CHAPTER 4 COMPUTER SYSTEM

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CHAPTER 4COMPUTER SYSTEM Von Neumann Model

• Topics
• Basic components of a computer
• Instruction processing
• Examples from the LC-3 simulator
• Changing the sequence of execution

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Basic Components of a Computer System

• The basic components are
• Central processing unit (CPU)
• Memory
• Input devices
• Output devices
• A clock controls all the operations performed in
  a computer system.
• A computer system is a synchronous machine.

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Von Neumann Model of a Computer System

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Central Processing Unit (CPU)

• The CPU consists of
• Arithmetic logic unit (ALU)
• Control unit (CU)
• Small set of storage areas, called registers
• Arithmetic Logic Unit (ALU)
• The ALU is the primary processing unit in a
  computer.
• It performs all the arithmetic and logic
  operations.
• Control Unit (CU)
• The CU controls all the operations performed
  by a computer and determines what operations the
  program calls for and in what order they need to
  be carried out.

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CPU (Continued)

• The CPU is responsible for
• The complete processing of information
• All the decision-making operations
• Word Size of a processor
• Number of bits the processor reads from or writes
  into the memory in one clock cycle
• Number of bits normally processed by ALU in one
  instruction
• Also width of registers
• LC-3 is a 16 bit processor

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Memory

• Memory provides the storage space for
• the program that is currently being executed
• the data that is needed during the execution of
  the program
• The memory unit consists of a number of memory
  locations of the same width.
• The number of bits that can be stored in a
  memory location corresponds to the memory width.
• Each memory location is identified by a unique
  address.
• Memory capacity Total number of memory
  locations memory width

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Interface to Memory
MAR Memory Address Register MDR Memory Data
Register
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Block Diagram of a Memory Unit

• memory width
• 7 6 5 4 3 2 1 0
• 0
• 1
• 2
• .
• MAR 10 bits . 8 bits
  MDR
• .
• 10-bit memory address . 8-bit data
• (A9 A0) . (D7 D0)
• .
• 1023
• For this example, memory capacity 210 x 8
  bits 1 Kbytes
• (The above figure does not show the control
  signals needed to control the memory read and
  memory write operations.)

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Memory - Basic Operations

• LOAD - Read data from a memory location
• Write the address of the location into the MAR.
• Send a read signal to the memory.
• Read the data from MDR.
• STORE - Write data to a memory location
• Write the data to the MDR.
• Write the address of the location into the MAR.
• Send a write signal to the memory.

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Input/Output (I/O) Devices

• Input devices transfer information from the
  outside world into the memory of a computer
  system.
• Examples Keyboard, disk drives, scanner,
  mouse
• Output devices transfer information from the
  memory of a computer system into the outside
  world.
• Examples Printers, display units,
  plotters, disk drives
• Some devices provide both input and output
• Examples disk, network
• Driver - Program that controls access to a
  device

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Buses in a Computer System

• Buses are the physical link between the various
• components within the computer.
• The computer employs mainly three types of buses
  
• Address bus
• Data bus
• Control bus

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Buses in a Computer System (Continued)

• Three types of buses in a computer system
• Address bus Carries the address generated by
  the CPU to the memory and I/O.
• Data bus Allows the transfer of data between
  the CPU, memory and I/O.
• Control bus Carries the control signals,
  generated by the control unit, to the
  various components in the system in order to
  ensure proper sequencing of data and
  instruction movement.

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Block Diagram of a Basic Computer Architecture

• CPU
•  
• Clock
• CU ALU
  Clock Generator
•  
• Address Bus p
•  
• Data Bus q
• Control Bus
  r
•  
•   Main Memory I/O Unit
•  
• I/O Bus (represents address, data,
  control)
•  
•   Keyboard Printer Secondary
  Memory
•  
• 
  

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Instruction

• The instruction specifies two items
• Opcode operation to be performed
• Operands data/locations to be used for operation
• An instruction is encoded as a sequence of bits.
  Often,
• but not always, instructions have a fixed length,
  such as
• 16 or 32 bits.
• Instruction Format The binary representation of
  an instruction

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Instruction (Continued)

• Control unit
• interprets the instruction
• generates sequence of control signals to carry
  out operation.
• A computers instructions and their formats is
  known as
• its Instruction Set Architecture (ISA).

