some booting questions

[one]

Will the boot-loader code be of any use later on after it loads the secondary/kernel. If not then why do most people say that the bootloader code should be relocated in memory to prevent overwriting?
Any insights on this will be appreciated.Also why does evrybody use a JUMP instruction followed by a NOP at the start of the bootloader? is the NOP required?

[Pype.Clicker]

the NOP is required so that you have 3 bytes of instructions before the Bios Parameters Block (for FAT-compatible organizations)

i'm not sure to know what you refer to when talking of "relocation", but two things are certains:
[*] anything that things it is a bootsector must be loaded at 0x0000:0x7C00, thus if your bootsector needs at a moment to load something else (for instance, a MBR that must defer the boot to the partition's bootsector), it must not stay at 0x0000:0x7C00.
[*] the code you're executing to load the system must not be overwritten before it jump to the code it has loaded.

[Tim]

A small clarification: the BIOS can load the boot sector to any segment:offset pair which evaluates to 0x7C00 linear. Common combinations are 0000:7C00 and 07C0:0000. Therefore you should reload CS, DS and SS at the start to be certain:

Code: Select all

[org 0]
; ...
; ... BPB stuff
; ...

entry:
    jmp 0x7C00:.start
.start:
    mov ax, cs
    mov ds, ax
    mov ss, ax
    sub sp, sp

[HOS]

entry:
jmp 0x7C00:.start

i think you want

Code: Select all

entry:
    jmp 0x07C0:.start

[Tim]

Ahem. You're right.