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CSCI 1800 Cybersecurity and International Relations

Computer Architecture

John E. Savage

Brown University

Overview

• Definition of computer

• Binary numbers

• Very brief history of computer

• Architecture of CPU – CPU and RAM

– Typical instructions

• Simple assembly level program

• Assembler and compiler

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The Computer

• A computer takes input, produces output, potentially storing data in the process.

• Most computers can be configured to perform many tasks, i.e. they are programmable.

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Very Brief History of Computers

• Mechanical computers have existed for millennia. – Abacus

• Electronic computers emerged in the 1940s. – Eniac

• Embedded computers are used in appliances. – iPod, smart phones

• Specialized control hardware less expensive to implement with a general purpose computer

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Binary Numbers

• Today computers store data as bits, 0s and 1s. • Addition/subtraction is done with binary numbers. • Here is a mapping of decimal to binary numbers 0 000 = 0*22 + 0*21 + 0*20 1 001 = 0*22 + 0*21 + 1*20 2 010 = 0*22 + 1*21 + 0*20 3 011 = 0*22 + 1*21 + 1*20 4 100 = 1*22 + 0*21 + 0*20 5 101 = 1*22 + 0*21 + 1*20 6 110 = 1*22 + 1*21 + 0*20 7 111 = 1*22 + 1*21 + 1*20

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4 = 22, 2 = 21, 1 = 20

Binary Numbers

• How can we represent 1019 in binary? – Find the largest power of 2 that is less than 1019

• 20 =1, 21 = 2, 22 = 4, 23 = 8, 24 = 16, 25 = 32, 26 = 64, 27 = 128, 28 = 256, 29 = 512, 210 = 1024

• Answer is 512 = 29

• Subtract 512 from 1019 = 507

– Find largest power of 2 that is less than 507 • Answer 256

• Subtract 256 from 507 = 251

– Repeat to obtain 1019 = 512+256+128+64+32+8+2+1

– 101910 = 2 9 +28 +27 +26 +25 +24 +23 +21 +20 = 111110112

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Computer Architecture

• Computers have central processing units (CPUs) and random access memories (RAMs).

• Programs stored in the RAM direct the action of the CPU. • The CPU implements the fetch-execute cycle

– Fetch an instruction and execute it.

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Random Access Memory (RAM)

• A word is a collection of bits, typically 32 bits.

• RAM is given command (read, write, no-op), input word, and an address. It produces an output word.

• read changes out_wrd

to word at address addr.

• write replaces word at

address addr with in_wrd.

• no-op makes no changes.

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Central Processing Unit (CPU)

• Fetch-execute cycle: Repeat forever: a) read an instruction from the RAM and b) execute it.

• Typical instructions: ADD, SUBTRACT, SHIFT LEFT, MOVE, COMPARE, JUMP, JUMPC (conditional), READ, WRITE

• A program is a set of instructions.

• A register is a storage location

• CPU has registers, e.g. rega , that regb, hold temporary results. – Permanent results stored in the RAM.

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Architecture of the CPU

• The CPU also has a program counter (PC) such as prog_ctr. PC holds location of next CPU instruction.

• Normally, instructions are executed from sequential RAM locations (change PC to PC+ 1). – ADD, SUB, SHIFT, COMPARE, MOVE cause PC to increment

• But program may jump to remote instruction by changing PC to new address, not from PC to PC+ 1. – JUMP and JUMPC (Jump Conditional)

– JUMPC allows program to choose between two different paths.

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Addition

• The instruction ADD A B adds the contents of registers A and B and puts the sum into A.

• E.g. Consider addition of 310 = 0112 and 510 = 1012.

011 = 0*22 + 1*21 + 1*20

101 = 1*22 + 0*21 + 1*20

1000 = 1*23 + 0*22 + 0*21 + 0*20

• Subtraction (SUB) implemented by adding a negative number.

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COMPARE and MOVE Instructions

• COMPARE A B compares contents of registers A and B and sets CMP = 1 (0) if same (different).

• MOVE A B moves the contents of register (or location) A to that of register (or location) B.

• SHIFT LEFT A performs left shift of contents of rega SHIFT LEFT 1011 = 0110

SHIFT LEFT 0110 = 1100

• SHIFT LEFT can be used to multiply two numbers.

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JUMP Instructions

• The program counter PC is a CPU register with the address of the next instruction in memory.

• Normally the PC is incremented on each step. – That is, the next instruction in memory is executed

• The JUMP and JUMPC (conditional) instructions change the contents of the PC. – The CPU jumps to a new point in the program.

• JUMP A sets PC to A. • JUMPC A sets PC to A if compare bit CMP is 1.

– This command allows for a conditional branch.

A Simple Multiplication Algorithm

• To multiply integers x and y, set u = 0, z = 0 and then add y to z a total of x times. – E.g. x = 3, y = 2, u = 0, z = 0.

COMMAND MEANING COMMENT 1. ADD y z Add y to z (z = 0 initially) 2. SUB x 1 Subtract 1 from x 3. COMPARE x u Compare x to u (u remains at 0) 4. JUMPC 6 If x = 0, jump to instruction 6. 5. JUMP 1 If x ≠ 0, jump to beginning of loop 6. DONE

External CPU Connections

• READ reads from external input, e.g. keyboard

• WRITE writes to external output, e.g. display

• These commands require a way to specify which device is connected to the CPU.

Assembly Language

• CPU instructions are bit sequences.

– Most computers operate on 32-bit words, though 64-bit is becoming more common.

– Some bits in a word represent an opcode (e.g. MOVE)

– Other bits represent addresses and values.

• Assembly languages use mnemonics, such as ADD, SHIFT LEFT, MOVE, COMPARE, JUMPC instead of bits.

• An assembler is a program that translates assembly language programs into binary CPU instructions.

Compilers

• High level languages are more expressive than assembly level languages.

– Fewer statements, each having more meaning.

• Compilers are programs to translate high level languages into machine level languages.

Review

• Definition of computer

• Brief history of computer

• Architecture of CPU

– CPU and RAM

– Typical instructions

• Simple assembly level program

• Assembler and compiler