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Pattpatelch04

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The Von Neumann Model

1

Chapter 4

The Von Neumann

Model

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

4-2

The Stored Program Computer

1943: ENIAC

Presper Eckert and John Mauchly -- first general electronic computer.

(or was it John V. Atanasoff in 1939?)

Hard-wired program -- settings of dials and switches.

1944: Beginnings of EDVAC

among other improvements, includes program stored in memory

1945: John von Neumann

wrote a report on the stored program concept,

known as the First Draft of a Report on EDVAC

The basic structure proposed in the draft became known

as the “von Neumann machine” (or model).

a memory, containing instructions and data

a processing unit, for performing arithmetic and logical operations

a control unit, for interpreting instructions

For more history, see http://www.maxmon.com/history.htm

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

4-3

Von Neumann Model

MEMORY

CONTROL UNIT

MAR MDR

IR

PROCESSING UNIT

ALU TEMP

PC

OUTPUT

Monitor

Printer

LED

Disk

INPUT Keyboard

Mouse

Scanner

Disk

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

4-4

Memory

2k x m array of stored bits

Address

unique (k-bit) identifier of location

Contents

m-bit value stored in location

Basic Operations:

LOAD

read a value from a memory location

STORE

write a value to a memory location

• • •

0000 0001 0010 0011 0100 0101 0110

1101 1110 1111

00101101

10100010

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

4-5

Interface to Memory

How does processing unit get data to/from memory?

MAR: Memory Address Register

MDR: Memory Data Register

To LOAD a location (A):

1. Write the address (A) into the MAR.

2. Send a “read” signal to the memory.

3. Read the data from MDR.

To STORE a value (X) to a location (A):

1. Write the data (X) to the MDR.

2. Write the address (A) into the MAR.

3. Send a “write” signal to the memory.

MEMORY

MAR MDR

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

4-6

Processing Unit Functional Units

ALU = Arithmetic and Logic Unit

could have many functional units.

some of them special-purpose

(multiply, square root, …)

LC-3 performs ADD, AND, NOT

Registers

Small, temporary storage

Operands and results of functional units

LC-3 has eight registers (R0, …, R7), each 16 bits wide

Word Size

number of bits normally processed by ALU in one instruction

also width of registers

LC-3 is 16 bits

PROCESSING UNIT

ALU TEMP

2

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

4-7

Input and Output

Devices for getting data into and out of computer memory

Each device has its own interface, usually a set of registers like the memory’s MAR and MDR

LC-3 supports keyboard (input) and monitor (output)

keyboard: data register (KBDR) and status register (KBSR)

monitor: data register (DDR) and status register (DSR)

Some devices provide both input and output disk, network

Program that controls access to a device is usually called a driver.

INPUT

Keyboard

Mouse

Scanner

D isk

OUTPUT

Monitor

Printer

LED

Disk

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

4-8

Control Unit Orchestrates execution of the program

Instruction Register (IR) contains the current instruction.

Program Counter (PC) contains the address

of the next instruction to be executed.

Control unit:

reads an instruction from memory

the instruction’s address is in the PC

interprets the instruction, generating signals

that tell the other components what to do

an instruction may take many machine cycles to complete

CONTROL UNIT

IRPC

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

4-9

Instruction Processing

Decode instruction

Evaluate address

Fetch operands from memory

Execute operation

Store result

Fetch instruction from memory

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

4-10

Instruction

The instruction is the fundamental unit of work.

Specifies two things:

opcode: operation to be performed

operands: data/locations to be used for operation

An instruction is encoded as a sequence of bits.

(Just like data!)

• Often, but not always, instructions have a fixed length,

such as 16 or 32 bits.

• Control unit interprets instruction:

generates sequence of control signals to carry out operation.

• Operation is either executed completely, or not at all.

A computer’s instructions and their formats is known as its

Instruction Set Architecture (ISA).

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

4-11

Example: LC-3 ADD Instruction

LC-3 has 16-bit instructions.

Each instruction has a four-bit opcode, bits [15:12].

LC-3 has eight registers (R0-R7) for temporary storage.

Sources and destination of ADD are registers.

“Add the contents of R2 to the contents of R6,

and store the result in R6.”

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

4-12

Example: LC-3 LDR Instruction

Load instruction -- reads data from memory

Base + offset mode:

add offset to base register -- result is memory address

load from memory address into destination register

“Add the value 6 to the contents of R3 to form a

memory address. Load the contents of that

memory location to R2.”

3

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4-13

Instruction Processing: FETCH

Load next instruction (at address stored in PC)

from memory

into Instruction Register (IR).

Copy contents of PC into MAR.

Send “read” signal to memory.

Copy contents of MDR into IR.

Then increment PC, so that it points to

the next instruction in sequence.

PC becomes PC+1.

EA

OP

EX

S

F

D

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

4-14

Instruction Processing: DECODE

First identify the opcode.

In LC-3, this is always the first four bits of instruction.

A 4-to-16 decoder asserts a control line corresponding

to the desired opcode.

Depending on opcode, identify other operands

from the remaining bits.

Example:

for LDR, last six bits is offset

for ADD, last three bits is source operand #2

EA

OP

EX

S

F

D

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

4-15

Instruction Processing: EVALUATE ADDRESS

For instructions that require memory access,

compute address used for access.

Examples:

add offset to base register (as in LDR)

add offset to PC

add offset to zero

EA

OP

EX

S

F

D

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

4-16

Instruction Processing: FETCH OPERANDS

Obtain source operands needed to

perform operation.

Examples:

load data from memory (LDR)

read data from register file (ADD) EA

OP

EX

S

F

D

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

4-17

Instruction Processing: EXECUTE

Perform the operation,

using the source operands.

Examples:

send operands to ALU and assert ADD signal

do nothing (e.g., for loads and stores) EA

OP

EX

S

F

D

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

4-18

Instruction Processing: STORE RESULT

Write results to destination.

(register or memory)

Examples:

result of ADD is placed in destination register

result of memory load is placed in destination register

for store instruction, data is stored to memory

write address to MAR, data to MDR

assert WRITE signal to memory

EA

OP

EX

S

F

D

4

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

4-19

Changing the Sequence of Instructions

In the FETCH phase, we increment the Program Counter by 1.

What if we don’t want to always execute the instruction that follows this one? examples: loop, if-then, function call

Need special instructions that change the contents of the PC.

These are called control instructions. jumps are unconditional -- they always change the PC

branches are conditional -- they change the PC only if some condition is true (e.g., the result of an ADD is zero)

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

4-20

Example: LC-3 JMP Instruction

Set the PC to the value contained in a register. This

becomes the address of the next instruction to fetch.

“Load the contents of R3 into the PC.”

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

4-21

Instruction Processing Summary

Instructions look just like data -- it’s all interpretation.

Three basic kinds of instructions:

computational instructions (ADD, AND, …)

data movement 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

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

4-22

Control Unit State Diagram

The control unit is a state machine. Here is part of a

simplified state diagram for the LC-3:

A more complete state diagram is in Appendix C.

It will be more understandable after Chapter 5.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

4-23

Stopping the Clock

Control unit will repeat instruction processing sequence

as 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 processor

and other resources.

To stop the computer:

• AND the clock generator signal with ZERO

• When control unit stops seeing the CLOCK signal, it stops processing.

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