hi
i was converting an ordinary floppy into bootable via the following code which i downloaded from some site i dont remember. it works perfectly well but i m unable to understand some parts of the assembly code .if some one will please explain my questioins
source: (there is a makefile also)
------------
.code16
.text
.org 0x00

.global _start <============== why this "start" begins with an underscore while other labels do not?
_start:
mov $0x07C0,%ax
mov %ax,%ds

call booting
xor %dx,%dx
call working
call idleloop
ret

booting:
mov $boot,%si
call message
call nl
ret

message:
cld
lodsb
or %al,%al
jz done
mov $0xE,%ah
mov $7,%bx
int $0x10
jmp message

done:
ret

nl:
mov $newline,%si
call message
ret

working:
call nl
call nl
mov $device,%si
call message
call waitenter
inc %dx
cmp $3,%dx
je keyboard
jmp working

waitenter:
call nl
movw $cont,%si
call message
xor %ax,%ax
int $0x16
cmp 'T',%al
jz taz
cmp 't',%al
jz taz
cmp $13,%al
jnz waitenter
ret

keyboard:
call nl
call nl
movw $kbd,%si
call message
call nl
movw cont,%si
call message
ret

taz:
call nl
call nl
movw tazq,%si
call message
jmp waitenter

idleloop:
hlt
jmp idleloop

boot:
.ascii "Loading UniOS 1.1 Beta..."
newline:
.byte 10, 13, 0
device:
.ascii "Device not responding to SCSI command."
cont:
.ascii "Hit ENTER to continue..."
kbd:
.ascii "Keyboard not responding to SCSI command."
tazq:
.ascii "Yes, TazQ is an Internet crack monkey on wheels."


<========================== what is the function of the following few lines?
padlen:
.byte boottag - .

.fill 10,1,0


.org 510
boottag:
.word 0xAA55

=============== it seems the above few lines are writing AA55 to 510... th byte of boot sector which ofcourse will tell the BIOS that it is a bootable.but if these three lines can do the job why do we need the whole program?they r never referenced in the program will only this three lines make it a bootable?

thanks in advance.