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bubach



Joined: 17 Sep 2004
Posts: 340
Location: Trollhättan, Sweden
bubach
Hello, i am trying to alter my FAT and directory entrys for a FAT12 floppy.
I have added info in both FAT's and in the directory for a 50kb big file named kernel.sys that will be located at the very beginning of the data area.
I have not written the actual file on the floppy yet, but i think it would still show me a "fake" kernel.sys with 50kb size in xp.
Instead xp reports that i have used 6kb of the floppy space, but no file exists.

This is my code:

Code:
     ;--------------------;
     ;   sector buffer    ;
     ;--------------------;
          temp_sector:
          times 512       db  0 

     ;---------------;
     ;    other      ;
     ;---------------;
          filename        db  'KERNEL  SYS'    
....
Code:
     ;-------------;
     ; reset drive ;
     ;-------------;
     reset:
          xor     ax, ax
          mov     dl, 0
          int     0x13
          jc      reset

     ;---------------;
     ;  read fat 1   ;
     ;---------------;
          mov     ax, 0x0201
          mov     bx, temp_sector
          mov     cx, 2
          mov     dx, 0
          int     0x13
          jc      reset

     ;--------------;
     ;  change it   ;
     ;--------------;
          inc     bx
          inc     bx
          inc     bx
          mov     byte [bx], 0x03
          inc     bx
          mov     byte [bx], 0x40
          inc     bx
          mov     byte [bx], 0x00
          inc     bx
          mov     byte [bx], 0x05
          inc     bx
          mov     byte [bx], 0x60
          inc     bx
          mov     byte [bx], 0x00
          inc     bx
          mov     byte [bx], 0x07
          inc     bx
          mov     byte [bx], 0x80
          inc     bx
          mov     byte [bx], 0x00

     ;----------------;
     ;   write it     ;
     ;----------------;
          mov     ax, 0x0301
          mov     bx, temp_sector
          mov     cx, 2
          mov     dx, 0
          int     0x13
          jc      reset



     ;-------------;
     ; reset drive ;
     ;-------------;
     reset2:
          xor     ax, ax
          mov     dl, 0
          int     0x13
          jc      reset2

     ;---------------;
     ;  read fat 2   ;
     ;---------------;
          mov     ax, 0x0201
          mov     bx, temp_sector
          mov     cx, 10
          mov     dx, 0
          int     0x13
          jc      reset2

     ;--------------;
     ;  change it   ;
     ;--------------;
          inc     bx
          inc     bx
          inc     bx
          mov     byte [bx], 0x03
          inc     bx
          mov     byte [bx], 0x40
          inc     bx
          mov     byte [bx], 0x00
          inc     bx
          mov     byte [bx], 0x05
          inc     bx
          mov     byte [bx], 0x60
          inc     bx
          mov     byte [bx], 0x00
          inc     bx
          mov     byte [bx], 0x07
          inc     bx
          mov     byte [bx], 0x80
          inc     bx
          mov     byte [bx], 0x00

     ;----------------;
     ;   write it     ;
     ;----------------;
          mov     ax, 0x0301
          mov     bx, temp_sector
          mov     cx, 10
          mov     dx, 0
          int     0x13
          jc      reset2



     ;-------------;
     ; reset drive ;
     ;-------------;
     reset3:
          xor     ax, ax
          mov     dl, 0
          int     0x13
          jc      reset3

     ;---------------------------;
     ;   read directory entry    ;
     ;---------------------------;
          mov     ax, 0x0201
          mov     bx, temp_sector
          mov     cx, 15
          mov     dx, 0x0100
          int     0x13
          jc      reset3

     ;---------------------------;
     ;  change directory entry   ;
     ;---------------------------;
          mov     cx, 11
          mov     dx, filename
     .l1:
          mov     [bx], dx
          inc     bx
          inc     dx
          loop    .l1

     ;--------------------;
     ;  file attributes   ;
     ;--------------------;
          mov     byte [bx], 5
          inc     bx

     ;---------------------------------------;
     ;   jump over reserved and time/date    ;
     ;---------------------------------------;
          mov     cx, 15
     .l2:
          inc     bx
          loop    .l2

     ;----------------------------------;
     ;   reversed data area location    ;
     ;----------------------------------;
          mov     byte [bx], 0x02
          inc     bx
          mov     byte [bx], 0x00
          inc     bx

     ;------------------------;
     ;   reversed filesize    ;
     ;------------------------;
          mov     byte [bx], 0x00
          inc     bx
          mov     byte [bx], 0xC8


     ;---------------------------;
     ;  write directory entry    ;
     ;---------------------------;
          mov     ax, 0x0301
          mov     bx, temp_sector
          mov     cx, 15
          mov     dx, 0x0100
          int     0x13
          jc      reset3




     ;---------------------------------;
     ;  write the file to data area    ;
     ;---------------------------------;
          ;...... not done yet    

but it does not work.
The info i used for finding out what to write was from this page:
http://www.mega-tokyo.com/osfaq2/index.php/FAT12%20document
i have used the values for the "example" file PROCESSA.TXT described in the document, as it starts at the beginning of the data area.
The things i have changed is the filename and filesize..

