II Esercitazione: S.O. & Fortran 90

Transcription

1 II Esercitazione: S.O. & Fortran 90 Francesco Battista Laboratorio di Calcolo di Aerodinamica a.a. 2016/17 DIMA, Sapienza University of Rome, Italy April 4, 2017

2 Outlines 1 Short description of computer architectures 2 how to use the compiler: - Ubuntu/Mac: terminal - Windows: MS-DOS/prompt command 3 Introduction to programming languages: Fortran 90

3 Computer architecture I Von Neumann architecture (1945) the information quantity is measured in Bit Bit (binary quantity 1/0, on/off) 8 bit = Byte

4 Computer architecture I - CPU/Processor: instruction frequency or clock frequency GHz; memory representation 32/64 bit (Intel, Pentium,... ) - Main memory: fast, not persistent and expensive (up to 16Gb). The most famous is the RAM (Random Access Memory read/write) - Secondary memory: slow, persistent and not expensive. Magnetic Hard Disk (fino a 4Tb). Optical CD, CD-ROM o DVD. - Input devices: keyboard, mouse, microphone, camera... - Output devices: screen, printer, loudspeaker...

5 Fundamental of Programming The CPU understands only binary commands. It is possible to speak with the CPU with a language more complex than the binary one: Language Level use machine very low never assembler low only for high efficiency Fortran, C, C++, Java high most part of applications familiar very high not yet ready different programs allow to write a code and to execute it we need a compiler and a text editor: 1 Ubuntu: gedit, or vim 2 Window: note pad or editor (from DOS) 3 Mac: textedit, or vim

6 Installation of compiler The compiler is not a software, but a translator the compiler can be downloaded from internet (look for gfortran ) to control if it works open the terminal digit gfortran and press enter if the message is gfortran: no input files the compiler is ok

7 Write a code: 4 steps - writing: write the program with an editor save on disk with extension.f90 - Compile: translate from fortran language to binary (fog CPU) the compiler i gfortran a file with extension.o is obtained - Link: translate from fortran language to binary (fog CPU) the linker is generally embeded in the compiler an executable file is obtained (Unix: without extension, Windows: extension.exe) - Execution and test: the program has to be executed (see after) if the execution is correct the program works otherwise start from beginning

8 Why Fortran? the first programming language with a compiler (1954), has been upgraded up to 2013 PRO: extensively used for numerical calculus, with suitable libraries CON: in some cases old philosophy (anni 50) alternative languages: C, C++ The version fortran 90 is not the older one. Most of the arguments presented in the course will work also with the other versions.

9 Why fortran? strongly oriented to the number crunching: long and complex calculus. data and operations are naturally mapped on the CPUs the writing rules allow optimisation in programming step new data types can easily be defined compact definition of the vectors and optimised operations among vectors

10 PROS & CONS The present standard includes 4 different languages all independent it is complex to execute not numerical statements C programmers are more than the Fortran one. GUI (Graphics Unit Interface) e DB (Data Base) accesses written in C Optimised for numerical calculus older and rigid than C, hence the compilers better optimise the code better in the use of type defined by user the object oriented programming can be used allow the use of the libraries written in other languages

11 The first program 1! File : hello. f90 2! The first program in Fortran 90 3! This code print a message on the screen 4 PROGRAM hello 5 6! Executive section 7 WRITE (*,*) Hello world! 8 9! End section 10 STOP 11 END PROGRAM hello not overcome column 71, use & to continue to the following row symbol! precedes the comments fortran is case INseNSItiVE

12 The first program The compiler ignores the text after symbol! up to the end of the line: comments comments are optionals, but they should be optimised: a huge number of useless comments is not recommended. how should you use the comments? variable meaning algorithm hypoteses Avoid obvious comments

13 Unità del programma: Programma principale A fortran code is organised in units - main unit - procedures (subroutines or functions) - modules - libraries... The main unit (only one) - it is automatically evoked at the execution of the program - it starts with PROGRAM program-name - it has to end with END, followed by PROGRAM program-name (optional) - it is good practice to always end the unit with type and name of 1! File : hello. f90 2! The first program in Fortran 90 3! This code print a message on the screen 4 PROGRAM hello 5 6! Executive section 7 WRITE (*,*) Hello world! 8 9! End section 10 STOP 11 END PROGRAM hello

14 The second code 1! File : assegnazione. f90 2! This program reads and prints a number on the screen 3 PROGRAM assegnazione 4 5! declaration section 6 IMPLICIT NONE 7 INTEGER :: i 8 9! execution section 10 WRITE (*,*) write and integer on the screen 11 READ (*,*) i WRITE (*,*) You write,i 14 15! end section 16 STOP 17 END PROGRAM assegnazione

15 Second code description declaration section first part of the code code name, and declaration of constants and variables not executable operation but needed for compiling and execution executive section heart of the code executable instruction variables declared in the declaration section are employed End section closure of the code

16 Declaration section INTEGRER:: i declaration of a variable integer - it assigns the name to the memory part where values are stored - the declaration need of types, attributes (optional) and a comma-separated list of the variable names. features of the name of the variables - allowed characters: a-z, A-Z, 0-9, _ - the first character has to be a letter - max name length is 31 characters it is good-practice to use the optional command IMPLICIT NONE clearer and more readable code: each variable is characterised typos are found by the compiler In the next lesson more detail will be provided about the declaration section

17 Execution section: Input/output command writing/reading operation of user - READ(*,*) and READ *, reading - WRITE(*,*) and PRINT *, writing these commands provide formatted data in input/output - formatted means that the code translates from the user text to the internal binary format - all the internal and external formats depend on the declared type READ(*,*) and READ *, are equivalent WRITE(*,*) and PRINT *, are equivalent

18 how to compile the code As soon as the code is edited it have to be compiled to obtain the executable gfortran name_file.f90 the file a.out is obtained gfortran -o name_exe name_file.f90 the file nome_exe is obtained program execution: Ubuntu/Mac:./a.out or./nome_exe Windows: a.out or nome_exe