GNU GAS: Label is not relatively referenced

Question

I am writing a little bootsector for learning purpose.

Here is boot.S

.code16
.text
    movw    $0xB800, %ax    /* 0xB000 is the text screen video memory         


        

Answer 1

Of course it is possible to relocate strings.

First of all, your -Ttext 0x7C00 is correct. Do not change it. At the beginning of your bootloader you should zero the segment registers (%ds, %es, %fs, %gs and %ss):

mov $0, %ax         // xor %ax, %ax would be more space efficient
mov %ax, %ds
mov %ax, %es
mov %ax, %fs
mov %ax, %gs
mov %ax, %ss

Next, you should set up a stack. Read: osdev wiki about stacks

Now to your question: It is usual to address strings using segmentation in the form of %ds:%si. When relocating the bootloader, just change %ds properly.
Assuming you call your string label:

mov $label, %si     // Always prefix a label address with a $

You can get a character from a string using the lodsb instruction (LOaD String Byte, which automatically increments %si for you:

lodsb               // Character is stored in `%al` now

Another recommendation is not to address the video memory manually. Use BIOS Interrupt 0x10.

Good luck with your bootloader!