Architecture of Central
Processing Unit (CPU)
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What is the role of CPU in
computer system;
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Role of CPU in computer system
The central processing unit (CPU) of a computer is a piece of
hardware that carries out the instructions of a computer program. It
performs the basic arithmetical, logical, and input/output operations
of a computer system. The CPU is like the brains of the computer -
every instruction, no matter how simple, must go through the CPU.
The CPU is sometimes also referred to as the central processor unit,
or processor for short.
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Describe functions of components of CPU,
i.e. Arithmetic and logic unit, control unit,
registers, cache, Internal buses (data bus,
address bus and control bus) with the
help of block diagram;
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Arithmetic Logic Unit
An arithmetic logic unit (ALU) is a major component of the
central processing unit of a computer system. It does all
processes related to arithmetic and logic operations that
need to be done on instruction words. In some
microprocessor architectures, the ALU is divided into the
arithmetic unit (AU) and the logic unit (LU).
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Control Unit (CU)
A control unit (CU) handles all processor control signals. It
directs all input and output flow, fetches code for instructions
from microprograms and directs other units and models by
providing control and timing signals. A CU component is
considered the processor brain because it issues orders to just
about everything and ensures correct instruction execution.
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Control Unit (CU)
CU functions are as follows:
• Controls sequential instruction execution.
• Interprets instructions.
• Guides data flow through different computer areas.
• Regulates and controls processor timing.
• Sends and receives control signals from other computer
devices.
• Handles multiple tasks, such as fetching, decoding, execution
handling and storing results.
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Registers
Register are used to quickly accept, store, and transfer data
and instructions that are being used immediately by the CPU,
there are various types of Registers those are used for various
purpose. Among of the some Mostly used Registers named as
AC or Accumulator, Data Register or DR, the AR or Address
Register, program counter (PC), Memory Data Register (MDR),
Index register, Memory Buffer Register.
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Cache Memory
Cache memory is a small-sized type of volatile computer
memory that provides high-speed data access to a processor
and stores frequently used computer programs, applications
and data. It is the fastest memory in a computer, and is
typically integrated onto the motherboard and directly
embedded in the processor.
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Bus
The electrically conducting path along which data is transmitted inside
any digital electronic device. A Computer bus consists of a set of
parallel conductors, which may be conventional wires, copper tracks
on a PRINTED CIRCUIT BOARD. Each wire carries just one bit, so the
number of wires determines the largest data WORD the bus can
transmit: a bus with eight wires can carry only 8-bit data words, and
hence defines the device as an 8-bit device.
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Bus
Types of Computer Bus
Data Bus: The data bus allows data to travel back and forth between
the microprocessor (CPU) and memory (RAM).
Address Bus: The address bus carries information about the location of
data in memory.
Control Bus : The control bus carries the control signals that make sure
everything is flowing smoothly from place to place.
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Describe the functions of following registers
and their types, i.e.
b. Special Purpose Register
i. Program Counter (PC)
ii. Instruction Register (IR)
iii. Memory Address Register (MAR)
iv. Memory Buffer Register (MBR)
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CPU Registers
• These are the high speed memory locations built into the
microprocessor.
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CPU Registers
• These are the high speed memory locations built into the microprocessor.
• The CPU uses these locations to store data and instructions temporarily for
processing.
• CPU processes, stores and transfer data from one component to another
with the help of registers.
• The number of register varies among computers.
• It increases the performance of CPU.
• Each register receives the information, holds it temporarily and pass it on,
as directed by CU.
• The size of register depends on the computer architecture.
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There are four types Special Purpose Register
i. Program Counter (PC)
ii. Instruction Register (IR)
iii. Memory Address Register (MAR)
iv. Memory Buffer Register (MBR);
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Special Purpose Registers
Program Counter (PC)
• Used to store the address of the next instruction to be fetched for
execution.
• When the instruction is fetched, the value of program counter is
incremented.
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RAM
2134 10110011 1 PC
2134
ADDRESS
2135 10100011 4
2136 11100011 2
2137
2138 11110000 3
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Special Purpose Registers
Instruction Register (IR)
• Instruction register is used to store the fetched the instruction.
• The instruction is also decoded in this register.
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RAM
2134 10110011 1 PC
2134
ADDRESS
2135 10100011 4
2136 11100011 2 IR
10110011
2137
2138 11110000 3
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Special Purpose Registers
Memory Address Register (MAR)
• Used to store memory address being used by the processor.
• When processor wants to READ data from or WRITE data in memory, it
stores the address of that memory location in this register.
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RAM
2134 10110011 1 PC
2134
ADDRESS
2135 10100011 4
2136 11100011 2 IR
10110011
2137
2138 11110000 3 MAR
2137
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Special Purpose Registers
Memory Buffer Register (MBR)
• Used to store data coming from or going from the memory.
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RAM PC
2134
2134 10110011 1
IR
10110011
ADDRESS
2135 10100011 4
2136 11100011 2 MAR
2137
2137 10101010
2138 11110000 3 MBR
10101010
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Describe the functions of following registers
and their types, i.e.
a. General Purpose Register
i. Accumulator (AC)
ii. Data Register (DR)
iii. Base Register (BR)
iv. Counter Register (CR);
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General Purpose Register
Accumulator (AX)
An accumulator is a register for short-term, intermediate storage
of arithmetic and logic data in a computer's CPU (central
processing unit). It is used for arithmetic and data operations.
Data Register (DX)
The data register is responsible for division and multiplication
operations.
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General Purpose Register
Base Register (BX)
It is used for arithmetic and data movement. It has special addressing
capabilities.
