fernlader/lboot.S

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

            .global     _start
            .text
            .code16

_start:     cli
            cld

            // we keep our text and data close to each other
            xor         %ax, %ax
            mov         %ax, %ds

            call        init_com1

            mov         $msg_start, %si
            call        print

            mov         $msg_a20, %si
            call        print
            call        enable_a20

            mov         $msg_unreal, %si
            call        print
            call        unreal

            mov         $msg_memmap, %si
            call        print
            call        memmap

            mov         $msg_fin, %si
            call        print
1:          hlt
            jmp         1b

.set        COM1,       0x3F8
.macro      com1_write  offset=0, byte
            mov         $COM1+\offset, %dx
            mov         $\byte, %al
            outb        %al, %dx
.endm

init_com1:  com1_write  1, 0x00 // clear interrupts
            com1_write  3, 0x80 // set DLAB to 1
            com1_write  0, 0x0C // 9600 baud rate
            com1_write  1, 0x00
            com1_write  3, 0x07 // 8 bit data + 1 parity bit
            ret

enable_a20: // TODO more thorough implementation
            inb         $0x92, %al
            or          $2, %al
            outb        %al, $0x92
            ret

unreal:     push        %ds
            lgdt        gdt_ptr

            mov         %cr0, %eax
            or          $0x01, %al
            mov         %eax, %cr0
            ljmp        $0x8, $1f

1:          mov         $0x10, %cx
            mov         %cx, %ds
            
            and         $0xFE, %al
            mov         %eax, %cr0
            ljmp        $0x0, $2f

2:          pop         %ds
            ret

memmap:     push        %bp
            mov         %sp, %bp
            sub         $24, %sp

            mov         %ss, %ax
            mov         %ax, %es
            mov         %sp, %di
            xor         %ebx, %ebx
            mov         $0x534D4150, %edx
            mov         $24, %ecx
            mov         $0xE820, %eax
            int         $0x15

            mov         %bp, %sp
            pop         %bp
            ret

print:      xor         %bx, %bx

1:          mov         $COM1+5, %dx
            inb         %dx, %al
            test        $0x20, %al
            jz          1b

            lodsb
            or          %al, %al
            jz          2f

            mov         $COM1, %dx
            outb        %al, %dx

            mov         $0x0E, %ah
            int         $0x10
            jmp         1b

2:          ret

gdt:        // entry 0: null descriptor
            .word       0
            .word       0
            .byte       0
            .byte       0
            .byte       0
            .byte       0
            // entry 1: code segment
            .word       0xFFFF
            .word       0
            .byte       0
            .byte       0b10011010
            .byte       0x8F
            .byte       0
            // entry 2: data segment
            .word       0xFFFF
            .word       0
            .byte       0
            .byte       0b10010010
            .byte       0x8F
            .byte       0
            .set        gdt_size, .-gdt
gdt_ptr:    .word       gdt_size-1
            .long       gdt

msg_start:  .asciz      "Netboot via fernlader v1 ...\r\n"
msg_a20:    .asciz      " * Enabling A20\r\n"
msg_unreal: .asciz      " * Unreal Mode\r\n"
msg_memmap: .asciz      " * Fetching Memory Map\r\n"
msg_fin:    .asciz      "Finished.\r\n"