COA.pptx

Computer Organization and Architecture

UNIT -1
Basic Structure of Computer

Difference Between Computer Architecture and Computer
Organization
• Computer Architecture is a functional description of requirements and design implementation for
the various parts of a computer. It deals with the functional behavior of computer systems. It
comes before the computer organization while designing a computer.
• Computer Organization comes after the decision of Computer Architecture first. Computer
Organization is how operational attributes are linked together and contribute to realizing the
architectural specification. Computer Organization deals with a structural relationship.

Figure 1: Difference between computer system and
computer organization

S. No. Computer Architecture Computer Organization
1. Architecture describes what the computer does. The Organization describes how it does it.
2. Computer Architecture deals with the functional behavior of computer systems. Computer Organization deals with a structural relationship.
3. In the above figure, it’s clear that it deals with high-level design issues. In the above figure, it’s also clear that it deals with low-level design issues.
4. Architecture indicates its hardware. Where Organization indicates its performance.
5.
As a programmer, you can view architecture as a series of instructions, addressing
modes, and registers.
The implementation of the architecture is called organization.
6. For designing a computer, its architecture is fixed first. For designing a computer, an organization is decided after its architecture.
7. Computer Architecture is also called Instruction Set Architecture (ISA). Computer Organization is frequently called microarchitecture.
8.
Computer Architecture comprises logical functions such as instruction sets, registers,
data types, and addressing modes.
Computer Organization consists of physical units like circuit designs, peripherals, and
adders.
9.
The different architectural categories found in our computer systems are as follows:
1.Von-Neumann Architecture
2.Harvard Architecture
3.Instruction Set Architecture
4.Micro-architecture
5.System Design
CPU organization is classified into three categories based on the number of address fields:
1.Organization of a single Accumulator.
2.Organization of general registers
3.Stack organization
10. It makes the computer’s hardware visible. It offers details on how well the computer performs.
11. Architecture coordinates the hardware and software of the system. Computer Organization handles the segments of the network in a system.
12. The software developer is aware of it. It escapes the software programmer’s detection.

CPU Architecture/ Functional Unit
Figure 2: CPU Architecture

• Memory holds both data and instructions.
• The arithmetic/logic gate unit is capable of performing arithmetic and logic operations on data.
• A processor register is a quickly accessible location available to a digital processor's central
processing unit (CPU). Registers usually consist of a small amount of fast storage, although some
registers have specific hardware functions, and may be read-only or write-only.
• The control unit controls the flow of data within the CPU - (which is the Fetch-Execute cycle).
• Input arrives into a CPU via a bus.
• Output exits the CPU via a bus.

• Parts of a CPU:
ALU - The arithmetic logic unit executes all calculations within the CPU
CU - control unit, coordinates how data moves around, decodes instructions
• Registers:
• a memory location within the actual processor that work at very fast speeds. It stores instructions which
await to be decoded or executed.
PC - program counter - stores address of the -> next <- instruction in RAM
MAR - memory address register - stores the address of the current instruction being executed
MDR - memory data register - stores the data that is to be sent to or fetched from memory
CIR - current instruction register - stores actual instruction that is being decoded and executed
ACC - accumulator - stores result of calculations
• Buses:
address bus - carries the ADDRESS of the instruction or data
data bus - carries data between processor and the memory
control bus - sends control signals such as: memory read, memory write
Together, these buses may be referred to as the “system bus” or the “front-side bus”

Figure 3: Hardwired control unit

Figure 4: Micro-programmed Control Unit

Instruction Format
• A computer will usually have a variety of instruction code format. It is
function of control unit within the CPU to interpret each instruction
code and provide the necessary control function needed to process
the instruction.
Syntax:-
Opcode Operand

1) Opcodes: An operation code field that specifies operation to be performed.
Such as add, subtract, complement, shift etc.
2) Operand Address: An address field that designates memory address or
processor register. Operations specified by computer instructions are
executed on some data stored in memory or processor.
operands resides in memory are specified by their memory
address. Operands residing in processor register are specified with a register
address.
I= Mode field
The mode field of an instruction specify a variety of alternatives for choosing the
operand from the given address. The basic computer has three instruction code
format:
I Opcode Operand

1. Memory reference.
2. Register reference
3. I/O instruction

1. Memory reference
I Opcode Operand Add
In memory reference instruction, if I=0 then it is direct addressing and if I=1 then it is indirect addressing.
2. Register reference
I 1 1 1 Register Operation
An operand from memory is not needed therefore 12 bits are used to specify operation or test to be executed.
15 14 12 11 0
15 14 12 11 0

