Newbie help - File I/O

Comments

• anthrax11

  December 2008

  Google is your friend
  
  http://www.arl.wustl.edu/~lockwood/class/cs306/books/artofasm/Chapter_13/CH13-1.html#top
  http://www.emu8086.com/assembly_language_tutorial_assembler_reference/8086_bios_and_dos_interrupts.html
  
• perfectlyround

  December 2008

  Thanks for the links! With their help, I have been able to come up with this code:
  
  [code].model small
  .data
  Filename db 'test.txt'
  FHndl dw ?
  Buffer db 80h dup(?)
  .stack 100h
  .code
  Program:
  mov ah, 3dh ;Open the file
  mov al, 0 ;Open for reading
  lea dx, Filename ;Presume DS points at filename
  int 21h ; segment.
  ; jc BadOpen
  mov FHndl, ax ;Save file handle
  
  LP: mov ah,3fh ;Read data from the file
  lea dx, Buffer ;Address of data buffer
  mov cx, 1 ;Read one byte
  mov bx, FHndl ;Get file handle value
  int 21h
  ; jc ReadError
  cmp ax, cx ;EOF reached?
  jne EOF
  mov al, Buffer ;Get character read
  call write ;Print it
  jmp LP ;Read next byte
  
  EOF: mov bx, FHndl
  mov ah, 3eh ;Close file
  int 21h
  ;jc CloseError
  
  PROC write
  push ax bx cx dx
  mov ah, 2
  mov dl, Buffer ;char to be printed
  int 21h
  pop ax bx cx dx
  ret
  ENDP
  
  
  End Program[/code]
  
  My issue now is actually printing the characters from the txt file to the console/cmd prompt. Am I on the right track here? I'm not sure if my write procedure is entirely accurate. Thanks again.
  
  
• anthrax11

  December 2008

  You're on the right track, but you've made some small mistakes, which is
  normal though, because asm is no easy language to learn
  
  I hope you're not stuck to that old Tasm assembler, because it has been
  obsolete for years now. I'd go with [link=http://www.flatassembler.net]Fasm[/link] instead.
  
  Here's the code with some changes to get it to assemble with Fasm.
  I can't get it to work in WinXP, but WinXP has bad DOS emulation too..
  [code]
  format MZ
  entry codeseg:main ; program entry point
  stack 100h
  
  segment dataseg
  [color=Green] ; a terminating zero is needed so that the function knows where the string ends[/color]
  Filename db 'test.txt'[color=Red],0[/color]
  FHndl dw ?
  [color=Green]; You're reading just one byte, so the buffer only needs one byte..[/color]
  Buffer db ?
  
  segment codeseg
  main:
  [color=Red] mov ax, dataseg
  mov ds, ax[/color]
  
  mov ah, 3dh ;Open the file
  mov al, 0 ;Open for reading
  lea dx, [Filename] [color=Red];Presume DS points at filename[/color]
  int 21h [color=Red]; segment.[/color]
  ; jc BadOpen
  mov [FHndl], ax ;Save file handle
  
  LP: mov ah,3fh ;Read data from the file
  lea dx, [Buffer] ;Address of data buffer
  mov cx, 1 ;Read one byte
  mov bx, [FHndl] ;Get file handle value
  int 21h
  ; jc ReadError
  cmp ax, cx ;EOF reached?
  jne EOF
  
  mov al, [Buffer] ;Get character read
  call write ;Print it
  jmp LP ;Read next byte
  
  EOF: mov bx, [FHndl]
  mov ah, 3eh ;Close file
  int 21h
  ;jc CloseError
  
  [color=Green]; Don't forget to exit![/color]
  [color=Red]exit:
  mov ax, 4c00h
  int 21h[/color]
  
  write:
  mov ah, 2
  mov dl, al ;char to be printed
  int 21h
  ret
  [/code]
  I hope this helps!
• perfectlyround

  December 2008

  Brilliant, anthrax11, you're a godsend! Yes, it's quite a tricky language to understand, but I seem to be getting the hang of it now. You've helped quite a bunch!
  
  One other quick thing, if it's not too much hassle to explain; I know in Java and the like you're able to format the way something is printed, with printf or whatever. Is there a simple way of doing that in assembly? For example, instead of printing the contents of that file one by one, is it possible to print it's contents (the numbers 1-6, for example), as:
  
  123
  456
  
  I'm only able to get things to print one by one, or string by string. That's pretty much the final thing I'd like to get this piece of code to do, I suppose I'll need a procedure to do that!
  
