simple question, but please answer

[elias]

ive seen many boot loaders with either ORG [0] or ORG [07C0], if might not be 07C0, but some other numbers, btu ims ure you guys know wat i mean. now i question is, where is the boot coded loaded to? 07C0:0000 or 0000:07C0?

[dronkit]

In real mode an absolute memory address is calculated multiplying the segment value by 16 and adding the offset, so:

0000:7c00 = (0000h * 16) + 7c00h = 7c00h
07c0:0000 = (07c0h * 16) + 0h = 7c00h

they reference to the exact same memory position

[Friend]

Basically, there are many different segment:offset pairs that the bios might load the code to, the best idea is to simply do a jmp at the start of your boot code and this will force cs=segment for example

jmp 0x0000:Start_Boot

would force cs=0x0000 and then jump to Start_Boot, so whatever segment you want to have simply put that first and it should load cs with that value.

hope this helps.

[Tom]

org 0 tellsNASM that the FritzOS bootsector is at org 0 - but it's not. So I do a jump from 0x7c00:start i.e. jump from 0x7C00 to the start lable.

That's just one way to load a bootsector...

[elias]

so i can have either
ORG [0]
or
ORG [07C0]?
as long as i jump to the first portion of code?

[Tom]

if you have a org 0 you have to do a "jmp 0x7C00:start"

if have a org 0x7c00 you can just do "jmp start"

[elias]

ok thanx, now jsut one more thing. is the address 7C00 or 07C0?

[Tom]

I typed it in correctly "0x7C00"

[grey wolf]

the correct addresses are 0x07C0:0x0000 and 0x0000:0x7C00.

so with ORG 0x0, you need your CS to be 0x07C0 ( jmp 0x07C0:start ) and with ORG 0x7C00 your CS will be 0x0 ( jmp 0x0000:0x7C00 )