ICS312 Set 15

FILE OPERATIONS

FILE PROCESSING

For the purposes of the following discussion, reading a file means copying all or part of it into memory

Writing a file means copying data from memory to a file

Rewriting a file means replacing a file's content with other data

File Handle When a file is created or opened in a

program, DOS assigns it a unique number called the file handle

This number is later used (in BX) to identify the file, so the program must save that information

Note: a file stream in C/C++/Java is the same thing as a file handle in Assembler

File Handle (Cont.)

There are 5 predefined file handles:

0     keyboard 1     screen 2     error output - usually the screen 3     COM1 4     printer

Additional user-defined files are assigned file handles starting with 5, 6, 7, etc.

File Errors There can be many types of errors in

using INT 21h file handling functions. Each error type is identified by a code number

In the following functions, if an error

occurs the CF is set and the code number is stored in AX

A list of common file-handling errors

Error Code:        Meaning: 1              invalid function number 2              file not found 3              path not found 5              access denied (file may already be open by another process)6              invalid file handle F              invalid drive specified

DOS FILE HANDLE INSTRUCTIONS

As with the DOS I/O functions, put the function number into the AH Register and execute INT 21h.

OPENING A FILE A file must be created before it can be used

To create a new file, or to rewrite an existing file, the user provides a filename and an attribute

DOS returns a file handle in AX(if no errors occur)

OPENING A NEW FILE

INT 21h, Function 3Ch DOS function:     Open a new file/rewrite a file

Input:     AH = 3Ch     DS:DX = address of filename (an ASCIIZ string ending with a zero byte)     CL = attribute (= 0 for write only, = 1 for read only, =2 for read & write)

Output:     If successful, CF (the carry flag) = 0 i.e. is off, and AX = file handle     If an error occurs: CF = 1 and AX = error code (3, 4, or 5)

OPENING A FILE (Cont)

The filename may include a path: A:\PROGS\PROG1.ASM

Possible errors:

1.Path doesn't exist

2.Access denied --- directory is full or file is read-only

OPENING A NEW FILE (Cont.)Example: Write instructions to open a new read-only file called "FILE1"

FNAME     DB 'FILE1', 0 HANDLE    DW ?

.CODE MOV AX,@DATA MOV DS, AX            ; initialize DS MOV AH, 3CH           ; open file function LEA DX, FNAME         ; copy address to DX MOV CL, 1             ; read-only attribute INT 21H               ; open the file JC OPEN_ERROR         ; jump if error MOV HANDLE, AX        ; handle or err code

OPENING AN EXISTING FILEINT 21h, function 3Dh DOS Function: Open an existing file

Input:     AH = 3Dh DX = offset of filename (an ASCII string ending with a zero byte) AL = access code: 0 = open for reading (where attribute is 1 or 2)         1 = open for writing (where attribute is 0 or 2)       2 = open for read/write (where attribute is 2)

Output:     If successful, AX = file handle    

If CF = 1, AX = error code (2, 4, 5, 12)

CLOSING FILES

A file should be closed after it has been processed. This frees the file handle for use with another file

If the file is being written, closing the file causes any data remaining in memory to be written to the file, and the file's time, date, and size will be updated in the directory entry

CLOSING FILES (Cont.1)

INT 21h, function 3Eh DOS Function: Close a file

Input:     AH = 3Eh     BX = file handle

Output:     Error if CF = 1, AX = error code (6)

CLOSING FILES (Cont.2)Example: Write instructions to close a file. Assume the file handle is stored in a variablecalled HANDLE

MOV AH, 3EH       ; close file function MOV BX, HANDLE     ; copy handle to bx INT 21H            ; execute closing JC CLOSE_ERROR     ; jump if error

The only error here would be if there is no file using the file handle (e.g., if the file had already been closed or an incorrect file handle had been stored.)

READING A FILE

The read file function reads a specified number of bytes from a file and stores them in memory

READING A FILE (Cont.1)INT 21h, function 3Fh DOS Function: Read a file

Input:     AH = 3Fh     BX = file handle     CX = number of bytes to read     DX = offset of memory bufferOutput:     AX = count of bytes actually read.     If AX = 0 or AX < CX, EOF (end of file) has occurred    If CF = 1, AX = error code (5, 6)

READING A FILE (Cont.2)Example: Write instructions to read a 512-bytesector from a file.

.DATA HANDLE DW ? BUFFER DB 512 DUP (0)

.CODE     . .     MOV AH, 3FH         ; read file     MOV BX, HANDLE      ; copy handle into BX    MOV CX, 512         ; set count to read     LEA DX, BUFFER INT 21H             ; AX = bytes read     JC READ_ERROR      ; jump if error

READING A FILE (Cont.3)

In some cases, we may want to read the entirefile (i.e., until EOF). In this case, after each read command, compare AX and CX to check for EOF:

CMP AX, CX             ; EOF? JE READ_LOOP          ; no, keep reading

WRITING A FILEINT 21h, function 40h DOS Function:     Write a file

Input:     AH = 40h     BX = file handle     CX = number of bytes to write     DX = offset of data to be written

Output:     AX = count of bytes written.     If AX < CX, error (disk full).    If CF = 1, AX = error code (5, 6)

WRITING A FILE (Cont.1)

It is possible that there is not enough room on the disk to accept the data. This is NOT an error, so the program must check for it by comparing AX and CX

Function 40h writes data to a file, but file handles 1 or 4 can be used to send data to the screen or printer

WRITING A FILE (Cont.2)

Example: Use function 40h to display a messageon the screen (as an alternative to fn 9).

.DATA MSG DB 'DISPLAY THIS MESSAGE‘LMES EQU $-MSG

.CODE . . MOV AH, 40H       ; write file MOV BX, 1           ; file handle for screen MOV CX, LMES       ; length of message (bytes) LEA DX, MSG         ; copy offset to DX INT 21H             ; display MSG executed

THE FILE POINTER The file pointer is used to locate a position in

a file When the file is opened, the file pointer is

positioned at the beginning of the file After a read operation, the file pointer

indicates the next byte to be read; after writing a new file, the file pointer is at EOF

The following function can be used to move the file pointer

THE FILE POINTER (Cont.1)DOS Function: Move File Pointer INT 21h, function 42h

Input:     AH = 42h     AL = movement code:         0 = relative to beginning of file        1 = relative to current file pos        2 = move relative to end of file     BX = file handle     CX:DX = number of bytes to move (a signed no.) Output:     DX:AX = new location in bytes from the beginning of the file         If CF = 1, AX = error code (1, 6)

THE FILE POINTER (Cont.2) In the case where we are moving the file from its current

position (AL = 1), if CX:DX is: < 0 => move pointer backward > 0 => move pointer forward If CX:DX is too large, the pointer could be moved past the

beginning or end of the file. This is not an error, but will cause an error when the next read or write is executed

If AL = 0 => move pointer from beginning of file (forward) If AL = 1 => move pointer from current position (forward or

backward) If AL = 2 => move pointer from end of file (backward)

THE FILE POINTER (Cont.3)Code to move the pointer to end of a file and determine It’s size:

MOV AH, 42H          ; move file pointer MOV BX, HANDLE  ; file handle MOV CX, 0          ; clear CX MOV DX, 0          ; 0 bytes to move MOV AL, 2           ; relative to end INT 21H            ; move pointer to end. DX:AX = file size JC MOVE_ERROR  ; error if CF = 1

Textbook Reading (Jones):

Chapter 18