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An editor lets you type and save program commands (or source code) in a file. Unless you’ve written a program completely in machine language, your source code may 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 Java).

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 processor and a separate executable file for an ARM processor.

Many Mac programs advertise themselves as a universal binary — which means the program actually consists of two executable files smashed into a single file:

 One executable file contains machine language code for the M-series processor (used in newer Mac computers)

 The second executable file contains machine language code for the Intel processor (used in old Mac computers)

Most compilers work only on one specific operating system and processor. So, a Windows compiler may only create programs that run under the Windows operating system. Likewise, a Linux compiler may only create programs that run under the Linux operating system.

If you write a program that runs under Windows, you can recompile it to run under Linux. Unfortunately, you may have to modify your program slightly (or a lot) to make it run under Linux.

Big companies, like Adobe and Microsoft, can afford to pay programmers to write and modify programs to run under different operating systems, such as macOS and Windows. Most smaller companies and individuals don’t have the time to rewrite a program to run under multiple operating systems. That’s why most small companies write programs for Windows — because it’s the largest market. If the program proves popular, they can later justify the time and expense to rewrite that program and compile it to run under macOS.

Choose your compiler carefully. If you use a compiler that can create only Windows programs, you may never be able to recompile that program to run on a different operating system, such as Linux or macOS. One reason Microsoft gives away its compilers for free is to trap people into writing programs that can run only under Windows. For example, if you write a program in C#, you may not be able to run that program on Linux or macOS without major modifications, which most people will probably never do.

To make it easy to create programs for multiple operating systems, you can use a cross-platform compiler. This means you can write a program once and then choose to compile it for two or more operating systems such as macOS and Windows or Android and iOS. Cross-platform tools make it easy to write the same program for multiple operating systems, but you may need to write additional code to take advantage of the unique features of each operating system.