CONTROL UNIT

The control unit maintains order within the computer system and directs the flow of traffic (operations) and data. The flow of control is indicated by the dotted arrows on figure 1-1. The control unit selects one program statement at a time from the program storage area, interprets the statement, and sends the appropriate electronic impulses to the arithmetic-logic unit and storage section to cause them to carry out the instruction.

The control unit does not perform the actual processing operations on the data. Specifically, the control unit manages the operations of the CPU, be it a single-chip microprocessor or a fill-size mainframe. Like a traffic director, it decides when to start and stop (control and timing), what to do (program instructions), where to keep information (memory), and with what devices to communicate (I/O). It controls the flow of all data entering and leaving the computer. It accomplishes this by communicating or interfacing with the arithmetic-logic unit, memory, and I/O areas. It provides the computer with the ability to function under program control. Depending on the design of the computer, the CPU can also have the capability to function under manual control through man/machine interfacing.

The control unit consists of several basic logically defined areas. These logically defined areas work closely with each other. Timing in a computer regulates the flow of signals that control the operation of the computer. The instruction and control portion makes up the decision-making and memory-type functions. Addressing is the process of locating the operand (specific information) for a given operation. An interrupt is a break in the normal flow of operation of a computer (e.g., CTRL + ALT + DEL). Control memory is a random-access memory (RAM) consisting of addressable storage registers. Cache memory is a small, high-speed RAM buffer located between the CPU and main memory; it can increase the speed of the PC. Read-only memory (ROM) are chips with a set of software instructions supplied by the manufacturer built into them that enables the computer to perform its I/O operations.

The control unit is also capable of shutting down the computer when the power supply detects abnormal conditions.

ARITHMETIC-LOGIC UNIT

The arithmetic-logic unit (ALU) performs all arithmetic operations (addition, subtraction, multiplication, and division) and logic operations. Logic operations test various conditions encountered during processing and allow for different actions to be taken based on the results. The data required to perform the arithmetic and logical functions are inputs from the designated CPU registers and operands.

The ALU relies on basic items to perform its operations. These include number systems, data routing circuits (adders/subtracters), timing, instructions, operands, and registers. Figure 1-2 shows a representative block diagram of an ALU of a microcomputer.

PRIMARY STORAGE (MAIN MEMORY)

The primary storage section (also called internal storage, main storage, main memory, or just memory) serves four purposes: . To hold data transferred from an I/O device to the input storage area, where it remains until the computer is ready to process it. This is indicated by the solid arrow on figure 1-1. . To hold both the data being processed and the intermediate results of the arithmetic-logic operations. This is a working storage area within the storage section. It is sometimes referred to as a scratch pad memory. 

. To hold the processing results in an output storage area for transfer to an I/O device.

Figure 1-2.\Representative block diagram of an ALU.

l To hold the program statements transferred from an I/O device. This area is called the program storage area.

Please note that the four areas (input, working, output, and program storage) are NOT fixed in size or location, but rather are determined by each individual program's requirements.

About now, you're probably wondering how the control unit is able to find these stored instructions and data items. To understand this, picture memory as a wall of post office boxes in a post office. Each box has a different number (address) and represents a specific storage location in memory, as shown in figure 1-3. Like the mail in a post office box, the contents of a storage location can change, but the number on the post office box or memory address does not change. In this manner, a particular program instruction or data item that is held in primary storage can be located by ^{knowing its address.} It is the responsibility of the programmer to assign descriptive names to these data items. This enables the computer program and the computer to keep track of the storage location address of each data item.

Primary storage can be classified by its physical or functional characteristics.

Memory Types by Physical Characteristics

Primary storage devices may be classified according to the type of magnetic or electronic principle they use to store data. Some of the more common types are magnetic core storage, semiconductor storage, and bubble storage.