Protected Mode

Protected mode is the 32 bit 'native' operating mode of Intel processors (and clones) since the 80386. It allows the developer to work with several virtual address spaces, each of which has 4GB of addressable memory and allows the system to enforce strict memory protection as well as restricting the available instruction set (so that your application cannot control the hard disk directly while the kernel can)

Protected mode unleashes the real power of your CPU, so you better get informed about it if you are considering writing an OS. However, it will prevent you from using virtually any of the BIOS interrupts (unless you have a V86 monitor).

Whether the CPU is in Real Mode or in protected mode is defined by the lowest bit of the CR0 register, so basically

;; make sure interrupts are disabled, etc.
mov eax, cr0
or al,1
mov cr0,eax

takes you to protected mode ... however you'll discover that there are many other things to be done before and after that operation to switch gracefully to pmode rather than resetting the CPU...

See Also

External Links

• OSRC: protected mode
• http://home.swipnet.se/smaffy/asm/info/embedded_pmode.pdf - pragmatic tutorial on pmode
• http://www.osdever.net/tutorials.php?cat=4&sort=1