fernlader2/src/nbp.S

            // vim: ft=gas:et:sw=12:ts=12:sts=12

            .include    "src/mode.S"

            .global     _start
            
            .text
            .code16
            // _start: entry point
_start:     cli
            cld

            mov         %ss, %ax
            mov         %ax, %cs:real_ss

            xor         %eax, %eax
            mov         %es, %ax
            shl         $4, %eax
            movzwl      %bx, %ebx
            add         %eax, %ebx
            mov         %ebx, %cs:PXENV

            xor         %eax, %eax
            xor         %ebx, %ebx
            mov         %sp, %bx
            mov         %ss:6(%bx), %ax
            mov         %ss:4(%bx), %bx
            shl         $4, %eax
            add         %eax, %ebx
            mov         %ebx, %cs:exPXE

            mov         %sp, %bp
            xor         %ax, %ax
            mov         %ax, %ds
            mov         %ax, %es

            // initialize our own BSS section
            mov         $_bss_start, %di
            mov         $_bss_end, %cx
            sub         %di, %cx
            xor         %al, %al
            rep stosb

            // a20_enable: Allow use of 'high' (>1Mb) memory
a20_enable: // Of all the ways to toggle A20, we only try the Fast A20 Gate.
            // This is known to cause problems on some ancient systems,
            // but as our bootloader exclusively runs on 64-bit machines,
            // we should not run into any of those systems.
            // Modern machines apparently don't even have the A20 line anymore.
            inb         $0x92, %al
            or          $2, %al
            outb        %al, $0x92

            // prot_enter: Set up GDT, switch into 32-bit protected mode.
prot_enter: lgdt        GDT_PTR

            PROT
            PROT32

            .code32
            jmp         main

            .data

            .global     PXENV
PXENV:      .long       0

            .global     exPXE
exPXE:      .long       0

            .global     real_ss
real_ss:    .word       0

            .global     GDT
GDT:        // entry 0: null descriptor
            .space      8, 0
            // entry 1: 16-bit code segment
            .byte       0xFF, 0xFF, 0, 0, 0, 0b10011010, 0x8F, 0
            // entry 2: 16-bit data segment
            .byte       0xFF, 0xFF, 0, 0, 0, 0b10010010, 0x8F, 0
            // entry 3: 32-bit code segment
            .byte       0xFF, 0xFF, 0, 0, 0, 0b10011010, 0xCF, 0
            // entry 4: 32-bit data segment
            .byte       0xFF, 0xFF, 0, 0, 0, 0b10010010, 0xCF, 0
            // TODO: 32-bit TSS
            .set        GDT_SIZE, . - GDT
GDT_PTR:    .word       GDT_SIZE - 1
            .word       GDT
            .word       0, 0, 0