Chapter 4 The von Neumann Model

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Chapter 4 The von Neumann Model
2
Digital Logic Hierarchy
FA
Mem
Now Begins Software
3
The Von Neumann Computer
INPUT Keyboard Mouse Scanner Card
reader Disk
John von Neumannproposed this model in 1946
Program instructions and data are both stored as
sequences of bits in a single shared memory
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Memory Interface

• Memory address register (MAR)
• Holds the memory address for the location that is
  to be read or written
• Memory data register (MDR)
• Holds the data that is read from memory
• Holds the data this is to be written to memory
• Memory write enable (WE)
• The control signal that is asserted to write to
  memory

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Using the Memory Block

• To read from memory
• Put the address from which you want to read into
  the MAR
• The memory then automatically reads that
  locations data into the MDR
• Read the data from the MDR
• To write to memory
• Put the address to which you want to write to
  into the MAR
• Put the data you want to write into the MDR
• Assert the WE signal

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Memory Terminology

• Address space Amount of data that can be stored
  (also called the memory size)
• Addressability Number of bits stored in each
  memory location
• 1 byte 8 bits
• 1 kilobyte (KB) 210 bytes 1,024 bytes
• 1 megabyte (MB) 220 bytes
• 1 gigabyte (GB) 230 bytes

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The Processing Unit

• Arithmetic and logic unit (ALU)
• ADD, SUB, MULT, etc.
• AND, OR, NOT, etc.
• The word length of a computer is the number of
  bits that the ALU can process
• Includes a small amount of memory very close to
  the ALU, often called a register file

INPUT
OUTPUT
Processing Unit
ALU
TEMP
Control Unit
PC
IR
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Input and Output

• Used to get information into and out of the
  computer
• External devices attached to a computer are
  called peripherals

INPUT Keyboard Mouse Scanner Card
reader Disk
OUTPUT Monitor Printer LED Disk
Processing Unit
ALU
TEMP
Control Unit
PC
IR
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Control Unit

• The control unit directs execution of the program
• Keeps track of where we are in the execution of
  the current program
• Program counter (PC) gives address for next
  instruction to be executed
• Also keeps track of where we are in the execution
  of the current instruction
• Instruction register (IR) contains the currently
  executing instruction
• Large finite state machine

INPUT
OUTPUT
Processing Unit
ALU
TEMP
Control Unit
PC
IR
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The LC-3 An Example von Neumann Machine

• Memory

• Processing Unit

PC

• Control

IR
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Anatomy of an Instruction ADD R6, R2, R6
16-bit instruction format.4-bit opcodes mean
there are at most 24 different instruction types.
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14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
0
0
0
1
1
1
0
0
1
0
0
0
0
1
1
0
unused
R6 R6 R2

• This is an operate instruction
• Not all bits have meaning in this particular
  instruction.

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The Instruction Cycle

• FETCH
• Obtain the next instruction from memory
• DECODE
• Examine the instruction, and determine how to
  execute it
• EVALUATE ADDRESS
• Compute the address of any needed memory location
  (put in MAR)
• FETCH OPERANDS
• Obtain the source operands needed for execution
• EXECUTE
• Carry out the operation of the instruction
• STORE RESULT
• Store the result in the designated destination

Not all instructions require all six phases
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The Fetch Phase

• Computer programs consist of instructions
• Each instruction is typically 1 machine word wide
• For example, LC-3 instructions are 16 bits
• Instructions are stored in memory
• Program ? sequence of instructions
• Stored in memory as 1s and 0s
• Called machine code

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Fetching an Instruction

• Copy PC contents into MAR, and increment PC

PC

• Wait for memory access
• Result will be placed into MDR by memory unit

• Copy MDR contents to IR

IR
?
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Decode

• Pick apart the instruction stored in the IR
• Control logic in control unit does this
• Determine
• Operation
• Operand sources
• Operand destination

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Fetch Operands
ADD R6,R2,R6

• Get sourceregister numbers (SR1 and SR2)from IR

PC

• Send SR1and SR2 to register fileas addresses

010
110
?

• Retrieve valuesaddressed bySR1 and SR2and send
  to ALU

IR
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Execute
ADD R6,R2,R6

• Control unittells ALU which operation to do,
  based on IR
• The ALU doesits operation

PC
IR
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Store Result
ADD R6,R2,R6

• Get destinationregister number(DR) from IR

?

• Send DR to registerfile

PC
110

• Store ALU resultinto register file
• at location pointedto by DR
• Control unit generatessignal that tells
  register
• file when to load

IR
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Anatomy of an Instruction LDR R2, R3, 6
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
0
1
1
0
0
1
0
0
1
1
0
0
0
1
1
0
EffectiveAddress R3 6 R2
MemEffectiveAddress

• This is a data movement instruction
• This requires the computation of an effective
  memory address
• For LDR, it is of the form base offset (e.g.,
  R3 6)

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Decode
LDR R2,R3,6

• Get sourceregister number (SR1) from IR

PC

• Send SR1 to register fileas an address

011


IR
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Evaluate Address
LDR R2,R3,6

• Retrieve valueaddressed bySR1 and
  theimmediate, thensend to addressadder

PC

• Add the contents of base register (R3) to
  theoffset (6)
• Put effective address in MAR

?
IR
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Fetch Operands
LDR R2,R3,6

• Read memory
• Put data from memory in MDR

?
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Store Result
LDR R2,R3,6

• Get destinationregister number(DR) from IR

?

• Send DR to registerfile

PC
010

• Store MDR result into
• Control unit generatessignal that tells
  register
• file when to load

IR
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Anatomy of an Instruction JMP R3
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
1
1
0
0
0
0
0
0
1
1
0
0
0
0
0
0
unused
unused
PC R3
This is a control instruction. It changes the
execution path of the CPU.
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Fetch Operands
JMP R3

• Get sourceregister number (SR1) from IR

PC

• Send SR1 to register fileas an address

011

• Retrieve valueaddressed bySR1 and sendto ALU

?

IR
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Execute
JMP R3

• The ALU doesits operationControl unittells it
  whichoperation to do

PC
IR
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Store Result
JMP R3

• Store ALU resultinto PCControl unit
  generatessignal that tells the PC when to do
  the load

?
PC
IR
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Von Neumann Summary

• Programs are stored in memory as instructions
• Our LC-3 machine has 16-bit instructions
• Data is also stored in the same memory
• A program is executed by
• Fetching next instruction from memory
• Decoding it
• Evaluating its address
• Fetching its operands
• Doing the requested operation and
• Storing the result

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The Von Neumann Bottleneck

• You may hear of the termVon Neumann Bottleneck
• All instructions have to be fetched from memory
• the path to memory is a bottleneck
• In spite of this, the Von Neumann model is the
  computing model that is used

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Stopping the Clock

• Control unit will repeat instruction processing
  sequenceas long as clock is running.
• If not processing instructions from your
  application,then it is processing instructions
  from the Operating System (OS).
• The OS is a special program
• that manages processorand other resources.
• To stop the computer
• AND the clock generator signal with ZERO
• When control unit stops seeing the CLOCK signal,
  it stops processing.