Overview of Boot Process

To get to Long Mode we need to follow the sequence:

• Load code into memory
• Relocate code and data to the appropriate locations
• Set up a Global Descriptor Table
• Switch to Protected Mode
• Set up an Interrupt Descriptor Table
• Set up some Page Descriptor Tables
• Enable Long Mode
• Enable Paging

Note the last three steps - unlike Protected Mode, if you want to use Long Mode then you must use Paging; even after you enable Long Mode it doesn't take effect until you also enable Paging. In many ways that's the most difficult part of getting the cpu working as a 64-bit processor.

The code covering this process can be found in the files:

The code in these files runs in 16-bit or 32-bit mode and is concerned with setting the system up to transfer to 64-bit mode. The final instruction in startup.s is a jump to the start of the 64-bit code in os.s.