• anthrax11

  December 2008

  If I understand correctly, you want to read in a specific number of bytes, print them,
  go to the next line and start over. Okay, for this you need to make the buffer longer
  and change cx accordingly before calling the file reading function.
  Next you need a function to which you can pass the number of bytes to be printed.
  There are at least two ways to do this. Either by printing out each character individually
  in a loop or by turning the buffer into a string and printing it out then. I'm bored and have
  no life, so I'll show you both:
  [code]
  ; 1)
  segment dataseg
  Buffer db 16 dup(?)
  NewLine db 13,10,"$"
  
  ...
  
  LP: mov ah,3fh ;Read data from the file
  lea dx, [Buffer] ;Address of data buffer
  mov cx, [color=Red]16[/color] ;Fill the buffer
  mov bx, [FHndl] ;Get file handle value
  int 21h
  ; jc ReadError
  cmp ax, 0 ;EOF reached?
  jne EOF
  
  ; Number of bytes read is in ax
  call write ;Print it
  jmp LP ;Read next byte
  
  ...
  
  write:
  mov cx, ax ;Use cx as a counter
  lea bx, [Buffer] ;Address of data buffer
  nextbyte:
  push cx ;Save the registers we use
  push bx
  mov ah, 2
  mov dl, [bx] ;Get byte from buffer
  int 21h
  pop bx
  pop cx
  inc bx
  dec cx
  jnz nextbyte
  
  ;Print a newline
  mov ah, 9
  lea dx, [NewLine]
  int 21h
  
  ret[/code]
  [code]; 2)
  ;Need one more byte for the terminating character
  segment dataseg
  Buffer db 16+1 dup(?)
  
  ...
  
  write:
  lea bx, [Buffer]
  add bx, ax ;Point to end of data buffer
  mov [bx], byte "$" ;Append terminating dollar sign
  
  ;Print the string
  mov ah,9
  lea dx, [Buffer]
  int 21h
  
  ;Print a newline
  mov ah, 9
  lea dx, [NewLine]
  int 21h
  
  ret
  [/code]
  
  When it comes to formatted output, then in DOS/asm it's probably best to
  make your own string handling functions. The only string functions DOS provides
  are those for getting characters into video memory, nothing more. In Windows you
  would also have wsprintf, plenty of console functions and other interesting functions
  which are also easy to use in assembly. Of course they are not as simple to use
  as those in Java & others, but they make things a lot easier. If you have the interest, then
  you should definitely look into programming under Windows, that way you can actually
  learn something relevant in today's world. Good luck!
• Copa7

  June 2009

  But in MASM the function Call write don't work. Can you give me another option to do that?
  
• anthrax11

  June 2009

  I think you'll have to post the whole source code that you are trying to assemble, because I can't see why "call write" shouldn't work. Use code tags too, please.
  
  If possible, avoid the hassle and start using [link=http://www.flatassembler.net]FASM[/link] or [link=http://www.nasm.us/]NASM[/link] instead.
• Copa7

  June 2009

  I haved see the problem now... But just print the first byte of file, don't print the others bytes... Can you help me? How I can read and print byte to byte? I need to know how can do that, because I have to print in binary, decimal and hex the bytes too...
  
  (Sorry about my english, but I'm portuguese xD)
  
• anthrax11

  June 2009

  You need a function for converting an integer to text with a way to give it the base number (2 for binary, 10 for decimal, 16 for hexadecimal). Here's one:
  [code]
  mov al, [Buffer] ; get byte from the buffer
  xor ah, ah ; set rest of ax to zero
  mov cx, 10 ; set base number for decimal
  call itoa
  
  ...
  
  
  ; print ax as base cx
  itoa:
  xor bx, bx
  
  div_loop:
  xor dx, dx ; dx = 0
  
  ; Divide ax:dx by cx,
  ; get quotinent in ax and remainder in dx
  div cx
  
  ; Convert digit to ASCII
  cmp dl, 10
  jb below10
  add dl, 'a'-10
  jmp continue
  below10:
  add dl, '0'
  continue:
  
  ; Write ASCII character to buffer
  mov [itoa_buffer+bx], dl
  inc bx
  
  ; Check if there was anything left to convert
  or ax, ax
  jnz div_loop
  
  ; Now show the result
  show_loop:
  mov ah, 2
  mov dl, [itoa_buffer+bx-1]
  push bx
  int 21h
  pop bx
  dec bx
  jnz show_loop
  
  itoa_ret:
  ret
  
  
  Buffer db ?
  itoa_buffer db 9 dup(?)
  [/code]