Tutorial by Examples

When Intel designed the original x86, the 8086 (and 8088 derivative), they included Segmentation to allow the 16-bit processor to access more than 16 bits worth of address. They did this by making the 16-bit addresses be relative to a given 16-bit Segment Register, of which they defined four: Code S...

Introduction When the 80286 was invented, it supported the legacy 8086 Segmentation (now called "Real Mode"), and added a new mode called "Protected Mode". This mode has been in every x86 processor since, albeit enhanced with various improvements such as 32- and 64-bit addressin...

Switching into Protected Mode is easy: you just need to set a single bit in a Control Register. But staying in Protected Mode, without the CPU throwing up its hands and resetting itself due to not knowing what to do next, takes a lot of preparation. In short, the steps required are as follows: ...

The unreal mode exploits two facts on how both Intel and AMD processors load and save the information to describe a segment. The processor caches the descriptor information fetched during a move in a selector register in protected mode. These information are stored in an architectural invisibl...