3. I/O instructions
• An I/O instruction does not need reference to memory and is
recognized by operation code with a one in the left most bit of an
instruction. The remaining 12 bits are used to specify the type of I/O
operation or test performed.
I 1 1 1 I/O operation
15 14 12 11 0

Addressing Modes
• The different ways in which location of operand is specified in an
instruction are referred to as addressing mode.
1) Immediate
2) Direct
3) Indirect
4) Register
5) Register indirect
6) Relative
7) Indexed

1. Immediate Addressing
• Operand is given explicitly in the instruction. For example ADD 5. this instruction add 5 to the
content of accumulator. No memory reference to fetch but in limited range.
Format:- Opcode Operand
2. Direct Addressing
• In this address field contains address of operand e.g.. ADD A
• Effective Address (EA) = Address field of A. This instruction (ADD A) add contents of A to the
accumulator.
• Look in memory at address A at for operand.
• Single memory reference to access data no additional calculation require to calculate effective
address.
Format:-
Memory
Opcode Address of A
Operand A

3. Indirect Addressing
• Memory cell pointed by address field and which contains address of
operand.
• Effective Address (EA) = Address field of A
• EA = [A]
• E.g. ADD A
• Format:-
memory
• It is Slower
Opcode Address
Address
Operand

4. Register Addressing
• Operand is present in register named in address field.
• Effective Address = R; EA = [R]
• Limited number of registers, Very small address field needed.
• Shorter instruction, Faster instruction fetch.
• No memory access, Fast execution.
• Multiple registers help to improve the performance.
• Format:-
Register
Opcode Register Address
Operand

5. Register Indirect Addressing
• Effective address = [R]
• Operand is in memory cell pointed by the content of register R. It is having
large address space. Fewer memory access than the direct addressing.
• Format:-
Register
memory
Opcode Register Add.
Address
Operand

6. Indexed Addressing
• Effective address = X+ [R]
• Address field holds two values i.e., X= Constant value (offset)
R= Register that holds address of memory location.
Format:-
Register Memory
Opcode Register R Const. value
Address Operand
+

7. Relative Addressing
• It is a version of displacement address. Where X is address part of the
instruction.
• Effective Address = X + PC
• PC = Program Counter
• Get the operand from X byte away from current location pointed by
PC.

Types of Instructions
• Computers may have instructions of several different addresses. The
no. of address field in the instruction format of a computer depends
on internal organization of its registers. Main 3 organizations are as
follows:
1. Single Accumulator Organization.
2. General Register Organization.
3. Stack Organization.

1. Single Accumulator Organization
• In this type of organization that specifies the arithmetic addition is by
an assembly language instruction as ADD X, where X is address of
operand. The ADD instruction in this case, result in the operation.
AC AC+M (x)
ADD X

2. General Register Organization
• The instruction format in this type of computer needs 3 register
address field. Thus, instruction for an arithmetic addition may be
written as.
ADD R1, R2, R3
R1 R2 + R3

3. Stack Organization
• The stack organization CPU uses stack which uses PUSH and POP
operation. These defined operation requires the address field.
For eg. PUSH X
POP X
For the above organization. There are 4 types of instructions-

Types of Instructions
1. Three Address Instruction
2. Two Address Instruction
3. One Address Instruction
4. Zero Address Instruction

1. Three Address Instruction
• Computers with three address instruction format can use each address
field to specify either a processor register or a memory operand.
• The program in assembly language that evaluate X= (A+B)*(C+D) as follows:
ADD R1, A, B = R1 M [A] + M [B]
ADD R2, C, D = R2 M [C] + M [D]
Mul X, R1, R2 = M[X] R1 * R2
It is assumed that the computer has two processor register. The symbol M[A]
denotes the operand at memory address symbolized by A.
• The advantage of three address format is that it results in short program
when evaluating arithmetic operations. The disadvantage is that the binary
coded instruction require to many bits to specify three address.

2. Two Address Instruction
• Two address instruction is most common in commercial system. Here,
again, each address field can specify either the processor register or a
memory word. The program to evaluate X= (A+B)*(C+D) as follows:
Mov R1, A = R1 M [A]
ADD R1, B = R1 R1 + M [B]
Mov R2, C = R2 M [C]
ADD R2, D = R2 R2 + M [D]
MUL R1, R2 = R1 R1 * R2
MOV X, R1 = M[X] R1

3. One Address Instruction
• One address instruction uses an accumulator register for all data
manipulation for the purpose of multiplication and division. There is a
need for second register and assume that the AC contains the result
of all operation. The program is to evaluate: X= (A+B)*(C+D) in one
address instruction:-
LOAD A AC M[A]
ADD B AC AC + M[B]
STORE T M[T] AC
LOAD C AC M[C]
ADD D AC AC + M[D]
MUL T AC AC * M[T]
STORE X M[X] AC

