Transition from Protected Mode to Long Mode (x86_64)

Topic Overview

Long mode transition

x86_64 requires protected mode with paging enabled, then activation of PAE and an LM-compatible page table before setting EFER.LME and far jumping to 64-bit code. For example, after MOV EAX, CR4; OR EAX, 0x20 enables PAE, CR3 must reference PML4 tables mapping your kernel.

Steps after protected mode

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• GDT: Needs 64-bit code descriptor (L bit set)
• Compatibility submode: First far jmp may land in compat before 64-bit CS
• Discard: Real mode and 16-bit segments after long mode entry
• Stack and IDT must use 64-bit handlers (IRETQ)

Compatibility submode

The first far jump after enabling LME may land in compatibility mode if the segment is a 32-bit code descriptor; a second far jump with a 64-bit CS clears compatibility state. Ensure PAE page tables include the high-half addresses if the kernel links above 2 GB. IRETQ is required for interrupts once running in true 64-bit mode.

Why for this exercise

You will do the address arithmetic long mode is built around: split a 48-bit virtual address across the four paging levels (PML4, PDPT, PD, PT: nine bits each), enforce canonical form, and compute physical addresses from installed mappings. This exercise requires PAE on before LME and a valid long-mode GDT entry for the far jump target.

Simulate x86_64 four-level paging address handling in C, driven by a command script on stdin.

Protocol (all numbers in hex, addresses treated as 64-bit values):

• split <vaddr> First check canonical form: bits 63-48 must all equal bit 47. If not, print "NONCANONICAL". Otherwise print "PML4=%llu PDPT=%llu PD=%llu PT=%llu OFF=0x%03llX" with the four 9-bit indices (vaddr >> 39, >> 30, >> 21, >> 12, each & 0x1FF) and the 12-bit offset.
• map <vaddr> <frame> Record one 4 KiB mapping for the page containing vaddr (frame up to 48 bits) and print "MAPPED PML4=%llu PDPT=%llu PD=%llu PT=%llu FRAME=0x%llX".
• trans <vaddr> Check canonical form first (print "NONCANONICAL" and skip if not). Then walk the four levels: if no mapping exists for the page, print "PAGEFAULT"; otherwise print "PHYS=0x%012llX" where the physical address is (frame << 12) | offset.

Process every command on stdin until EOF, printing one line per command.