section .realmode global rm_hcf rm_hcf: ; Load BIOS IVT lidt [.rm_idt] ; Jump to real mode jmp 0x08:.bits16 .bits16: bits 16 mov ax, 0x10 mov ds, ax mov es, ax mov fs, ax mov gs, ax mov ss, ax mov eax, cr0 btr ax, 0 mov cr0, eax jmp 0x00:.cszero .cszero: xor ax, ax mov ds, ax mov es, ax mov fs, ax mov gs, ax mov ss, ax sti .hang: hlt jmp .hang bits 32 .rm_idt: dw 0x3ff dd 0 global rm_int rm_int: ; Self-modifying code: int $int_no mov eax, dword [esp+4] mov eax, dword [0 + eax*4] mov dword [.int_offset], eax ; Save out_regs mov eax, dword [esp+8] mov dword [.out_regs], eax ; Save in_regs mov eax, dword [esp+12] mov dword [.in_regs], eax ; Save GDT in case BIOS overwrites it sgdt [.gdt] ; Save IDT sidt [.idt] ; Load BIOS IVT lidt [.rm_idt] ; Save non-scratch GPRs push ebx push esi push edi push ebp ; Jump to real mode jmp 0x08:.bits16 .bits16: bits 16 mov ax, 0x10 mov ds, ax mov es, ax mov fs, ax mov gs, ax mov ss, ax mov eax, cr0 and al, 0xfe mov cr0, eax jmp 0x00:.cszero .cszero: xor ax, ax mov ss, ax ; Load in_regs mov dword [ss:.esp], esp mov esp, dword [ss:.in_regs] pop gs pop fs pop es pop ds popfd pop ebp pop edi pop esi pop edx pop ecx pop ebx pop eax mov esp, dword [ss:.esp] ; Indirect interrupt call pushf call far [ss:.int_offset] ; Load out_regs mov dword [ss:.esp], esp mov esp, dword [ss:.out_regs] lea esp, [esp + 10*4] push eax push ebx push ecx push edx push esi push edi push ebp pushfd push ds push es push fs push gs mov esp, dword [ss:.esp] ; Restore GDT o32 lgdt [ss:.gdt] ; Restore IDT o32 lidt [ss:.idt] ; Jump back to pmode mov eax, cr0 or al, 1 mov cr0, eax jmp 0x18:.bits32 .bits32: bits 32 mov ax, 0x20 mov ds, ax mov es, ax mov fs, ax mov gs, ax mov ss, ax ; Restore non-scratch GPRs pop ebp pop edi pop esi pop ebx ; Exit ret align 16 .esp: dd 0 .out_regs: dd 0 .in_regs: dd 0 .gdt: dq 0 .idt: dq 0 .rm_idt: dw 0x3ff dd 0 dw 0 .int_offset: dd 0