BA471 - Software

Stair & Reynolds: Software: Systems and Apps.

• Operating System: "software to manage system resources" (Stair & Reynolds, p. 146):
• File system.
• Memory allocation/deallocation.
• Task-on-time (multi-tasking).
• I/O.
• Security/access.
  
• Users (Stair & Reynolds, p. 145 (bottom): users ≠ people. Also, multi-user systems are not just "typical for larger, mainframe computers."
  

• Kernel + utility programs: Kernel loads into memory (Booting).
• Some brands:
• Windows xxx.
• OS2: IBM's answer to DOS/Windows - now defunct.
• UNIX: Many flavors (e.g., Sun-Solaris, HP-UX, Silicon Graphics-IRIX, IBM-AIX, DEC-ULTRIX).
• Linux: born on Sep. 17th, 1991, developed by Linus Torvalds while at Helsinki University of Technology.
• 1980s: Richard Stallman and the Free Software Foundation develop the GNU OS. By 1990 they have all the utilities but lack a kernel.
• 1991: Linus Torvalds builds a kernel, calls it Linux.
• Others distribute the Linux kernel with the GNU utilities and call the whole package 'Linux'.
• Richard Stallman insists that it should be called GNU/Linux.

• Mac OS X BSD (UNIX + Apple's desktop).
• Others: See Stair & Reynolds, pp. 149-156 & Table 4.3.

• Trends:
• 2002: UNIX (server) OS expenditures > Windows/Linux combined.
• 2007: Windows and Linux catch up.

• Why is Linux welcome (mostly on the server)?
• Allows equipment savings while retaining UNIX advantages (e.g., United Parcel Service).
• Allows UNIX advantages on Intel architectures.
• Creates competition ==> forces quality improvements across the board.
  

• Third party server/OS hosting:

• Economies of scale:

• Optimized hardware infrastructure (machines, connections, UPS, system backups, etc.)
• Optimized skills.
• Optimized disaster control.
• Frees up resources for core competency.
• Incur only variable cost; e.g., Sun Grid Computing. (1$/CPUhour).

• Programming: Stair & Reynolds (p. 171): "constructing statements that communicate instructions to be executed by a computer."
• How to let a computer greet the world?
• VB.NET:

Public Class Hello

  Shared Sub Main()
    System.Console.WriteLine("Hello, world...")
  End Sub

End Class

• C:

#include <stdio.h>

void main(void)
{
 printf("\n\nHello, world...\n\n");
}

• Your computer as a binary state machine: programming as specifying binary state transitions.
• Generations of programming languages (Stair & Reynolds, p. 172): 
• Machine language (first generation). 
• Assembly language (second generation). 
• High-level procedural (third generation): 
• FORTRAN (science & engineering).
• COBOL (admin.).
• C (System software, modern application software). 
• C++/Java /.NET languages (VB, C#, J#) (object-oriented programming).
• Perl, Python, Tcl, PHP (scripting languages).
  
• Lisp and Prolog (artificial intelligence).
• ...and thousands more.

• Fourth-generation: 

• Dedicated, domain-specific languages.

• Structured Query Language (SQL).

• Extensible Markup Language (XML) Query language.

• Fifth generation: natural language based, speculative, rather experimental.

• Compiled, interpreted and byte-compiled languages (Stair & Reynolds. p. 173).
  

• Compiler: takes all program statements and translates them into machine language (e.g., FORTRAN, C/C++).
• Interpreter: takes a program statement, translates it and submits to the CPU for execution (e.g., Tcl, Perl, Python, PHP).
• Some consequences:

• Interpreted code gets translated every time it is run. Compiled code gets translated only once --> compiled codes run faster.
• Interpreted code requires fewer lines since it has less stringent syntactic rules --> is written faster.
  
• To port your code to another platform you need a compiler or interpreter for that platform.
• Maintaining compiled code for multiple platforms is more work than maintaining interpreted code for multiple platforms.
  

• The case of Java: byte-compiled:

• Java compiler compiles down to platform-independent binary, nonmachine language (!!) code.
• Java Virtual Machine (JVM) interpreter translates each byte-compiled instruction to machine language and submits to the CPU for execution.
• Note: requires the JVM on each platform. JVMs are made available by platform manufacturers ==> 'high transportability.'
• Stair & Reynolds, p. 174 "Java is a programming language developed by Sun Microsystems that can run on any OS and on the Internet."