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Computer architecture, part 1: What we’ll cover for this lecture topic: – What is a CPU? – How is a program executed by the CPU? – Boolean logic – Binary circuits that can add! COMP 4—Power Tools for the Mind What’s in the box? 1 Central Processing Unit (CPU): – Carries out the program’s instructions! • Operates on data it finds in the computer’s memory. – Also called Microprocessor • Includes all binary circuits that carry out arithmetic & logic operations---reduced to a single IC. – CPU has four key parts that we will examine: • Control Unit • Arithmetic & Logic Unit • Registers • Clock And, of course, wires that connect everything together. COMP 4—Power Tools for the Mind What’s in the box? 2 —CONTROL UNIT (CU): circuitry for coordinating machine’s activities. Controls sequence of operations. —ARITHMETIC & LOGIC UNIT (ALU): circuitry to perform data manipulation (arithmetic & logic). —Registers: Temporary storage areas. Holds information applicable to the current operation. —Clock: Triggers start and stop of all CPU operations. (heartbeat) CPU Two main functional units: COMP 4—Power Tools for the Mind CU ALU clock Registers What’s in the box? 3 • CPUs support a set of very simple instructions that typically fall into the following categories: • Data movement (load, store, copy…) • Arithmetic/logical (add, subtract, compare..) • Program control (branch, halt…) – Very primitive commands (operations) executed by the CPU [logical structure] – These commands are implemented as electronic binary circuits [physical structure] which can transform the 0s and 1s. BASIC INSTRUCTION SET COMP 4—Power Tools for the Mind What’s in the box? 4 Sample of a partial Basic instruction set Instr: Meaning: STO ADD SUB MUL DIV INC CMP JMP Store data in a particular memory location Add two numbers together Subtract one number from another Multiply two numbers Divide two numbers Increment a number by adding 1 Compare two numbers to see if they are equal Jump to a specific position in the instruction code • Instructions are given to the processor in the form of a program … so it knows what circuits to use, in what order; and from where the data should be read or to where it should be stored. COMP 4—Power Tools for the Mind What’s in the box? 5 More specifically: • A program consists of a sequence of instructions. • EACH instruction specifies both: – The operation to perform – The address of the data it will transform in that operation (if necessary). • Instructions are stored and processed in machine language--also called microcode. • Like everything else (e.g. like ASCII characters, pixels of an image, …) machine language consists solely of bit patterns. COMP 4—Power Tools for the Mind What’s in the box? 6 Machine language or Machine Code a/k/a microcode – ML bit patterns are based directly on CPU’s instruction set (on its binary circuits). So who created them? – CPU chip: designed to recognize certain bit patterns as representing certain ML instructions, which correspond directly to certain available binary circuits. – Each ML instruction contains a fixed-length instruction code that: • Identifies the operation to perform: op code • Tells the CPU how to determine the operands For example: assume a 4-bit op-code + two 6-bit operands = one 16 bit instruction COMP 4—Power Tools for the Mind What’s in the box? 7 E.G.: An ADD instruction in a 16-bit machine language: 0101 110011 111100 Op-code: Operands: 0101 (RAM or Register addresses) 110011 111100 • Before it can process a M.L. instruction, the CPU must fetch it from main memory. • The CPU must keep track of its position in the instruction code. It uses a bookmark of sorts. • The offset at which the next instruction starts is stored in a special register, called the Program Counter or Instruction Pointer. Important digression… – who knows if bits in memory are data or program … ? – increments to memory address of next instruction ... COMP 4—Power Tools for the Mind What’s in the box? 8 Putting it all together Processor: simple minded! repeats same steps over & over... MACHINE CYCLE: “Instruction-Execution cycle” Processing of a single machine-level instruction (one Op Code) in a basic machine. Instruction Cycle: in the Control Unit 1. FETCH • Fetches instruction (from memory) at address given by instruction pointer; copies it into CU storage register. 