4. Zero Address Instruction
• Step organized computes and do not use an address field for the
instruction addition and multiplication.
• The PUSH and POP instructions however need an address field to specify
the operand that communicate with the stack.
• The program to evaluate the same operation in Zero address instructions
as follows:-
PUSH A TOS A
PUSH B TOS B
ADD TOS A+B
PUSH C TOS C
PUSH D TOS D
ADD TOS C+D
MUL TOS (A+B) * (C+D)
POP X M[X] TOS

Look ahead carry generator
• The adder produce carry propagation delay while performing other arithmetic operations like multiplication
and divisions as it uses several additions or subtraction steps. This is a major problem for the adder and
hence improving the speed of addition will improve the speed of all other arithmetic operations. Hence
reducing the carry propagation delay (Propagation delay is the amount of time required for a signal to be
received after it has been sent; it is caused by the time it takes for the signal to travel through a medium.) of
adders is of great importance.
• There are different logic design approaches that have been employed to overcome the carry propagation
problem.
• Like, ripple carry adder, carry look-ahead adder etc.
• One widely used approach is to employ a carry look-ahead which solves this problem by calculating the
carry signals in advance, based on the input signals. This type of adder circuit is called a carry look-ahead
adder. Here a carry signal will be generated in two cases:
1. Input bits A and B are 1,
2. When one of the two bits is 1 and the carry-in is 1.

• In ripple carry adders, for each adder block, the two bits that are to be added are available instantly.
However, each adder block waits for the carry to arrive from its previous block. So, it is not possible to
generate the sum and carry of any block until the input carry is known. The block waits for the block to
produce its carry. So there will be a considerable time delay which is carry propagation delay.
Ripple Carry Adders
Figure 5: Ripple Carry Adder

• Consider the above 4-bit ripple carry adder.
The sum is produced by the corresponding full adder as soon as the input signals are applied to it. But the
carry input is not available on its final steady-state value until carry is available at its steady-state value.
Therefore, though the carry must propagate to all the stages in order that output and carry settle their final
steady-state value. The propagation time is equal to the propagation delay of each adder block, multiplied by
the number of adder blocks in the circuit.
• For example, if each 3 full adder stage has a propagation delay of 20 nanoseconds, then will reach its final
correct value after 60 (20 × 3) nanoseconds. The situation gets worse, if we extend the number of stages
for adding more number of bits.

If you are asked to calculate the time after which the output sum bit or
carry bit becomes available from any particular full adder, then it is
calculated as-
• Time After Which Carry Bit Cx Becomes Available-
• Required time = Total number of full adders till full adder producing Cx X Carry propagation delay
of full adder.
• Time After Which Sum Bit Sx Becomes Available-
• Required time = Time taken for its carry in to become available + Sum propagation delay of full
adder
= { Total number of full adders before full adder producing Sx X Carry propagation delay of full adder
} + Sum propagation delay of full adder

Problem-01:
• A 16-bit ripple carry adder is realized using 16 identical full adders.
The carry propagation delay of each full adder is 12 ns and the sum
propagation delay of each full adder is 15 ns. The worst case delay of
this 16 bit adder will be ______?
A) 195 ns
B) 220 ns
C) 250 ns
D) 300 ns

We consider the last full adder for worst case delay.
Time after which output carry bit becomes available from the last full adder
= Total number of full adders X Carry propagation delay of full adder
= 16 x 12 ns
= 192 ns
Time after which output sum bit becomes available from the last full adder
= Time taken for its carry in to become available + Sum propagation delay of
full adder
= { Total number of full adders before last full adder X Carry propagation
delay of full adder } + Sum propagation delay of full adder
= { 15 x 12 ns } + 15 ns
= 195 ns

Carry Look-ahead Adder :
•
A carry look-ahead adder reduces the propagation delay by introducing more complex hardware.
In this design, the ripple carry design is suitably transformed such that the carry logic over fixed
groups of bits of the adder is reduced to two-level logic. Let us discuss the design in detail.
Figure 6: Carry Look-ahead Adder

• Consider the full adder circuit shown above with corresponding truth table.
We define two variables as ‘carry generate’ Gi and ‘carry propagate’ Pi
then,

Figure 7: Look-ahead Carry Generator

Advantages and Disadvantages of Carry Look-Ahead Adder :
Advantages –
• The propagation delay is reduced.
• It provides the fastest addition logic.
Disadvantages –
• The Carry Look-ahead adder circuit gets complicated as the number of
variables increase.
• The circuit is costlier as it involves more number of hardware.

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