Any idea of what i have done wrong? Or a suggestion of a simpler way of doing this?

PS: if anything is terrible wrong in this code, i blame it at i did this coding _really_ late at night.. Wink

Thanks in advance...
/ Christoffer

_________________
BOS homepage: http://bos.asmhackers.net/


Last edited by bubach on 13 Feb 2012, 14:53; edited 1 time in total
Post 12 Nov 2004, 10:01
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mike.dld



Joined: 03 Oct 2003
Posts: 235
Location: Belarus, Minsk
mike.dld
Post 12 Nov 2004, 17:48
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bubach



Joined: 17 Sep 2004
Posts: 340
Location: Trollhättan, Sweden
bubach
Thanks, i had a quick look, didn't get much except it uses DOS not BIOS like me. But i better study it more carfully.

One more thing, how do i get params, from the dos prompt in a .com file? Nothing to complex, i need it for either -b or -k and then a filename. Any guides on this?

/ Christoffer


Last edited by bubach on 13 Feb 2012, 14:58; edited 1 time in total
Post 12 Nov 2004, 18:13
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Matrix



Joined: 04 Sep 2004
Posts: 1171
Location: Overflow
Matrix
what do you say searching for it like:

What is fat-12
Post 12 Nov 2004, 19:04
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mike.dld



Joined: 03 Oct 2003
Posts: 235
Location: Belarus, Minsk
mike.dld
.com-file has addresses from $100, so those $100 bytes from the beginning are used as PSP block. Sorry if I'd mistaken.
Post 12 Nov 2004, 19:33
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Matrix



Joined: 04 Sep 2004
Posts: 1171
Location: Overflow
Matrix
mike.dld wrote:
.com-file has addresses from $100, so those $100 bytes from the beginning are used as PSP block. Sorry if I'd mistaken.


no , you're right, dos loads it there. while bootprog usually loads @ 7c00h
Post 12 Nov 2004, 19:40
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mike.dld



Joined: 03 Oct 2003
Posts: 235
Location: Belarus, Minsk
mike.dld
ok Wink
so parameters are at offset $80 IIRC
Post 12 Nov 2004, 23:44
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bubach



Joined: 17 Sep 2004
Posts: 340
Location: Trollhättan, Sweden
bubach
Thanks, but i already solved it.. Wink

for future forum-searches, here's how i did it:
Code:
       boot            db  0
       kernel          db  0
       hyphen          db  0
       filename        db  '           ',0

;-----------------------------------------------;
;  read in arguments from the command line...   ;
;-----------------------------------------------;
    mov     cl, [ds:0x80]
   or      cl, cl
      je      print_usage
         mov     si, 0x81
    mov     di, filename
        mov     bx, 0

     param_loop:
   lodsb
     ;     or      al, al
     ;     je      continue
   cmp     al, 13
      je      continue
    cmp     al, '-'
   jne     .c1
         mov     [hyphen], 1
         jmp     .c5
     .c1:
        cmp     [hyphen], 1
         jne     .c5
         cmp     [boot], 1
   je      .c3
         cmp     [kernel], 1
         je      .c3
         cmp     al, 'b'
   jne     .c2
         mov     [boot], 1
   jmp     .c5
     .c2:
        cmp     al, 'k'
   jne     print_usage
         mov     [kernel], 1
         jmp     .c5
     .c3:
        cmp     al, ' '
   je      .c5
         cmp     bx, 11
      ja      print_usage
;          cmp     al, '.'                        ; Old code, becasue i was
;          jne     .c4                            ; thinking wrong. This converts:
;          cmp     bx, 8                          ; FILE.TXT to:
;          je      .c5                            ; FILE    TXT

;          push    cx
;          mov     cx, 8
;          sub     cx, bx
;          mov     al, ' '
;          rep     stosb
;          inc     bx
;          pop     cx
;          jmp     .c5
     .c4:
    stosb
       inc     bx
     .c5:
         loop    param_loop


     ;---------------;
     ;  continue..   ;
     ;---------------;
     continue:
      cmp     al, 13
      jne     .next
       mov     al, 0     ; Make the filename ASCIIZ Wink
        stosb
     .next:
    cmp     [boot], 1
   je      write_boot
          cmp     [kernel], 1
         je      write_kernel
    


Last edited by bubach on 13 Feb 2012, 15:00; edited 1 time in total
Post 13 Nov 2004, 13:24
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bubach



Joined: 17 Sep 2004
Posts: 340
Location: Trollhättan, Sweden
bubach
Is there a way of finding out a file-size, when using handle not FCB?

/ Christoffer
Post 13 Nov 2004, 16:55
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Matrix



Joined: 04 Sep 2004
Posts: 1171
Location: Overflow
Matrix
you're jokeing are you? this is not funny anymore.
Post 13 Nov 2004, 18:17
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bubach



Joined: 17 Sep 2004
Posts: 340
Location: Trollhättan, Sweden
bubach
Huh?? What is funny about my questions?
Post 14 Nov 2004, 12:42
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