Simply stating, it is an Operating System that does nothing but displays a simple message "Hello World". The source code is written in assembly language (NASM), and can be booted from a floppy disk.
The reader must be familiar with all the OS terminology and boot loader mechanism along with a minimal knowledge of a 16 bit Assembly language and working knowledge with NASM (freely downloadable) and the MSDOS program name Debug.Exe.
Using the Code
Without describing anything, let me write the boot loader code.
[BITS 16] [ORG 0x0000]
mov ax, 0x07C0
mov ds, ax
mov es, ax
mov fs, ax
mov gs, ax
mov ax, 0x0000
mov ss, ax
mov sp, 0xFFFF
mov si, msgEnd
or al, al
mov ah, 0x0E
mov bh, 0x00
mov bl, 0x07
msgHello db 0x0D, 0x0A, "Hello World", 0x0D, 0x0A, 0x00
msgEnd db 0x0D, 0x0A, "That's all folks!!!", 0x0D, 0x0A, 0x00
TIMES 510-($-$$) DB 0
Save the above code in a file, say boot.asm.
Next we have to generate a RAW Binary code file for the above code. So I used NASM. Assuming that NASM is in the system path, we write the following at the COMMAND prompt:
D:\>NASM boot.asm -o boot.bin -f bin
Next we need a floppy disk from which we will boot the OS. So place a floppy disk in the A Drive and before continuing, please backup any data that may be on the floppy Disk, because we would require to format the floppy in this step:
D:\>format a: /q
I have quick formatted the disk to save a lot of time, but full format will also do.
Now we need to copy our OS binary to the floppy disk. So we use the Debug.Exe program as follows:
-W 100 0 0 1
On issuing the
-W option in debug program, the binary is RAW written to the boot sector of the floppy disk after which we quit the debug application.
So we are done with our OS. Now restart the system and boot from the floppy disk. We will see the following output on the screen:
That's all folks!!!
And then the computer halts as it now receives the
Points of Interest
The first point to note here is that this is not a real OS but a kernel boot-loader. And thus the activities I can do here are limited to 512 kb (1 sector). So my code size does not exceed 512 kb which includes the data section.
Apart from that, I guess the code itself is self explanatory. The only tricky part to note is that the loader always tries to search for the Signature
0xAA55 in the boot loader to mark it as valid for loading. So at the end of the 510 byte of my code (and data) space, the extra 2 bytes are provided for the 2 byte signature for the loader to verify my code as valid.
The only problem in this OS is that after the OS is copied to the floppy disk, Windows or MSDOS starts thinking that the floppy is not formatted and then onwards, every time I try updating the code above, I have to first format the floppy and then copy the boot sector binary (boot.bin).
I welcome any suggestions to modify the code above so that the formatted data doesn't get destroyed.
- 23rd July, 2008: Initial post