Page 1 of 1
Boot loader explanation?
Posted: Sun Sep 13, 2009 6:12 am
by Aurdal
I found this tutorial about boot loaders, located at
http://inglorion.net/documents/tutorial ... ootsector/, but it left me with some questions:
Here's the bootloader source. I've commented every action with what I gather from the tutorial it does. Please do let me know if I assume wrong somewhere.
However, I know I don't understand the following:
- The"xor bx, bx" command
- Why CL is set to 1
- Why DH is set to 0
Code: Select all
[BITS 16] ;tell the assembler that its a 16 bit code
[ORG 0x7C00] ;Origin, tell the assembler that where the code will
;be in memory after it is been loaded
; Kernel info
KERNEL_START equ 1 ; Disk block where kernel starts
KERNEL_SIZE equ 1 ; Kernel size in disk blocks
KERNEL_SEGMENT equ 1000h ; Segment where kernel will be loaded
; Load kernel
mov ax, 200h + KERNEL_SIZE ; Set AH to 2 (OPcode to read), and AL to KERNEL_SIZE
push word KERNEL_SEGMENT ; Send KERNEL_SEGMENT to stack
pop es ; Retrieve KERNEL_SEGMENT from stack and store in ES,
; which is where "int 13h" will look for the segment to read from.
xor bx, bx ; I don't understand this, but since BX is the offset,
; I assume this somehow sets BX = 0?
mov cx, KERNEL_START + 1 ; Set CH = KERNEL_START, Set CL = 1 (Why?)
mov dx, 0 ; Set head = 0 (Why?), Set drive number = 0 (Floppy A)
int 13h ; Invoke BIOS to OPcode(2:Read), flag if error.
jnc ok ; Jump to ok: if not carry flag
jmp $ ; Infinite loop if carry flag
ok:
jmp KERNEL_SEGMENT:0 ; Jump to Kernel, located at 1000h with offset 0
TIMES 510 - ($ - $$) db 0 ; Fill the rest of sector with 0
DW 0xAA55 ; Add boot signature at the end of bootloader
Thanks in advance.
Re: Boot loader explanation?
Posted: Sun Sep 13, 2009 6:32 am
by Cjreek
Hi,
You should read
Int 13h and
CHS on Wikipedia or I guess the osdev wiki contains similar articles.
writes 0 to bx.
do you know how the xor composition works? if you've got 2 binary numbers, like
1001 and 1101
then
because if you xor 2 equal digits you'll get 0 else 1. So if you xor a number with itself you get always zero.
I hope I explained it good enough
Re: Boot loader explanation?
Posted: Sun Sep 13, 2009 6:39 am
by Aurdal
Thanks for the quick response, Cjreek.
I'll make sure to read the wikipedia articles you provided. And yes your explanation of xor was satisfactory, I believe I understand it now. Thanks.
EDIT:
Just to make sure:
Code: Select all
xor gives 1 while or gives 1
if and only if if either of the
only one of the numbers are 1:
numbers are 1:
1001 1001
xor 0101 or 0101
-------- --------
1100 1101
Am I right?
Re: Boot loader explanation?
Posted: Sun Sep 13, 2009 7:02 am
by Creature
Aurdal wrote:
Just to make sure:
Code: Select all
xor gives 1 while or gives 1
if and only if if either of the
only one of the numbers are 1:
numbers are 1:
1001 1001
xor 0101 or 0101
-------- --------
1100 1101
Am I right?
I believe that is correct. I also hope you're referring to the bits with 'numbers'
.
Re: Boot loader explanation?
Posted: Sun Sep 13, 2009 7:23 am
by Aurdal
Yes, I was refering to the individual bits. (I just realised the connection digits in decimal and bits in binary. lalalala... :p )
Also is there a reason to use "xor bx, bx" instead of just "mov bx,0"?
EDIT: Am I right in believing these two pieces of code do the same thing?
Code1:
Code: Select all
; Load Kernel
mov ah, 02h ; Tell int 13h to read
mov al, KERNEL_SIZE ; How much to read
mov ch, KERNEL_START ; Track to read
mov cl, 1 ; Sector to read
mov dh, 0 ; Head to read
mov dl, 0 ; Device to read (0 = Floppy A)
mov es, KERNEL_SEGMENT ; ES:BX - Where to load
xor bx, bx ; Set BX = 0
int 13h ; Invoke read, flag if error
jnc ok ; Jump to "ok:" if no error
jmp $ ; Infinite loop if error
Code2:
Code: Select all
; Load kernel
mov ax, 200h + KERNEL_SIZE ; Set AH to 2 (OPcode to read), and AL to KERNEL_SIZE
push word KERNEL_SEGMENT ; Send KERNEL_SEGMENT to stack
pop es ; Retrieve KERNEL_SEGMENT from stack and store in ES,
xor bx, bx ; Set BX = 0;
mov cx, KERNEL_START + 1 ; Set CH = KERNEL_START, Set CL = 1 (Why?)
mov dx, 0 ; Set head (DH) = 0 (Why?), Set drive number (DL) = 0 (Floppy A)
int 13h ; Invoke BIOS to OPcode(2:Read), flag if error.
jnc ok ; Jump to ok: if not carry flag
jmp $ ; Infinite loop if carry flag
Re: Boot loader explanation?
Posted: Sun Sep 13, 2009 10:05 am
by DLBuunk
Thes two pieces of code do the same thing, however, code1 is, once compiled, larger. The xor bx,bx is the same reason: xor bx,bx is 2 bytes machinecode, while mov bx,0 results in 3 bytes machinecode.
Re: Boot loader explanation?
Posted: Sun Sep 13, 2009 10:16 am
by Aurdal
I get it now. My thanks go out to everyone who has replied in this thread.