Arithmetic/Logic Unit performs arithmetic operations and logical operations and controls
the speed of those devices. As you know, arithmetic operations such as addition,
subtraction, multiplication, and division are fundamental to all mathematical operations.
Logical operations are comparisons. That is, the ALU compares two pieces of data to
see whether one is equal to (=), greater than (>), or less than (<) the other.

The control unit controls the flow of information and instructs the computer how to carry
out program instructions. It directs the movement of data and instructions between the
main memory and the ALU. It also carries out the controlling of input and output devices
and passing of data to the ALU for computation.

Registers store very small amounts of data and instructions for short periods of time.
They hold various types of information such as data, instructions, memory addresses
and the intermediate results of calculations. Essentially, they hold the data and
instructions that the CPU is currently working with and they can be thought of as being
the CPU’s workspace. As soon as a particular instruction or piece of data is finished with,
the next ones immediately replace it and information that results from the processing is
returned to main memory. Data and instructions do not go into either the Arithmetic/Logic
Unit (ALU) or the Control Unit (CU). The ALU works on the data held in the registers
acting on the instructions that are also held there.

Efficient processing and timely output is important to an organisation. Let’s consider
some factors which affect the processing speed of a processor.

Execution of an instruction takes place during a machine cycle. The time which a
machine cycle occurs is measured in fractions of a second.
Early computer operating speeds were measured in milliseconds (thousands of a
second) range. New computers operate nanoseconds (millionth of a second) and
picoseconds (trillionth of a second).

It is also measured in terms of how many instructions are executed in a second. This
is known as MIPS (millions of instructions per second)

Each CPU produces a series of electronic pulses at a predetermined rate, known as
clock speed, which affects machine cycle time.
Clock speed is measured in megahertz.
Hertz – One cycle or pulse per second
Megahertz (MHz) – Measurement of cycles in millions of cycles per second
Gigahertz (GHz) – Billions of cycles per second

Data is moved within a computer system in units called bits. A bit is a binary digit – 0
or 1. Number of bits the CPU can process at one time is known as the wordlength of
the CPU. The amount of data that can be transferred between devices in the same
machine cycle depends on the wordlength.

Data is transferred from the CPU to other system components through bus lines, the
physical writing that connects the computer system components. The number of bits
a bus line can transfer at any one time is known as bus line width.

Machine cycle time, clock speed, wordlength and bus line width affects the processing
speed of the CPU. When selecting a CPU, organizations must balance the benefits of
speed with cost. CPUs with faster clock speeds and machine cycle times are usually
more expensive than slower ones. CPU speed is also related to complexity. As
discussed in the next section, having a high speed cache memory can also increase the
speed of the CPU.