Introduction 1 (Read Chap. 1) What is Programming? For some given problem: design a solution for it -- identify, organize & store the problem's data -- develop algorithms (procedures) to process it; code this solution as a program; test the program; maintain the program (fix it, upgrade it, …). What is a program? — a collection of statements that — implement the design plan, and — are written in a programming language — a language that the computer can understand. CS 104: Applied C++ OCD in C++ debug "real world"

What kinds of statements do computers understand? A computer only understands a language specially designed for it called machine language. Machine-language statements are stored in a computer’s memory, which is a sequence of two-state devices (on-off switches). They are retrieved from memory and executed one at a time. The "character set" for machine language: __ representing "off" __ representing "on" 2 RISC CISC 0 1

Machine language programs thus consists of strings of bits ( ) Example (hypothetical) binary digits

4 Bits are usually grouped into bytes (8 bits) and words (e.g., 32 or 64 bits). Each machine language instruction might be stored in one word. opcode instruction #1 first operandsecond operand e.g., "Store" 1 in memory location with this address

5 So a sequence of machine language instructions might be stored in a sequence of consecutive words instruction #1 #4 #3 #2

SPARC executable machine code … this goes on for another 1600 lines... Intel Pentium executable machine code … this goes on for another 74 lines... A Real Machine-Language Example 6 int main() { int x, y; x = 1; y = x + 2; return 0; } C++

Early Computers Required a programmer to write in machine language. –Very easy to make mistakes! –And they were hard to find! –Programs were not portable. They could only be run on one kind of machine! Result: programming was very difficult and programs weren't widely used. 7

An Early Innovation Create a machine-language program called an assembler to input each assembly language instruction and translate it into machine language LOAD x ADD 2 STORE y Assembler 8 Devise a set of mnemonics (abbreviations), one for each machine language instruction; this was called an assembly language.

Intel Pentium assembly language: _main: pushl %ebp movl %esp,%ebp subl $24,%esp call ___main movl $1,-4(%ebp) movl -4(%ebp),%eax addl $2,%eax movl %eax,-8(%ebp) xorl %eax,%eax jmp L2.align 4 L2: movl %ebp,%esp popl %ebp ret Intel Assembler 9 The Real Example SPARC assembly language: main: save%sp, -120, %sp mov1, %o0 st%o0, [%fp-20] ld[%fp-20], %o0 add%o0, 2, %o1 st%o1, [%fp-24] mov0, %i0 b.LL2 nop mov0, %i0 b.LL2 nop.LL2: ret restore Sun Assembler int main() { int x, y; x = 1; y = x + 2; return 0; }

Assembly Languages Allowed a programmer to use mnemonics, which were more natural than binary. +Much easier to read programs +And much easier to find and fix mistakes –Still not portable to different machines –Still quite difficult to write, read, and debug programs 10

Next Major Advance: High Level Languages & Compilers To improve on assembly language: Devise a set of statements called a high-level language that are closer to human language and methods of writing expressions and a program called a compiler to translate them into machine language. 11 Why not just use human language? It’s too complex and ambiguous; e.g., “Time flies like an arrow” 1950s FORTRAN COBOL LISP

Compilers vs. Assemblers An assembler translates one assembly-language statement into one machine-language statement. A compiler translates one high-level statement into multiple machine-language statements, so it is much more difficult to write a correct compiler than an assembler. LOAD b ADD c STORE temp1 LOAD a MULT temp1 STORE z Assembler z = a * (b + c); Compiler 12

Advantages of High-Level Languages With programming in high-level languages (e.g., C++): +Programs are much easier to read. +Mistakes are much easier to find and fix. +Programs are (or can be) portable from one machine to another (provided they conform to the language standard). Just need a compiler for that language written in the machine language of that machine.  Not so simple that just anyone can use them (otherwise this course wouldn't exist) 13

Objectives in Programming A program to solve a problem should be: +correct (it actually solves the problem) +readable (understandable by another person) +user-friendly (designed in a way that is easy for its user to use). 14 Grading criteria Later +efficient (doesn’t waste time or space)

Fredrick P. Brooks, Jr. (1931- ) We enjoy designing things because we are created in the image of God. The woes of programming: Products become obsolete too quickly. 15 The computer is a powerful and rewarding tool to use. The joys of programming: The “mindless” details can be excessively tedious.