programming-for-dummies

Figuring Out Programming 21
Converting source code with
an assembler or compiler
An editor lets you type and save program commands (source code) in a file.
Unless you’ve written a program completely in machine language, your source
code might as well have been written in Swahili because processors don’t
understand any language other than machine language.
So to convert your source code into machine language commands, you have
to use an assembler (if you wrote your program commands in assembly language)
or a compiler (if you wrote your program commands in the C language
or a high-level language, like BASIC).
Book I
Chapter 1
Getting Started
Programming a
Computer
After converting your source code into equivalent machine language commands,
an assembler or compiler saves these machine language commands
in a separate file, often called an executable file, or just an EXE file. When you
buy a program, such as a video game or an antivirus program, you’re really
buying an executable file. Without an assembler or a compiler, you can’t
create your program.
Compilers translate source code into machine language, which is the native
language of a specific processor. But what if you want your program to run
on different processors? To do this, you have to compile your program into
machine language for each different processor. You wind up with one executable
file for each processor, such as an executable file for an Intel Core
Duo 2 processor and a separate executable file for a PowerPC processor.
Many Macintosh programs advertise themselves as a Universal Binary — the
program actually consists of two executable files smashed into a single file:
✦ One executable file contains machine language code for the PowerPC
processor (used in older Macintosh computers).
✦ The second executable file contains machine language code for the Intel
processor (used in new Macintosh computers).
Most compilers work only on one specific operating system and processor.
So a Windows compiler can only create programs that run under the
Windows operating system. Likewise, a Linux compiler can only create programs
that run under the Linux operating system.
If you write a program that runs under Windows, you can re-compile it to run
under Linux. Unfortunately, you may have to modify your program slightly
(or a lot) to make it run under Linux.