Programmers who are used to DOS programming will need to read this section. Most DOS programmers are used to "real mode" programming, whereas DJGPP programs run in "protected mode". Unix programmers will understand protected mode programming, since they've always been in protected mode.
Real mode is when the CPU is running in 8086 compatible mode. Protected mode is when the CPU is running in, well, the not quite as compatible mode. The purpose of protected mode is not to protect your program. The purpose is to protect everyone else (including the operating system) from your program.
What does this mean to the programmer? Well, basically it means that you can't just expect that everything in the computer is there for you to mess with. You can't just take over an interrupt. You can't just change the video settings. You can't just change the CPU's operating mode.
Here are some definitions you'll need to know.
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