2. COPY & DECODE • Copies op code into instruction register, operands into address registers. • Interprets instruction. ALU is invoked to perform decoded operation for any ALU ops (enters Execution Cycle) 3. ADVANCE INSTRUCTION POINTER (counter) • Pointer incremented; contains memory address of next instruction that will be fetched. 9 Imprecise data representation, and a vast simplification-but general concepts are correct. A single Instruction cycle (Add) 3 , 2b 000001 Advance CONTROL UNIT 000011 pointer 110011 Pointer 0101 Instr. reg. 1 Fetch 2a Copy & Decode 0101 110011 111100 PROCESSOR ALU Enter execute cycle Address reg 111100 Address reg Storage register 0101 1100 1111 1100 #000001 #000010 0111 1110 0111 1110 #000011 #000100 85 data shown in base 10 MEMORY #110011 …. 65 data shown in base 10 #111100 …. 10 Imprecise data representation, and a vast simplification-but general concepts are correct. Another single Inst Cycle (Divide) 3 , 2b 000011 Advance CONTROL UNIT 000101 pointer 111001 Pointer 0111 Instr. reg. 1 Fetch 2a Copy & Decode 0111 111001 111110 PROCESSOR ALU Enter execute cycle Address reg 111110 Address reg Storage register 0101 1100 1111 1100 #000001 #000010 0111 1110 0111 1110 #000011 #000100 85 data shown in base 10 MEMORY #110011 …. 65 data shown in base 10 #111100 …. 11 Execution Cycle: by the CU/ALU – Execution cycles vary, depending on the op code. • Eg: load register R03 with contents of memory cell 47: Just for culture: The CU causes the load to occur. • CU activates ALU circuitry, which performs the actual Op Code. EG: MUL, ADD, DIV, … E.G: an arithmetic operation: ADD 1. LOAD • Data copied from memory to ALU register. 2. ADD • Data values are added in ALU adder circuitry. 3. COPY • Result stored in ALU accumulator. 4. STORE • Result copied from ALU to memory. 12 Execution cycle PROCESSOR C.U. Enter execute cycle AL U 85 + 65 = 150 3 Result copied to Accumulator 2 Data values Adder added 85 Register 65 Register 150 4 Accumulator 1 Data loaded to registers MEMORY 0101 1100 1111 1100 #000001 #000010 0111 1110 0111 1110 #000011 #000100 COMP 4—Power Tools for the Mind 85 data shown in base 10 #110011 …. 65 data shown in base 10 #111100 …. 150 #111110 …. Result stored in memory What’s in the box? 13 Boolean logic • Boolean algebra: solve complex math problems by reducing them to questions answered by Y/N. – Logic gate: Mathematical operation that handles only data that is represented in one of only two states: TRUE/FALSE – Physical gate: electronic device that transforms binary input(s) to produce a single binary output, according to its transformation rule. – Many computer binary circuits employ complex Boolean logic to perform various tasks. • Such circuits can test the truth of propositions by making comparisons, as well as carry out complex calculations. • Gates: AND, OR, NOT COMP 4—Power Tools for the Mind What’s in the box? 14 Truth tables STRICT Parents AND AND truth truth table table COOL Parents OR truth table Homework? Clean? Homework? Clean? 2 Inputs 1 Output 0 0 1 1 0 1 0 1 0 0 0 1 2 Inputs 1 Output 0 0 1 1 0 1 0 1 0 1 1 1 Stubborn child NOT truth table 1 Input 1 Output 0 1 1 0 – Engineers design binary computer circuits using gates based on truth tables. – Gates: building blocks of all digital devices! – Such circuits can carry out +, –, *, /, comparisons, and many complex operations. COMP 4—Power Tools for the Mind What’s in the box? 15 Digital Logic • Remember, the fundamental part of the digital computer is the switch. • A switch simply lets current either flow completely or not at all (binary) : – – – – Relay: mechanical switch Vacuum tubes: electronic switch Transistors: electronic switch ICs: use many transistors on a circuit board • Speed of computer determined by how fast a switch “switches”! COMP 4—Power Tools for the Mind What’s