How The Computer Works: The CPU and Memory (2024)

How Computers Work: The CPU and Memory

Figure 0 shows the parts of a computer:

The Central Processing Unit: (CPU), Buses, Ports and controllers, ROM; Main Memory (RAM); Input Devices; Output Devices; Secondary Storage; floppy disks, hard disk, CD-ROM

Figure 0: Inside The Computer

This part of the reading will examine the CPU, Buses, Controllers,and Main Memory. Other sections will examine input devices, outputdevices, and secondary memory.

The Central Processing Unit (CPU)

Figure 1: The Central Processing Unit

The computer does its primary work in a part of the machine we cannotsee, a control center that converts data input to information output. Thiscontrol center, called the central processing unit (CPU), is a highly complex, extensive set of electronic circuitry that executes stored programinstructions. All computers, large and small, must have a central processing unit. As Figure 1 shows, the central processing unit consists of twoparts: The control unit and the arithmetic/logic unit. Each part has a specific function.

• Equal-to condition. In a test for this condition, the arithmetic/logic unitcompares two values to determine if they are equal. For example: If thenumber of tickets sold equals the number of seats in the auditorium,then the concert is declared sold out.
  
• Less-than condition. To test for this condition, the computer comparesvalues to determine if one is less than another.For example: If the number of speeding tickets on a driver's record is less than three, then insurance rates are $425; otherwise, the rates are $500.
  
• Greater-than condition. In this type of comparison, the computer determines if one value is greater than another. For example: If the hours aperson worked this week are greater than 40, then multiply every extrahour by 1.5 times the usual hourly wage to compute overtime pay.

A computer can simultaneously test for more than one condition. Infact, a logic unit can usually discern six logical relationships: equal to, lessthan, greater than, less than or equal to, greater than or equal to, and notequal.

The symbols that let you define the type of comparison you want the computer to perform are called relational operators. The most commonrelational operators are the equal sign(=), the less-than symbol(<), and the greater-than symbol(>).

• Registers: Temporary Storage Areas

Registers are temporary storage areas for instructions or data. They arenot a part of memory; rather they are special additional storage locationsthat offer the advantage of speed. Registers work under the direction of the control unit to accept, hold, and transfer instructions or data and performarithmetic or logical comparisons at high speed. The control unit uses adata storage register the way a store owner uses a cash register-as a temporary,convenient place to store what is used in transactions.

Computers usually assign special roles to certain registers, including theseregisters:

• An accumulator, which collects the result of computations.
• An address register, which keeps track of where a given instruction orpiece of data is stored in memory. Each storage location in memory isidentified by an address, just as each house on a street has an address.
• A storage register, which temporarily holds data taken from or about tobe sent to memory.
• A general-purpose register, which is used for several functions.

Memory and Storage

Memory is also known as primary storage, primary memory, main storage,internal storage, main memory, and RAM (Random Access Memory); all these terms are used interchangeably by people in computer circles. Memory is the part of the computer thatholds data and instructions for processing. Although closely associatedwith the central processing unit, memory is separate from it. Memory storesprogram instructions or data for only as long as the program they pertain tois in operation. Keeping these items in memory when the program is notrunning is not feasible for three reasons:

• Most types of memory only store items while the computer is turnedon; data is destroyed when the machine is turned off.
• If more than one program is running at once (often the case on largecomputers and sometimes on small computers), a single program can not lay exclusive claim to memory.
• There may not be room in memory to hold the processed data.

How do data and instructions get from an input device into memory?The control unit sends them. Likewise, when the time is right, the controlunit sends these items from memory to the arithmetic/logic unit, where anarithmetic operation or logical operation is performed. After beingprocessed, the information is sent to memory, where it is hold until it isready to he released to an output unit.

The chief characteristic of memory is that it allows very fast access toinstructions and data, no matter where the items are within it. We will discuss the physical components of memory-memory chips-later in thischapter.

To see how registers, memory, and second storage all work together, let ususe the analogy of making a salad. In our kitchen we have:

• a refrigerator where we store our vegetables for the salad;
• a counter where we place all of our veggies before putting them on the cutting board for chopping;
• a cutting board on the counter where we chop the vegetables;
• a recipe that details what veggies to chop;
• the corners of the cutting board are kept free for partially chopped piles ofveggies that we intend to chop more or to mix with other partially choppedveggies.
• a bowl on the counter where we mix and store the salad;
• space in the refrigerator to put the mixed salad after it is made.

The process of making the salad is then: bring the veggies from thefridge to the counter top; place some veggies on the chopping board accordingto the recipe; chop the veggies, possibly storing some partially choppedveggies temporarily on the corners of the cutting board; place allthe veggies in the bowl to either put back in the fridge or put directlyon the dinner table.

