Linking first and second stage bootloader

Question

I am writing a two stage bootloader Here is my boot.asm

[org 0x7c00]
[bits 16]
xor ax, ax
mov ds, ax
mov es, ax
xor bx, bx
mov ah, 0x0E
mov al, 'A'
int 0x10
jmp 0x8000
cli
hlt
times 510 - ($-$$) db 0
dw 0xAA55

And boot2.asm

[org 0x8000]
[bits 16]
xor ax, ax
mov ds, ax
mov es, ax
xor bx, bx
mov ah, 0x0E
mov al, 'B'
int 0x10

I compile it using

nasm -f bin -o boot.bin boot.asm
nasm -f bin -o boot2.bin boot2.asm

It compiles without any error or warning. But how will I put stage 2 at 0x8000 and link stage1 and stage2 to work together?

According to your code stage2 is as 0x7e00 not 0x8000. Your first stage has to read the second stage into memory using something like [Int 13/ah=2](http://www.ctyme.com/intr/rb-0607.htm) — Michael Petch, Apr 03 '19 at 05:27
This is an example of a first stage reading a second stage and using NASM to generate a 1.44MB disk image: https://stackoverflow.com/a/54894586/3857942 — Michael Petch, Apr 03 '19 at 05:34
Oh I am developing on android phone!!! You must be surprised but its true, im a using termux terminal emulator for android. — Gyan Prakash, Apr 23 '19 at 15:06

score 2 · Answer 1 · edited May 09 '19 at 17:15

Likely you are asking how to combine the first and second stage into one file. If so:
On Linux:

cat boot.bin boot2.bin > final_file.file_format

On Windows:

copy /b boot.bin+boot2.bin  final_file.file_format

To load the second stage from the bootloader you can use the following code:

mov ah, 0x02      ; Read disk BIOS call 
mov cl, 0x02      ; sector to start reading from
mov al, 1         ; number of sectors that will be read (modify if your second stage grows)
mov ch, 0x00      ; cylinder number
mov dh, 0x00      ; head number
xor bx, bx
mov es, bx        ; ES=0x0000
mov bx, 0x8000    ; ES:BX(0x0000:0x8000) forms complete address to read sectors to
; DL should contain the boot disk number passed to the bootloader by the BIOS
int 0x13          ; Make BIOS disk services call (Int 0x13/AH=2) to read sectors
; For simplicity assume the disk read was successful
jmp 0x0000:0x8000 ; FAR JMP to second stage and ensure CS=0x0000
                  ; since CS is not guaranteed to be 0x0000 when control is transferred
                  ; to our bootloader

Martin Rosenau · Answer 2 · 2023-05-26T05:39:29.933

1

But how will I put stage 2 at 0x8000 ...

Unfortunately, I don't use "nasm" but other assemblers. But I'd expect that you have to change [org 0x7e00] to [org 0x8000].

... and link stage1 and stage2 to work together?

That's not as easy as you think:

The BIOS will load one sector (510 bytes plus 2 bytes 0xAA55) into the memory at 0x7C00. Using a normal BIOS there is no possibility to load more data!

The code in these 510 bytes ("stage 1") has to load the "state 2" into the memory: It may use functions ah=2 or ah=0x42 of int 0x13 to do so.

If you have your own floppy format, this is quite simple:

You store "stage 2" in the second sector of the floppy disk and load the second sector.

If you want to load "stage 2" from a file system (e.g. from a file from a FAT-formatted disk), this is more tricky.

edited May 26 '23 at 05:39

answered Apr 03 '19 at 05:44

Martin Rosenau

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Its OK, But how will i do this for example make.sh – Gyan Prakash Apr 03 '19 at 07:39
*Unfortunately, I don't use "masm" but other assemblers* - Neither does the OP, this is a NASM question. – Peter Cordes May 26 '23 at 02:08
1

@PeterCordes It seems like this was a typo of mine. I should have written "**n**asm". I corrected that typo. I wanted to say that the assembler typically used by me does not know the `org` keyword at all. – Martin Rosenau May 26 '23 at 05:38

Linking first and second stage bootloader

2 Answers2