in the box? 16 Transistor How does a transistor work? If input voltage is low (binary 0) the switch opens and the current cannot pass through. Input Voltage Current trying to flow through Transistor (Switch) If input voltage is high (binary 1), the switch closes and the current passes through freely. COMP 4—Power Tools for the Mind What’s in the box? 17 Logic Gates Transistors are only switches. They simply determine whether or not current can pass through a wire. Transistors are used to create logic gates. These gates perform logic operations on the input voltages and give the result in form of an output voltage. Let’s construct a few simple gates to see how this works… COMP 4—Power Tools for the Mind What’s in the box? 18 NOT gate When input is low, then switch is open, and high current flows to output. Steady current (high) When input is high, then switch is closed, and the current is grounded (instead of passing into output), therefore the output voltage is low. Output Voltage Input Voltage logical operator: NOT Low input becomes high output. High input become low output. COMP 4—Power Tools for the Mind IN OUT 0 1 1 0 Ground (low) What’s in the box? 19 NAND gate (NOT AND) IN1 IN2 OUT 0 0 1 0 1 1 1 0 1 1 1 0 Input Voltage 1 Input Voltage 2 If both inputs are High, then current passes through, and output is Low. If either input is Low, switch closes, and output is High. Steady current (high) Ground (low) COMP 4—Power Tools for the Mind Output Voltage What’s in the box? 20 NOR gate (NOT OR) If both inputs are Low, then neither switch is closed, and Steady current output is High. (high) If either input is High, a switch opens, and output is Low. Output Voltage Input Voltage 1 IN1 IN2 OUT 0 0 1 0 1 0 1 0 0 1 1 0 Input Voltage 2 Ground (low) COMP 4—Power Tools for the Mind Ground (low) What’s in the box? 21 Digital Logic • We have only created NOT, NAND, and NOR. We wanted ANDs and ORs as well… • Just feed output of NAND and NOR into NOT gate. • Actually, all boolean logic expressions can be constructed just from NAND or from NOR – complete! • Demo: 4-bit half adder COMP 4—Power Tools for the Mind What’s in the box? 22 How can binary circuits ADD? 1 1 OR 1 AND Output Bit + 0 0 AND 0 NOT 1 1 You will not have to reconstruct diagrams! But you should know how to evaluate Boolean ops. 0 Carry Bit 16 How can binary circuits ADD? 1 1 OR 0 AND Output Bit + 1 1 0 AND 1 NOT 0 1 Carry Bit 17 What about other operations? • Subtraction – Addition using two’s complement representation • Multiplication – Repeated addition (with bit shifting) • Division – Repeated subtraction (with bit shifting) Basically, a computer just adds. COMP 4—Power Tools for the Mind What’s in the box? 25 Important digression! – What kind of code do programmers use? Source Code – What is the only kind of code a CPU Machine Code understands? or Object code – So, what has to occur before the CPU Translation can execute a source-code program? or Compilation High-level language (source code) Machine language (object code) e.g.: Turing Translator (Compiler) COMP 4—Power Tools for the Mind What’s in the box? 26 An Analogy Carbon-based unit JOHNNY, age 4 (obeys orders to transform raw material into tasty sandwiches.) Basic Instruction Set: Simple commands he can do: Go, Bring, Unwrap, … Child brain, hands, skills he uses. Grandpa’s commands: “Please make 2 p&j sandw’s on rye!” Mom: Grandpa language to Child Talk Child-talk instructions to Johnny that he understands: GO to pantry. BRING bread & PB to kitchen. UNWRAP bread. PULL OUT 4 slices …. 20 Silicon-based unit CPU (obeys orders to Johnny transform raw data into meaningful info.) STO; ADD; SUB; MUL; DIV; INC; CMP; JMP… what Johnny Basic Instruction Set: can do Primitive commands (log’l) it can do Computer circuits (phy’l) it uses Source code Grandpa’s command Answer := Num1 + Num2 Translator prog: Mom translates 0010 00001100 0111 0010 00001101 1000 0101 0111 1000 0010 0011 0010 00100001 as Source to object Object code (Low-level ML) Child talk so CPU can understand it: LOAD M012 R07 M013 R08 binary codes LOAD ADD R07 R08 R02 STORE R09 M033 21