The refrigerator is the equivalent of secondary (disk) storage. It canstore high volumes of veggies for long periods of time. The counter topis the equivalent of the computer's motherboard - everything is done on thecounter (inside the computer). The cutting board is the ALU - the work gets done there. Therecipe is the control unit - it tells you what to do on the cuttingboard (ALU). Space on the counter top is the equivalent of RAM memory -all veggies must be brought from the fridge and placed on the counter topfor fast access. Note that the counter top (RAM) is faster to access thanthe fridge (disk), but can not hold as much, and can not hold it for longperiods of time. The corners of the cutting board where we temporarily storepartially chopped veggies are equivalent to the registers. The corners ofthe cutting board are very fast to access for chopping, but can not holdmuch. The salad bowl is like a temporary register, it is for storing thesalad waiting to take back to the fridge (putting data back ona disk) or for taking to the dinner table (outputting thedata to an output device).

Now for a more technical example.let us look at how a payroll program uses all three types ofstorage. Suppose the program calculates the salary of an employee. Thedata representing the hours worked and the data for the rate of pay areready in their respective registers. Other data related to the salary calculation-overtime hours, bonuses, deductions, and so forth-is waitingnearby in memory. The data for other employees is available in secondarystorage. As the CPU finishes calculations about one employee, thedata about the next employee is brought from secondary storage intomemory and eventually into the registers.

The following table summarizes the characteristics of the various kinds ofdata storage in the storage hierarchy.

Storage	Speed	Capacity	Relative Cost ($)	Permanent?
Registers	Fastest	Lowest	Highest	No
RAM	Very Fast	Low/Moderate	High	No
Floppy Disk	Very Slow	Low	Low	Yes
Hard Disk	Moderate	Very High	Very Low	Yes

Modern computers are designed with this hierarchy due to thecharacteristics listedin the table. It has been the cheapest way to get the functionality.However, as RAM becomes cheaper, faster, and even permanent, we maysee disks disappear as an internal storage device. Removable disks, likeZip disks or CDs (we describe these in detail in the online reading onstorage devices) will probably remain in use longer as a means to physicallytransfer large volumes of data into the computer. However, even thisuse of disks will probably be supplanted by the Internet as the major(and eventually only) way of transferring data. Floppy disks drives are alreadydisappearing: the new IMac Macintosh from Apple does not come with one. Withinthe next five years most new computer designs will only include floppydrives as an extra for people with old floppy disks that they must use.

Figure 2: The Machine Cycle

Before an instruction can be executed, program instructions and datamust be placed into memory from an input device or a secondary storagedevice (the process is further complicated by the fact that, as we noted earlier, the data will probably make a temporary stop in a register). As Figure2 shows, once the necessary data and instruction are in memory, the central processing unit performs the following four steps for each instruction:

1. The control unit fetches (gets) the instruction from memory.
2. The control unit decodes the instruction (decides what it means) anddirects that the necessary data be moved from memory to the arithmetic/logic unit. These first two steps together are called instructiontime, or I-time.
3. The arithmetic/logic unit executes the arithmetic or logical instruction.That is, the ALU is given control and performs the actual operationon the data.
4. Thc arithmetic/logic unit stores the result of this operation in memory or in a register. Steps 3 and 4 together are called execution time,or E-time.

The control unit eventually directs memory to release the result to anoutput device or a secondary storage device. The combination of I-timeand E-time is called the machine cycle. Figure 3 shows an instructiongoing through the machine cycle.

Each central processing unit has an internal clock that produces pulsesat a fixed rate to synchronize all computer operations. A single machine-cycle instruction may be made up of a substantial number ofsub-instructions, each of which must take at least one clock cycle. Each type of centralprocessing unit is designed to understand a specific group of instructionscalled the instruction set. Just as there are many different languages thatpeople understand, so each different type of CPU has an instruction set itunderstands. Therefore, one CPU-such as the one for a Compaq personalcomputer-cannot understand the instruction set from another CPU-say,for a Macintosh.

Figure 3: The Machine Cycle in Action

Figure 4: Memory Addresses Like Mailboxes

The location in memory for each instruction and each piece of data isidentified by an address. That is, each location has an address number, likethe mailboxes in front of an apartment house. And, like the mailboxes, theaddress numbers of the locations remain the same, but the contents(instructions and data) of the locations may change. That is, new instructions or new data may be placed in the locations when the old contents nolonger need to be stored in memory. Unlike a mailbox, however, a memorylocation can hold only a fixed amount of data; an address can hold onlya fixed number of bytes - often two bytes in a modern computer.

Figure 4 shows how a program manipulates data in memory. A payrollprogram, for example, may give instructions to put the rate of pay in location3 and the number of hours worked in location 6. To compute theemployee's salary, then, instructions tell the computer to multiply the datain location 3 by the data in location 6 and move the result to location 8.The choice of locations is arbitrary - any locations that are not already spokenfor can be used. Programmers using programming languages, however,do not have to worry about the actual address numbers, because each dataaddress is referred to by a name. The name is called a symbolic address. Inthis example, the symbolic address names are Rate, Hours, and Salary.

Now that we see conceptually how a computer works, we will look at the hardware components that make up the internals os a modern computer. Click here to continue the required reading.