Central Processing Unit

1
Chapter 4

• Central Processing Unit
• CPU

2
Learning outcomes

• By the end of this Chapter you will be able to
• explain the components of the CPU and their
  functions
• explain the instruction format and the
  instruction cycle
• illustrate How the CPU execute instructions
• explain the details of the execution of
  instructions
• Explain different ways of improving computer
  performance
• Clock frequency,
• Cache memory,
• Pipelining,
• CISC and RISC

3
Additional Reading

• Essential Reading
• Stalling (2003) Chapters 3.1 - 3.4, 4.2, 12.4,
  13.4
• Further Reading
• Burrell (2004) Chapters 5 and 8
• Schneider and Gersting (2004) Chapters 5
• Brookshear (2003) Chapter 2

4
Introduction (1)

• Before
• how information is stored in an abstract level.
• Now
• How information is processed in the computer?
• To run a program
• First it is turned into machine code which
  consists of 1s and 0s.
• The machine code is loaded into the main memory
  and then executed by the CPU.
• How ?

5
Central Processing Unit

• The central processing unit (CPU) is the brain
  of the computer. It
• interprets instructions to the computer (control
  unit),
• performs the arithmetic and logical processing
  (ALU)

6
Central Processing Unit - CPU

• The machine-code program can be divided into 2
  parts instructions and data.
• Instructions ask the CPU to take a particular
  action (do a job).
• Data (numbers, characters, letters, sounds,
  colours, etc.)
• The CPU execute a machine-code program as
  follows
• it fetches an instruction, execute it
• then goes to fetches the next instruction and
  execute it.
• It follows fetch-execute cycle until all
  instructions are executed.

7
Basic Instruction Cycle
Fetch next instruction
Execute instruction
Halt
Start
8
(No Transcript)
9
Component of the CPU

• Registers
• Program counter
• Accumulator
• Arithmetic Logic unit.
• Control Unit.

10
Registers

• Memory Address Register (MAR)
• Stores the address of the cell the CPU is going
  to execute.
• Memory Buffer Register (MBR)
• Contains instruction or data just read from the
  memory.
• Or data that is about to be written in the
  memory.
• Instruction Register (IR)
• Holds the instruction just fetched from the main
  memory.

11
Program Counter - PC

• In contains the address of the next
  instruction.
• i.e. I 100 1101

I tells CPU to execute the instruction stored
in the address 1101.
PC 1101
12
Arithmetic Logic Unit- ALU

• It performs all arithmetic operations and Boolean
  logical operations.

13
Control Unit

• It is the portion that allows things to happen.
• It controls all operations.

It tells CPU to execute the instruction stored
in the address 1101.
control unit
PC 1101
14
ALU, Registers and Control Unit Relationships
Data are presented to the ALU in registers.
Registers
Performs operations and put the result back in
registers
ALU
Control unit
Control operations.
15
CPU and System Bus
MAR
Address bus
Registers
ALU
Data bus
MBR
Control unit
Control bus
16
Instruction Format

• Op-code
• Op-code indicates what the kind of operation to
  be performed.
• Operands
• Specifies the things that is to be operated on
• It is an address of a cell where some data are
  stored.

instruction
17
Example of Instruction
4 bits
12 bits
16-bit instruction format
0001 load the Accumulator (AC) from a cell in
main memory 0010 store the content of AC in a
cell in main memory 0101 Add to the AC the
content of a cell in the main memory For example
0001 000100000000 ? load AC with the data
stored in 000100000000.
18
(No Transcript)
19
(No Transcript)
20
Example 8-bit Processor

• Address Instruction
• 00000 001 10000
• 00001 010 10000
• 00010 100 10000
• 00011 110 10001
• 00100 111 00000
• 10000 000 00001
• 10001 000 11111

21
Six-Stage Instruction cycle
Fetch real operand From memory
Decode I
Store result In M.M
DI
FO
WO
EI
FI
Perform Operation And store Result in A register

CO
Calculate operand address
22
Reading From The Memory
23
Writing to Memory
24
Enhancing Computer Performance

• Desirable to make computers run faster.
• How can this be achieved?
• In a computer all information processing is done
  by the CPU.
• The speed of the CPU is the number of
  micro-operations it can perform in a second.

25
CPU Speed

• CPU consists of a set of registers, an ALU and
  Control Unit.
• CPU micro-operations are controlled by the
  control unit.
• The control unit issues a sequence of control
  signals at a fixed frequency.
• The control unit is able to do that as it is
  connected to a clock.

26
Clock

• A clock is a micro-chip that regulates the timing
  and speed of all computer functions.
• It includes a crystal that vibrates at a certain
  frequency when electricity is applied to it.
• The clock transmits a regular sequence of
  alternating 1s and 0s.

27
Clock speed

• Also called clock rate, the speed at which a
  microprocessor executes instructions.
• Every computer contains an internal clock that
  regulates the rate at which instructions are
  executed and synchronizes all the various
  computer components.
• The CPU requires a fixed number of clock cycles
  to execute each instruction.
• The faster the clock, the more instructions the
  CPU can execute per second.
• Clock speeds are expressed in Megahertz (MHz) or
  Gigahertz (GHz).

28
Control Unit - Clock

• Control unit can issue one or more control
  signals in one clock cycle.
• This will enable the CPU to do one
  micro-operation per cycle, or a number of
  micro-operations simultaneously.
• Recent processor have a clock with frequency 2
  GHz (2230 Hz)
• (2 230 micro-operation/ sec)

29
Cache Memory

• Main memory is slower than CPU.
• There is another clock between MM and CPU to
  co-ordinate the events on the system bus.
• If the CPU is connected directly to the main
  memory it will be slowed down by the lower clock
  rate of the bus.
• To ovoid this, a cache memory which can operate
  at nearly the speed of the CPU is put in
  between.

