: To access data in the middle of the tape, all the data before the piece you want must be accessed and discarded. Although modern streaming-tape systems have the capability of skipping over segments of tape, the tape must physically move through the read/write heads. Any physical movement of this type is time consuming. See FIGURE 5.4.

A disk drive is a cross between a compact disc player and a tape recorder. A read/write head (similar to the record/playback head in a tape recorder) travels across a spinning magnetic disk, retrieving or recording data. As on a compact disc, the heads travel directly to the information desired; as on a tape, the information is stored magnetically.

Disks come in several varieties, but all of them consist of a thin disk made out of magnetic material. The surface of each disk is logically organized into tracks and sectors. Tracks are concentric circles around the surface of the disk. Each track is divided into sectors. Each sector holds a block of information as a continuous sequence of bits. See FIGURE 5.5(a). The figure depicts the original layout of data on a disk, in which each track has the same number of sectors, and each sector holds the same number of bits. The blocks of data nearer the center were more densely packed. On modern disks, there are fewer sectors near the middle and more toward the outside. The actual number of tracks per surface and the number of sectors per track vary, but 512 bytes or 1024 bytes is common. (The power of 2 strikes again.) The locations of the tracks and sectors are marked magnetically when a disk is formatted; they are not physically part of the disk.

The read/write head in a disk drive is positioned on an arm that moves from one track to another. See FIGURE 5.5(b). An input/output instruction specifies the track and sector. When the read/write head is over the proper track, it waits until the appropriate sector is beneath the head; it then accesses the block of information in that sector. This process gives rise to four measures of a disk drive’s efficiency: seek time, latency, access time, and transfer rate. Seek time is the time it takes for the read/ write head to get into position over the specified track. Latency is the time it takes for the specified sector to spin to the read/write head. The average latency is one-half the time for a full rotation of the disk. For this reason, latency is also called rotation delay. Access time is the sum of seek time and latency. Transfer rate is the rate at which data is transferred from the disk to memory.

Now let’s look at some of the varieties of disks. One classification of disk is hard versus floppy. These terms refer to the flexibility of the disk itself. The original floppy disk, introduced in the 1970s, was 8” in diameter and even its case was floppy. By the time of the rise in personal computers in the late 1970s, the floppy disk had been reduced in size to 5 1/4” in diameter. Later, generic “floppy” disks were 3 1/2” in diameter, encased in a hard plastic cover, and capable of storing 1.44 MB of data. Floppy disks are no longer used in modern computers, having been replaced by removable storage such as flash drives (see below). Historically, though, they explain the use of the contrasting term hard disks.

Hard disks actually consist of several disks—this sounds strange, so let’s explain. Let’s call the individual disks platters. Hard disks consist of several platters attached to a spindle that rotates. Each platter has its own read/write head. All of the tracks that line up under one another are called a cylinder (see Figure 5.5(b)). An address in a hard drive consists of the cylinder number, the surface number, and the sector. Hard drives rotate at much higher speeds than floppy drives do, and the read/write heads don’t actually touch the surface of the platters but rather float above them. A typical hard disk drive rotates at 7200 revolutions per minute. Laptop hard disks usually spin at 5400 RPM, conserving battery power. The disks in high-performance servers may run at 15,000 RPM, providing lower latency and a higher transfer rate.

The world of compact discs and their drivers looks like acronym soup. The ad we examined used the acronym DVD+/−RW. In addition, we have to decipher CD-DA, CD-RW, and DVD.

Let’s look for a moment at the acronym CD. CD, of course, stands for compact disc. A CD drive uses a laser to read information that is stored optically on a plastic disk. Rather than having concentric tracks, a CD has one track that spirals from the inside out. As on magnetic disks, this track is broken into sectors. A CD has the data evenly packed over the whole disk, so more information is stored in the track on the outer edges and read in a single revolution. To make the transfer rate consistent throughout the disk, the rotation speed varies depending on the position of the laser beam.

The other letters attached to CD refer to various properties of the disk, such as formatting and whether the information on the disk can be changed. CD-DA is the format used in audio recordings; CD-DA stands for compact disc–digital audio. Certain fields in this format are used for timing information. A sector in a CD-DA contains 1/75 of a second of music.

CD-ROM is the same as CD-DA, but the disk is formatted differently. Data is stored in the sectors reserved for timing information in CD-DA. ROM stands for read-only memory. As we said earlier, read-only memory means that the data is permanent and cannot be changed. A sector on a CD-ROM contains 2 KB of data. CD-ROM capacity is in the neighborhood of 600 MB.

CD-R stands for recordable, allowing data to be written after it is manufactured. The contents of a CD-R cannot be changed after data is recorded on it. A CD-RW is rewritable, meaning that it can have data recorded on it multiple times.

The most common format for distributing movies is now a DVD, which stands for digital versatile disk (although the acronym generally stands on its own these days). Because of its large storage capacity, a DVD is well suited to hold multimedia presentations that combine audio and video.

DVDs come in multiple forms: DVD+R, DVD−R, DVD+RW, and DVD-RW, and each of these may be preceded by DL. As we noted in describing the ad, the + and − refer to two competing formats. As with CD, R means recordable and RW means rewritable. DL stands for dual layer, which nearly doubles the capacity of a DVD. DVD−R has a capacity of 4.7 GB, while DL DVD-R can hold 8.5 GB. More recently, Blu-Ray disks with 25 GB capacity and DL 50 GB capacity have been introduced. Writable versions are also available. The name Blu-Ray refers to its use of a blue laser instead of the red laser in CD and DVD drives.

Note that the × used in rating CD and DVD speeds (such as 8× or 10×) indicates the relative speed of access compared with a standard CD or DVD player. When evaluating these devices, be aware that the higher speeds listed represent maximums that are usually attainable only when retrieving data from certain parts of the disk. They are not averages. Therefore, faster may not be better in terms of the added cost.

IBM introduced the flash drive in 1998 as an alternative to floppy disks. FIGURE 5.6 shows a flash drive (or thumb drive), which uses flash memory, a nonvolatile computer memory that can be erased and rewritten. To use a flash drive, it is simply plugged into a USB (universal serial bus) port.

Flash memory is also being used to build solid-state disks (SSDs) that can directly replace a hard disk. Because an SSD is all electronic and has no moving parts, it is faster and consumes less power than a hard disk. Even so, its storage elements can eventually wear out, meaning that it can suffer failures just as a hard disk can.