Recently im learning how to write a boot sector, here is the complete code that i am learning:
org 07c00h
mov ax, cs
mov ds, ax
mov es, ax
call DispStr
jmp $
DispStr:
mov ax, BootMessage
mov bp, ax
mov cx, 16
mov ax, 01301h
mov bx, 000ch
mov dl, 0
int 10h
ret
BootMessage: db "Hello, OS!"
times 510-($-$$) db 0
dw 0xaa55
a very simple code if you know how to boot a system. the result is a line Hello OS! displayed on the screen, the only thing that i dont know is the first line: org 07c00h.
The book tells me that the line of code let the compiler to locate the address to the 7c00h place, but the explanation is very ambiguous, and I really don't know whats the use of it here. what in the world does the line org 07c00h do here?
I tried to remove the line, and use nasm to create a bin file, then use the bochs to boot the bin file. Nothing different from the previous one: "hello OS!" displayed on the screen too.
Can i say that the first line does nothing here? What's the use of org xxxx?
It is where you have an assembler and linker in one step. The org tells the assembler which tells the linker (in these cases often the same program) where in physical memory space to put the code that follows. When you use a C compiler or some other high level language compiler you often have separate compile and link steps (although the compiler often calls the linker for you behind the scenes). The source is compiled to a position independent object file with some of the instructions left unimplemented waiting on the link step. The linker takes objects and a linker script or information from the user describing the memory space and from there then encodes the instructions for that memory space.
User786653 set it quite well it tells the assembler something it cant figure out on its own the memory space/address where these instructions are going to live in case there is a need to make position dependent encodings in the instructions. Also it uses that information in the output binary if it is a binary that includes address information, for example elf, srec, ihex, etc.