CSCE 121 200 Fall 2013 Prof Jennifer L

CSCE 121 -200 Fall 2013 Prof. Jennifer L. Welch

Simplified Model of a Computer • • • How data is represented and stored How data is operated on How a program is stored How the program is executed on the data How programs are translated into machine language CSCE 121 -200 2

How Data is Represented • Fundamental unit of computer storage is a bit, data entity that can take on two values, 0 and 1 • Given a sequence of bits (01010001), what does it represent? • Depends on the code – Base 10 integer: 1, 010, 001 – Base 2 integer: 1*20 + 1*24 + 1*26 = 81 (in base 10) – ASCII: The letter ‘Q’ • Must know the code to decode a sequence • Other codes: – Negative numbers (e. g. , 2’s complement) – Numbers with fractional parts (floating point) CSCE 121 -200 3

How Data is Stored • Byte: a group of 8 bits; 28 = 256 possibilities – 0000, 00000001, …, 11111110, 1111 • Memory: long sequence of locations, each big enough to hold one byte, numbered 0, 1, 2, 3, … • Address: the number of the location • Contents of a location can change • Use consecutive locations to store longer sequences – e. g. , 4 bytes = 1 word 1 1 0 0 0 0 1 1 0 1 0 0 1 1 0 …. . byte 0 byte 1 CSCE 121 -200 byte 2 4

Limitations of Finite Data Encodings • Overflow: number is too large – suppose 1 byte stores integers in base 2, from 0 (0000) to 255 (1111). If the byte holds 255, then adding 1 to it results in 0, not 256 • Roundoff error: – insufficient precision (size of word): try to store 1/8, which is. 001 in base 2, with only two bits – nonterminating expansions in current base: try to store 1/3 in base 10, which is. 333… – nonterminating expansions in every base: irrational numbers such as π CSCE 121 -200 5

Kinds of Storage The computer reads and writes the data stored in it. From fastest to slowest: • cache: super-fast • main memory: random access, equally fast to access any address • disk: random access, but significantly slower than main memory • tape: sequential access, significantly slower than disk CSCE 121 -200 6

How Data is Operated On • Central processing unit (CPU) consists of: • arithmetic/logic unit (ALU): – performs operations on data (e. g. , add, multiply) • registers: special memory cells, hold data used by ALU, even faster than cache • control unit: – figures out what the ALU should do next – transfers data between main memory and registers CSCE 121 -200 7

Machine Instructions Goal: add the number stored in address 3 and the number stored in address 6; put the result in address 10. Control unit does the following: 1. copies data from main memory address 3 into some register, say 1: LOAD 3, 1 2. copies data in main memory address 6 into some register, say 4: LOAD 6, 4 3. tells ALU to add the contents of registers 1 and 4, and put result in some register, say 3: ADD 1, 4, 3 4. copies data in register 3 into main memory address 10: STORE 3, 10 LOAD, ADD and STORE are machine instructions. How does the control unit know which instruction is next? The program! CSCE 121 -200 8

How a Program is Stored • Program: list of machine instructions using some agreed upon coding convention. • Example: ADD 1 4 0 0 1 opcode 1 st operand 0 1 0 0 2 nd operand 3 0 0 1 1 3 rd operand • Program is stored same way data is stored! CSCE 121 -200 9

How a Program is Executed • The control unit has – instruction register: holds current instruction to be executed – program counter: holds address of next instruction in the program to be fetched from memory • Program counter tells where the computer is in the program. Usually the next instruction to execute is the next instruction in memory • Sometimes we want to JUMP to another instruction (e. g. , if or while) – unconditional JUMP: always jump to address given – conditional JUMP: only jump if a certain condition is true (e. g. , some register contains 0) CSCE 121 -200 10

Machine Cycle • fetch next instruction, as indicated by the program counter (PC), and increment PC • decode the bit pattern in the instruction register – figure out which circuitry needs to be activated to perform the specified instruction • execute the specified instruction by copying data into registers and activating the ALU to do the right thing – a JUMP may cause the PC to be altered CSCE 121 -200 11

Diagram of Architecture CPU: PC: R 1: IR: R 2: R 3: control unit ALU R 4: bus 0 main memory: 1 2 3 4 5 data … first instr. program second instr. … third instr. 95 96 CSCE 121 -200 97 98 99 100 … 12

Evolution of Programming Languages • Machine languages: all in binary – machine dependent – painful for people • Assembly languages: allow symbols for operators and addresses – still machine dependent – slightly less painful – assembler translates assembly language programs into machine language • High-level languages: such as Fortran, C, Java, C++ – machine independent – easier for people – compiler translates high-level language programs into machine language CSCE 121 -200 13

Compilation Challenges faced by a compiler: • one high-level instruction can correspond to several machine instructions – Ex: x = (y+3)/z; • fancier data structures, such as – arrays: must calculate addresses for references to array elements – structs: similar to arrays, but less regular • fancier control structures than JUMP – while, repeat, if-then-else, case/switch CSCE 121 -200 14

Compilation Challenges faced by a compiler: • functions (a. k. a. procedures, subroutines, methods): must generate machine language code to: – copy input parameter values – save return address – start executing at beginning of function code – copy output parameter values at the end – set PC to return address CSCE 121 -200 15

Compilation Process • Lexical analysis: break up strings of characters into logical components, called tokens, and discard comments, spaces, etc. – Ex: “total = sum + 55. 32” contains 5 tokens • Parsing: decide how the tokens are related – Ex: “sum + 55. 32” is an arithmetic expression, “total = sum + 55. 32” is an assignment statement • Code generation: generate machine instructions for each high-level instruction. • The resulting machine language program, called object code, is written to disk CSCE 121 -200 16

Linking • Linker: combines results of compiling different pieces of the program separately. If pieces refer to each other, these references cannot be resolved during independent compilation. • Combined code is written to disk. main … declares x … invokes p … function p … refers to x … CSCE 121 -200 17

Loading • Loader: when it’s time to run the program, the loader copies the object code from disk into main memory – location in main memory is determined by the operating system, not the programmer • Loader initializes PC to starting location of program and adjusts JUMP addresses • Result is an executable CSCE 121 -200 18