Counter Register (CX)
It is used for counting purpose. It acts as a counter for repetition of
loops.
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Various operations performed
by CPU
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Compare three types of
instructions of CPU, i.e. data
transfer instructions, data
processing instructions and
program control instructions;
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Data Transfer Instructions
• Data transfer instructions are the instructions which
transfers data in the microprocessor.
• These instructions move data between registers, or
between memory and registers.
• These instructions copy data from source to destination.
• While copying, the contents of source are not modified.
• They are also called copy instructions.
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Data Processing Instructions
The data processing instructions manipulate data
within registers. They include move instructions,
arithmetic instructions, shifts, logical instructions,
comparison instructions, and multiply instructions.
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Program Control Instructions
These instructions used to change the sequence of
instruction of a program are called control transfer
instructions. These instructions transfer the
execution control to certain part of program
instead of next instruction. Some examples of
these operations are JUMP and JUMPZ (Jump if
zero) etc.
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Compare different types of CPU
instruction formats, i.e. zero
address instruction, one
address instruction and two
address instruction;
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What is instruction set?
An instruction is a statement that tells the computer to do
something. The way an instruction is given is called instruction
format.
A computer has a variety of instruction formats. The control unit
of CPU is responsible to interpret the instruction code. The control
unit also provides the necessary control function required to
process the instructions. A simple instruction format consists of 16
bits.
There are two parts of instruction code format:
• Operand code
• Address of Operand
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Instruction Code Format
1. Operand code
The operand code specifies the operation to be performed by the
computer such as ADD, SUB, MOV etc. It takes 4 bits. An operand can be
a value or register number on which the operation is performed.
Opcode Operand Reference 1 Operand Reference 2
16 bits
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Instruction Code Format
2. Address of Operand
The address of operand refers to a location in main memory where the
value is stored.
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Types of instruction formats
Different types of instruction formats are as follows:
Two-Address instruction format
This format uses two address fields. Each address field can specify either
a register or memory address. It is the most common instruction format.
The example of this instruction format are MOV, ADD, MUL etc.
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How Two-Address instruction format works
Suppose A=2, B=4, C=3 AND D = 6
X = (A+B) * (C+D)
MOV R1 2 A 2
MOV R1, A
ADD R1 6 B 4
ADD R1, B
MOV R2 3 C 3
MOV R2, C
ADD R2, D ADD R2 9 D 6
MUL R1 54 R2 9
MUL R1,R2
MOV X 54 R1 54
MOV X, R1
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Types of instruction formats
Different types of instruction formats are as follows:
One-Address instruction format
This format uses only one address field. It uses one accumulator register
(AC) for all data manipulation. A second register is required for
multiplication and division.
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How One-Address instruction format works
Suppose A=2, B=4, C=3 AND D = 6
X = (A+B) * (C+D) LOAD A 2
LOAD A ADD B+A 6
ADD B STORE T=B+A 6
STORE T
LOAD C 3
LOAD C
ADD C+D 9
ADD D
MUL T MUL T*(C+D) 54
STORE X=T 54
STORE X
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Types of instruction formats
Different types of instruction formats are as follows:
Zero-Address instruction format
In zero-address instruction format, an address field is absent in the
instruction. A stack-organized computer does not use address field for
the instruction like ADD and MUL. However, the instructions such as
PUSH and POP require an address field to specify the operand that
communicates with the stack.
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How Zero-Address instruction format works
Suppose A=2, B=4, C=3 AND D = 6
X = (A+B) * (C+D)
POP X 54
MUL (6)*(9)
PUSH A
PUSH B ADD (6+3)
ADD PUSH D 6
PUSH C PUSH C 3
PUSH D
ADD (4+6)
ADD
PUSH B 4
MUL
PUSH A 2
POP X BIEKCS.BLOGSPOT.COM
Describe
fetch – decode – execute cycle
with the help of a diagram;
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Fetch Execute Cycle?
The fetch execute cycle is the basic operation (instruction) cycle of a
computer (also known as the fetch decode execute cycle).
During the fetch execute cycle, the computer retrieves a program
instruction from its memory. It then establishes and carries out the
actions that are required for that instruction.
The cycle of fetching, decoding, and executing an instruction is
continually repeated by the CPU whilst the computer is turned on.
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How this cycle works
Step Fetch execute cycle steps Simplified description
The PC contains the address of the memory
PC has address of next
1 location that has the next instruction which has to
instruction
be fetched
This address is then copied from the PC to
2 PC copied to the MAR
the MAR via the address bus
The contents (instruction) at the memory location Lookup MAR and get
3 (address) contained in MAR are then copied into contents. Copy contents
the MDR into the MDR
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Fetch Execute Cycle
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Differentiate between Complex Instruction Set
Computer (CISC) and Reduced Instruction Set
Computer (RISC) Architecture;
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Difference between RISC and CISC
RISC CISC
1. RISC stands for Reduced Instruction Set 1. CISC stands for Complex Instruction Set
Computer. Computer.
2. CSIC processor has complex instructions that
2. RISC processors have simple instructions take up multiple clocks for execution. The average
taking about one clock cycle. The average clock clock cycle per instruction (CPI) is in the range of 2
cycle per instruction (CPI) is 1.5 and 15.
3. Execution time is very less 3. Execution time is very high
4. The decoding of instructions is simple. 4. Decoding of instructions is complex
5. The most common RISC microprocessors are 5. Examples of CISC processors are the
Alpha, ARC, ARM, AVR, MIPS, PA-RISC, PIC, System/360, VAX, PDP-11, Motorola 68000
Power Architecture, and SPARC. family, AMD, and Intel x86 CPUs.
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