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
Depending on the design of the
computer, the CPU can also have the capability to
function under manual control through man/machine
The control unit consists of several basic logically
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.
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
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
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.