30
Cache and Main Memory
Word transfer
Word transfer
CPU
Cache
Main memory

• CPU repeatedly accesses a particular small part
  of the main memory.
• In a short time a copy of this portion of the
  main memory is kept in the cache.

31
Read and Write with Cache

• Read a word from the main memory?
• The CPU checks whether the word is in the cache.
• If yes, the word is delivered to the CPU.
• If not, a block of the main memory containing the
  desired word is read into the cache and then
  passed to the CPU.
• Write data to the main memory?
• The CPU writes the data to the cache.
• Then, the cache writes the data to the main
  memory.

32
Pipelining

• Introducing parallelism into the sequential
  machine-instruction program.
• A number of instructions can be executed in
  parallel.
• Programs can run faster.

33
How does the CPU runs a program?

• The CPU runs a program by repeatedly performing
  an instruction cycle.
• Simple case
• CPU fetches an instruction from the main memory.
• Executes the instruction
• Called instruction cycle
• (fetch-execute-cycle)

34
Example Fetch-Execute-Cycle

• A two-stage cycle.
• Suppose we have 3 instruction I1, I2, I3.
• Without pipelining this will take 6 time units.
• With pipelining it will take only 4 time units.
• Why?

35

• Without pipelining
• Using pipelining

1 2 3 4 5 6
Fetch I1 I2 I3
Execute I1 I2 I3
1 2 3 4 5 6
Fetch I1 I2 I3
Execute I1 I2 I3
36
Six-Stage Instruction Cycle without pipelining
5 instructions A, B, C, D, E
1 2 3 4 5 6 7 . 12 ..
S1 A B
S2 A
S3 A
S4 A
S5 A
S6 A B
24 25 . 30
E




D E
37
Six-Stage Instruction Cycle with pipelining
5 instructions A, B, C, D, E
time stages 1 2 3 4 5 6 7 8 9 10
S1 A B C D E
S2 A B C D E
S3 A B C D E
S4 A B C D E
S5 A B C D E
S6 A B C D E
It takes 6 time unit to finish the instruction A,
and the other 4 instruction require 1 more time
unit each to finish there execution Therefore the
time required is 6 4 10
38
n-Stage Instruction Cycle

• Suppose we have m instruction
• Without pipelining
• nm
• With pipelining
• nm-1 time units.
• Explanation of the formulas
• The first instruction takes n time unit to be
  executed completely. The other (m-1) instruction
  will require one time unit for each one of them
  to be executed completely. Therefore the time
  requires to execute m instruction in n-stage
  cycle is nm-1.

39
Disadvantage of pipelining

• Data hazards.
• Structural hazards
• Control hazards

40
Data Hazards

• Data hazards occur when data is modified.
• For example an operand is modified and read soon
  after. Because instruction may not finished
  writing to the operand, the second instruction
  may use incorrect data.

41
Example
1st intr 2nd instr
1st intr 2nd instr
1st intr 2nd instr
42
Structural hazards

• Conflict in hardware resources
• Occurs when a part of the processors hardware is
  needed by two or more instructions at the same
  time
• Memory location etc, ..

43
Control hazards

• occur when the processor is told to branch
• ie, if a certain condition is true, jump from
  one part of the instruction stream to another one
  - not necessarily the next one sequentially.
• In such a case, the processor cannot tell in
  advance whether it should process the next
  instruction This can result in the processor
  doing unwanted actions.

44
Exercise

• What are the difficulties of pipelining in a
  conditional branch?

An unconditional branch is effectively just
one instruction in a straight sequence of
instructions so the pipeline can keep flowing.
With conditional branch, the processor
has to make a decision which path it has to take.
This can cause a problem if this decision depends
on the result of an instruction which has not yet
finished its path through the pipeline.
In this case the processor may proceed along
the wrong path and have to back up i.e. empty the
pipe and start again.
45
Strategies to reduce the number of times the
pipeline breaks

• Instruction buffers
• to fetch both possible instructions
• Prediction logic
• To fetch the most likely next instruction
• Delayed branch instructions
• Delays branch instructions
• By executing subsequent non-branch instruction
  irrespective of the branch outcome

46
Aims of RISC

• Reduce the number of instructions
• To simplify control unit
• freed chip used to allocate large number CPU
  registers.
• Small instruction format ? fast decoding
• Addressing is referred to internal registers, not
  to the main memory
• Hence, Operands is faster
• Compiler generates better machine code.
• However, RISC programs have more instructions

47
Aims of CISC

• Large number of complex instructions
• Decoding is slower,
• Instructions have different addressing mode.
• Hence, fetching operands are complicated
• However, instructions are more expressive than
  RISC.
• Programming at assembly level is simpler
• CISC programs have less instruction than RISC

48
RISC Vs CISC computers
RISC CISC
Less instructions Fixed length instruction More registers Register to register computation, only load and store access memory More transistors on memory registers More instructions per program Reduce the number of cycles per instruction More instructions Variable length instrutions Less registers Memory to memory operations More transistors to store complex instructions Less instructions per program More
49
Summary

• Components of CPU
• Instruction format (op-code operand )
• How the CPU execute instructions
• How to write a machine code program
• Enhance computer performance
• Cache memory
• Pipelining
• Problems with pipelining
• Risc Vs Cisc