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Instruction Processing
Fetch instruction from memory

Decode instruction
Evaluate address
Fetch operands from memory
Execute operation
Store result
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Instruction Processing FETCH
F

• Load the instruction from memory into
• Instruction Register
• Copy contents of PC into MAR
• MAR ? PC
• Increment PC so that PC points to the next
  instruction in sequence
• PC ? PC 1
• Send read control signal to memory
• MDR ? Memory MAR
• Copy contents of MDR into IR
• IR ? MDR

D
EA
OP
EX
S
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Example Instruction LDR Instruction

• LDR R2, R3, 6 Loads (reads) data from memory
• into the destination register R2

Memory address Base register SEXT
(offset) Memory address R3 6 Load the
contents of the memory location R3 6 to R2
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Instruction Processing DECODE
F

• First identify the opcode
• Identify the operands
• Example LDR R2, R3, 6
• Opcode IR1512 0110
• Operands
• Offset IR50 6 (6 decimal 000110
  binary)
• Base register IR86 011 R3

D
EA
OP
EX
S
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Instruction Processing EVALUATE ADDRESS
F

• For instructions that require memory access,
• compute address used for memory access.
• Example LDR R2, R3, 6
• Address SEXT (offset) Base Register
• MAR ? 6 R3

D
EA
OP
EX
S
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Instruction Processing FETCH OPERANDS
F

• Obtain source operands needed to
• perform operation.
• Example LDR R2, R3, 6
• Load data from memory (LDR)
• MDR ? Memory MAR

D
EA
OP
EX
S
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Instruction Processing EXECUTE
F

• Perform the operation, using the source
• operands.
• Example LDR R2, R3, 6
• No operation done in this step

D
EA
OP
EX
S
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Instruction Processing STORE RESULT
F

• Write results to destination.
• Destination can be a register or
• Memory.
• Example LDR R2, R3, 6
• Data read from the memory is placed in
• destination register.
• R2 ? MDR

D
EA
OP
EX
S
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Changing the Sequence of Instructions

• In the FETCH phase, we increment PC by 1.
• Need special instructions called control
  instructions to
• change the contents of the PC during the Execute
• phase.
• jumps (unconditional)
• Always change the PC
• branches (conditional)
• PC is changed only if the specified condition
  is
• true

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Examples LC-3 Control Instructions

• JMP R2 PC ? R2
• RET PC ? R7
• BRzp NEXT if the last result was zero or
  positive then
• PC ? address corresponding to NEXT

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Instruction Processing Summary

• Three basic kinds of instructions
• Arithmetic/logic instructions (ADD, AND, )
• Data transfer instructions (LD, ST, )
• Control instructions (JMP, BRnz, )
• Six basic phases of instruction processing
• F ? D ? EA ? OP ? EX ? S
• Not all phases are needed by every instruction
• Phases may take variable number of machine cycles

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Example Instruction LDR Instruction

• LDR R2, R3, 6 R2 ? mem R3 6
• Opcode IR1512 0110
• Operands
• Base register R3 given by IR86 Offset 6
  given by IR50
• Destination R2 given by IR119

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Instruction Processing LDR R2, R3, 6

• Fetch
• MAR ? PC
• PC ? PC 1
• MDR ? Memory MAR
• IR ? MDR
• Decode
• Identify the opcode
• Identify the operands
• Execute
• MAR ? 6 R3
• MDR ? Memory MAR
• R2 ? MDR

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Example Instruction ADD Instruction

• ADD R6, R2, R6 R6 ? R2 R6
• Opcode IR1512 0001
• Operands
• Source1 R2 given by IR86 Source2 R6
  given by IR20
• Destination R6 given by IR119

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Instruction Processing ADD R6, R2, R6

• Fetch
• MAR ? PC
• PC ? PC 1
• MDR ? Memory MAR
• IR ? MDR
• Decode
• Identify the opcode
• Identify the operands
• Execute
• R6 ? R2 R6

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Example Instruction JMP Instruction

• JMP R3 PC ? R3
• Opcode IR1512 0001
• Operand Base register R3 given by IR86

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Instruction Processing JMP R3

• Fetch
• MAR ? PC
• PC ? PC 1
• MDR ? Memory MAR
• IR ? MDR
• Decode
• Identify the opcode
• Identify the operand
• Execute
• PC ? R3