[PPT] - Introduction to Computer Science CSCI 109 Readings St. Amant, Ch. PowerPoint Presentation

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Introduction to Computer Science

CSCI 109

Andrew Goodney

Fall 2019

Readings

St. Amant, Ch. 3

Lecture 2: Architecture Sept. 9th, 2019

SLIDE 2

Reminders

uTake the survey if you haven’t already

u https://tinyurl.com/y2cot2r5

v Password: CS109Fall2019

uHW #1 out by EoD today. Due 9/23 uQuiz #1 is on 9/16. It will cover material taught

n 8/26 and 9/9.

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Schedule

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Computer Architecture

uThe von Neumann architecture uThe Central Processing Unit (CPU) uStorage uInput and Output

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Reading:

St. Amant Ch. 3

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Motivation

u What do computers do?

v Math with binary numbers

u So what do we need to build a computer?

v Place to store binary numbers v Way to do math

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“Memory”

u Very generic term u ”Place to store a binary number”

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ENIAC ~660 bits (<1 bit/ft^3) Magnetic core memory 1024 bits (3.2 x 10^4 bits/ft^3) Modern DRAM (10^16 bits/ft^3)

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Arithmetic/Logic

u “math” we need to do with numbers in memory

v ADD v SUBTRACT v MULTIPLY v DIVIDE v AND,OR,XOR,NOT v Etc…

u Assume we can build a circuit that can do this u Takes numbers represented as digital (electrical) values, produces

results as the same

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OP2

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Start building a circuit…

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bus

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Instructing the CPU

u Now we can make instructions… u Instructions are binary numbers that tell the circuit what to

do

u Select the 1st operand, 2nd operand, destination and function u With a series of such instructions the circuit can perform

arbitrary computations

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Where to get the instructions?

u Instruction Memory

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Controller ALU

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How to compute?

u Fill instruction memory with desired program u Initialize data memory u Run an instruction (given by program counter)

v Then increment program counter v Run next instruction, increment program counter…

u Some early computers were pretty much just this

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Toward the Von Neumann Architecture

u Separate instruction and data memory is inefficient/inflexible u Von Neumann proposed combined data and instruction

memory

u Also want to support larger and heterogeneous memory

utside the physical CPU

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Simplified Von Neumann CPU

u Controller + ALU + Registers = CPU

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I/O Bus Memory Bus

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The Central Processing Unit (CPU)

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u Controller + ALU = Central Processing Unit (CPU) u CPU has a small amount of temporary memory within it

v Registers v A special register called the program counter (PC)

u CPU performs the following cycle repeatedly

Fetch Instruction Decode Instruction Execute Instruction

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Fetch-Decode-Excecute

u Fetch

v Get the next instruction from memory

u Decode

v Send the proper signals from the controller to the ALU and Registers

u Execute

v Let the ALU do its work to produce a result

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Typical Controller Tasks

u Read an instruction from memory u Direct ALU to do some arithmetic or logic operation u Transfer data from one place to another u Prepare for next instruction to be read u Send a directive to input or output device

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Typical ALU Tasks

u Perform an arithmetic operation on the contents of

registers e.g., add R1 and R2 and put the result in R1 (R1 = R1 + R2)

u Perform a logical operation on the contents of registers

e.g., compare R1 and R2 (R1 < R2 ?)

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Improving the CPU

u Faster clock (~GHz on modern computers) u Specialized ALU (e.g., GPU) or more than one ALU u Multiprocessing (more than one CPU) u Streamlining Controller-ALU cooperation (pipelining) u More complex ALU instructions

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Storage: Addressing and Random Access

u How is storage organized ? u Linear ordering v Each stored item has a an address (which is a number) v To retrieve an item you have to know this number v Retrieval is by address

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Storage: Modular and Hierarchical

u Analogy with office files v Each (physical) file has a number v When a file is needed, you ask for it by number v In the office, files may be stored in shelves, on tables, in

cabinets, etc.

u Storage is modular: as long as the person who wants the file

knows its number and as long as the storekeeper can find the file, doesn’t matter exactly how physical files are stored

u Or where they are stored: Files may be stored in a basement

– not easy to reach

u So latency (time to access) differs

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The Nature of Computer Storage

u Only one item can be stored at one memory location u Random access – all locations in one type of memory

are (on the average) equally slow (or fast) in terms of access speed

u Storage is hierarchical – add new levels of memory

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The Storage Hierarchy

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Cheaper & larger Faster

Registers RAM (memory) Secondary Storage (Disk Space)

