The document discusses the CPU cycle and its components. The CPU, which executes programs stored in memory, consists of an arithmetic logic unit, control unit, and registers. It performs two phases - instruction and execution. In the instruction phase it fetches and decodes instructions, while the execution phase carries out the instruction and stores results. Various measures gauge processing speed like clock speed and multiprocessing improves it by executing instructions simultaneously. Main memory provides the CPU a working storage area for programs and data in forms like RAM, ROM, and cache.

1. Reusable learning object on the
CPU cycle
BY
Jeanne Ndatirwa
March 24th, 2013

2. Introduction
• The CPU which is often described as the “brain of the
computer”.
• Most CPUs are collections of digital circuits imprinted
on silicon wafers, or chips, no bigger than the tip of a
pencil eraser.
• The increased speed the current in a CPUS travels at,
can be increased by reducing the distance between
points which has resulted in ever smaller chips (see
Moore’s Law).
• The CPU cycle is closely associated with primary
storage also known as main memory.

3. Purpose of the CPU
• The job of the CPU is to execute programs
• A program is a list of unambiguous instructions
designed to be followed mechanically by a
computer.
• Once the CPU executes a program, it is stored in a
computer’s main memory (RAM or random
access memory).
• The CPU can also store information in memory by
specifying the information to be stored and the
address of the location where it is to be stored.

4. Hardware Components
The CPU consists of three elements:
The arithmetic/logic unit (ALU): performs mathematical
calculations and makes logical comparisons.
The control unit: sequentially accesses program
instructions, decodes them, and coordinates the flow of
data in and out of the ALU.
Registers (storage area): are high-speed storage areas
used to temporarily hold small units of program
instructions and data immediately before, during, and
after execution by the CPU.

5. Components in action
Executing any machine-level instruction involves two
phases: instruction and execution.
The instruction phase consists of:
1. Fetch instruction: the computer reads the next
program instruction to be executed and any necessary
data into the processor.
2. Decode instruction: instruction is decoded and passed
to the appropriate processor execution unit.
The time it takes to perform the instruction phase is
called the instruction time (I-time).

6. Components in action
The execution phase consists of
3. Execute instruction: hardware
element, freshly fed with an instruction and
data, carries out the instruction.
4. Store results: results are stored in registers
The time it takes to complete the execution
phase is called the execution time (E-time).

7. Processing

8. Processing
Fetch
Instructions from
Memory
Execute Decode
ALU Control Unit

9. Processing
Various measures are used to gauge processing
speed:
1. Machine cycle time: measured by how many
instructions are executed in one second, also
measured in nanoseconds or picoseconds.
2. Clock Speed: series of electronic pulses
produced at a predetermined rate that
affects machine cycle, measured in mega or
gigahertz.

10. Processing
Multiprocessing: involves the simultaneous execution of two
or more instructions at the same time.
Coprocessor: the part of the computer that speeds processing
by executing specific types of instructions while the CPU
works on another processing activity.
Multicore Microprocessor: a processor that combines two or
more independent processors into a single computer as they
share the workload.
Parallel Computing: simultaneous execution of the same task
on multiple processors to obtain results faster.
Grid Computing: use of a collection of computers , usually
owned by multiple individuals or organizations to work in a
coordinated manner to solve a common problem.

11. Memory characteristics and function
Main memory provides the CPU with a working
storage area for programs and data.
Computer memory takes several forms:
• Random access memory (RAM): a form of
memory in which instructions or data can be
temporarily stored.
• Ready-only memory (ROM): a non-volatile form
of memory.
• Cache memory: a type of high-speed memory
that a processor can access more rapidly than
main memory.

12. Knowledge Assessment

13. Credits
• http://www.teach-
ict.com/gcse_computing/ocr/212_computing_hardware/cpu/
miniweb/pg2.htm
• http://math.hws.edu/javanotes/c1/s1.html
• http://www.teach-
ict.com/gcse_computing/ocr/212_computing_hardware/cpu/
quiz/penalty_cpu.htm
• Stair, R. & Reynolds, G “Principles of information
systems”, 10th ed., pp. 91-97.