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March 05, 2008



It's great to see that "real mode" is still supported. My own operating system relies on some BIOS routines through vm86 mode, and until a BIOS is written to support protected mode, things aren't likely to change.

Maybe sometime in the future someone will write a BIOS that boots into protected mode, and provides some services for the bootloader and OS kernel to do their start-up routines. Really the only routines needed are disk access, screen output, and keyboard input. Once the kernel has control, it's own drivers can take over. The BIOS could use an internal IDT and GDT till the OS takes over.


I think you made a mistake in #5. Initial CS after reset on 386+ processors is F000 and the descriptor cache is FFFF0000 - the instruction pointer, EIP, being 0000FFF0.

Yuhong Bao

"The segment limits (64KB for real mode) are cached but are not reset when switch CS"
Should be "switching mode". BTW, this is often called "unreal mode".

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