Can anyone find the problem with my code?

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Rottbott

Joined: 03 Sep 2003
Posts: 3

Rottbott 03 Sep 2003, 10:44

Hi.. new here. I decided to have a go at writing the OS the other day, but this is my first ever excursion into assembly and I'm not very good at it yet Smile

Anyway I found various tutorials and have come up with this very basic code to go in the boot sector of a floppy, load more code stored in sector 2, and jump to it. However, it doesn't work, despite being more or less the same as code I found in 2 of the tutorials (just adapted for FASM). It just sits there with a black screen and blinking cursor where it should be displaying some coloured ASCII characters. So, can anyone here see if I've made some obvious glaring mistake?

The code:

BOOT SECTOR

Code:

format binary
use16

; This boot sector just loads code in sector 2 and jumps to it.
jmp start
start:
      mov ax, 0x1000
      mov es, ax
  mov bx, 0

       mov dl, 0
   mov dh, 0
   mov ch, 0
   mov cl, 2
   mov al, 1

       mov ah, 02h

     int 13h

 jmp 0x1000:0

SECTOR 2 CODE:

Code:

format binary
use16
org 0x0

; Display some ASCII characters
       jmp start
start:
     mov ax, 0xb800
      mov gs, ax
  mov byte [gs:0], 0x45
   mov byte [gs:2], 0x4C
   mov byte [gs:4], 0x49
   mov byte [gs:6], 0x4C
   mov byte [gs:8], 0x45
   mov cx, 5
   attribloop:
             mov si, cx
          add si, cx
          sub si, 1
           mov byte [gs:si], 0x1f
  loopw attribloop
loop1:
      jmp loop1

I have one theory about what's wrong - that the "jmp 0x1000:0" in the boot sector code is a relative jump, jumping 1000 ahead of where it is rather than to an absolute memory location. If so, how do I do an absolute jump?

03 Sep 2003, 10:44

Rottbott

Joined: 03 Sep 2003
Posts: 3

Rottbott 03 Sep 2003, 16:30

Nevermind! Like most bugs, it was just me being incredibly stupid! The code itself is actually fine. Problem is that loads off of the SECOND sector, which is actually sector number 1 since they start at zero. I had been writing my code to sector number 2, i.e. the third sector. What an idiot!

03 Sep 2003, 16:30

lagg070988

Joined: 09 Jul 2011
Posts: 5

lagg070988 15 Jul 2011, 22:54

sometimes it happens

15 Jul 2011, 22:54

bitshifter

Joined: 04 Dec 2007
Posts: 796
Location: Massachusetts, USA

bitshifter 16 Jul 2011, 02:35

And sometimes we reply to 8 year old posts...

> 03 Sep 2003, 16:30 Rolling Eyes

16 Jul 2011, 02:35

typedef

Joined: 25 Jul 2010
Posts: 2893
Location: 0x77760000

typedef 16 Jul 2011, 03:04

And sometimes are just sometimes

16 Jul 2011, 03:04

xleelz

Joined: 12 Mar 2011
Posts: 86
Location: In Google Code Server... waiting for someone to download me

xleelz 16 Jul 2011, 22:55

I wonder how that dude's doing with OS dev now?

16 Jul 2011, 22:55

me239

Joined: 06 Jan 2011
Posts: 200

me239 17 Jul 2011, 07:09

xleelz wrote:

I wonder how that dude's doing with OS dev now?

That was 8 years ago. He could be making the next Windows for all we know LOL

17 Jul 2011, 07:09

nop

Joined: 01 Sep 2008
Posts: 165
Location: right here left there

nop 17 Jul 2011, 09:55

me239 wrote:

xleelz wrote:
I wonder how that dude's doing with OS dev now?
That was 8 years ago. He could be making the next Windows for all we know LOL

yep i gues your thinking his initial screwup with sector numbers makes him well qualified for that job Wink

17 Jul 2011, 09:55

me239

Joined: 06 Jan 2011
Posts: 200

me239 17 Jul 2011, 19:01

nop wrote:

me239 wrote:

xleelz wrote:
I wonder how that dude's doing with OS dev now?
That was 8 years ago. He could be making the next Windows for all we know LOL
yep i gues your thinking his initial screwup with sector numbers makes him well qualified for that job

Hey, he could have learned fast in the EIGHT years that passed.

17 Jul 2011, 19:01

lagg070988

Joined: 09 Jul 2011
Posts: 5

lagg070988 25 Sep 2011, 15:39

bitshifter wrote:

And sometimes we reply to 8 year old posts...

> 03 Sep 2003, 16:30

jajajja come on, is just for fun...

25 Sep 2011, 15:39

p3rlphr33k

Joined: 22 Dec 2009
Posts: 17
Location: Grand Forks ND

p3rlphr33k 08 Apr 2012, 18:48

Can someone make explain or comment the code at the beginning of this post? I am still learning and I am trying to understand what is happening and what went wrong. Thank you to whoever can explain the code.

_________________
-p3rlphr33k-
(Soon to be ASMphr33k)

08 Apr 2012, 18:48

kalambong

Joined: 08 Nov 2008
Posts: 165

kalambong 09 Apr 2012, 09:22

p3rlphr33k wrote:

Can someone make explain or comment the code at the beginning of this post? I am still learning and I am trying to understand what is happening and what went wrong. Thank you to whoever can explain the code.

Everything starts from 0 (zero)

09 Apr 2012, 09:22

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