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The Unabridged Pentium 4 IA32 Processor Genealogy by Bob Colwell, Tom Shanley

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Protected Mode Interrupt/Exception Handling

General

In Real Mode, the OS permits the processor to execute only a single, rather than multiple programs. In this case, all BIOS and disk services and interrupt service routines exist solely to support the program that is executing. This being the case, there is no need to restrict access to these services when the program executes a software interrupt instruction.

The Protected Mode environment, on the other hand, exists to support multitasking OSs. When the currently executing task attempts to access a procedure using the software interrupt instruction, the processor must check to ensure that this program is permitted to access the target procedure. If access is denied, a GP exception is generated. ...

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