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Trade-offs

u An aside… u Identifying trade-offs is a fundamental engineering skill u Understanding and balancing trade-offs is part of design process u Not always easy to manage! u Conflicting interests u Speed vs. space is very common tradeoff in CS

v So if you want faster execution you need more memory

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Volatile Storage, Special Memory

u Volatile: memory is erased when power turned off v Registers, RAM (often called primary storage) u Non-volatile: memory intact when power turned off v Secondary storage (disk) u Booting and ROM (read-only memory) u Cache: small, fast memory between registers and RAM

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Cache

u Small (but bigger than registers) u Volatile u Fast (not as fast as registers, but faster than RAM) u What to keep in the cache ? v Things that programs are likely to need in the future v Locality principle:

u Look at what items in memory are being used u Keep items from nearby locations (spatial locality) u Keep items that were recently used (temporal locality)

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Input and Output

u Input: Interrupt-driven

v e.g., Key strokes are slow, CPU treats them like special

events

u Output: Write to special memory (e.g., video memory)

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SLIDE 27

Modern Computer Architecture diagram

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Controller ALU I/O Devices (USB, etc)

CPU

Registers & Program Counter

Memory DRAM Disk L2 Cache L3 Cache Boot ROM I/O Controller

Disk controller

On die, but not part of "CPU"

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Programming a CPU

u How to compute? u Develop a series of low-level instructions

v Using the registers and/or main memory for storage v Using only low-level operations made available by the particular CPU

u ”Assembly language”

v Or maybe even machine code (probably not, though)

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Creating Assembly (Machine) Code

u Adding four numbers in C is easy

a = b + c + d + e

u Equivalent assembly code

add b, c, a OR add b, c, a add a, d, a add d, e, f add a, e, a add a, f, a

u Which one is better ? Are the two equivalent ?

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Programs at Different Abstraction Levels

u C code

a = b + c

u Assembly code

add b, c, a

u Machine code 00000010001100100100000000100000

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Programs at Different Abstraction Levels

u C code

a = b + c

u Assembly code

add b, c, a

u Machine code 00000010001100100100000000100000

6 bits – opcode 5 bits: source register 1 5 bits: source register 2 5 bits: destination register 5 bits: shift amount 5 bits: functions

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Typical Operations

u ADD Ri Rj Rk

Add contents of registers Ri and Rj and put result in register Rk

u SUBTRACT Ri Rj Rk

Subtract register Rj from register Ri and put result in register Rk

u AND Ri Rj Rk

Bitwise AND contents of registers Ri, Rj and put result in register Rk

u NOT Ri

Bitwise NOT the contents of register Ri

u OR Ri Rj Rk

Bitwise OR the contents of registers Ri, Rj and put result in register Rk

u SET Ri value

Set register Ri to given value

u SHIFT-LEFT Ri

Shift bits of register Ri left

u SHIFT-RIGHT Ri

Shift bits of register Ri right

u MOVE Ri Rj

Copy contents from register Ri to register Rj

u LOAD Mi Ri

Copy contents of memory location Mi to register Ri

u WRITE Ri Mi

Copy contents of register Ri to memory location Mi

u GOTO Mi

Jump to instruction stored in memory location Mi

u COND_GOTO Ri Rj Mi

If Ri > Rj, jump to instruction stored in memory location Mi

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SLIDE 33

A Typical Day at PaniCorp

u Central processing: Connie and Alun u The bus: Buster u Storage (Memory): Mr. Lager u Input and output

Play out a typical instruction cycle with your friends

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Read St. Amant Ch. 3

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M100 LOAD M1 R1 M101 SET R2 0 M102 SET R3 1 M103 SET R6 0 M104 ADD R1 R2 R4 M105 SUB R1 R3 R5 M106 MOVE R5 R1 M107 MOVE R4 R2 M108 COND_GOTO R1 R6 104 M109 WRITE R2 M2 M110 END What does this program do?

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PC R1 R2 R3 R4 R5 R6 M1 M2 3