Chapter Thirteen (Part 5)
|Table of Content||
Chapter Thirteen (Part 7)
MS-DOS PC-BIOS AND FILE I/O (Part 6)
MS-DOS "New" Filing Calls
18.104.22.168 - Open File
22.214.171.124 - Create File
126.96.36.199 - Close File
188.8.131.52 - Read From a File
184.108.40.206 - Write to a File
220.127.116.11 - Seek (Move File Pointer)
18.104.22.168 - Set Disk Transfer Address (DTA)
- Find First File
22.214.171.124 - Find Next File
126.96.36.199 - Delete File
188.8.131.52 - Rename File
184.108.40.206 - Change/Get File Attributes
220.127.116.11 - Get/Set File Date and Time
18.104.22.168 - Other DOS Calls
Starting with DOS v2.0 Microsoft introduced a set of file handling procedures which (finally) made disk file access bearable under MS-DOS. Not only bearable but actually easy to use! The following sections describe the use of these commands to access files on a disk drive.
File commands which deal with filenames (Create Open Delete Rename and others) are passed the address of a zero-terminated pathname. Those that actually open a file (Create and Open) return a file handle as the result (assuming of course that there wasn't an error). This file handle is used with other calls (read write seek close etc.) to gain access to the file you've opened. In this respect a file handle is not unlike a file variable in Pascal. Consider the following Microsoft/Turbo Pascal code:
program DemoFiles; var F:TEXT; begin assign(f 'FileName.TXT'); rewrite(f); writeln(f 'Hello there'); close(f); end.
The file variable "f" is used in this Pascal example in much the same way that a file handle is used in an assembly language program - to gain access to the file that was created in the program.
All the following DOS filing commands return an error status in the carry flag. If the carry flag is clear when DOS returns to your program then the operation was completed successfully. If the carry flag is set upon return then some sort of error has occurred and the AX register contains the error number. The actual error return values will be discussed along with each function in the following sections.
22.214.171.124 Open File
Function (ah): 3Dh Entry parameters: al- file access value 0- File opened for reading 1- File opened for writing 2- File opened for reading and writing ds:dx- Point at a zero terminated string containing the filename. Exit parameters: If the carry is set ax contains one of the following error codes: 2- File not found 4- Too many open files 5- Access denied 12- Invalid access If the carry is clear ax contains the file handle value assigned by DOS.
A file must be opened before you can access it. The open command opens a file that already exists. This makes it quite similar to Pascal's Reset procedure. Attempting to open a file that doesn't exist produces an error. Example:
lea dx Filename ;Assume DS points at segment mov ah 3dh ; of filename mov al 0 ;Open for reading. int 21h jc OpenError mov FileHandle ax
If an error occurs while opening a file the file will not be opened. You should always check for an error after executing a DOS open command since continuing to operate on the file which hasn't been properly opened will produce disastrous consequences. Exactly how you handle an open error is up to you but at the very least you should print an error message and give the user the opportunity to specify a different filename.
If the open command completes without generating an error
DOS returns a file handle for that file in the
ax register. Typically
should save this value away somewhere so you can use it when accessing the file later on.
126.96.36.199 Create File
Function (ah): 3Ch Entry parameters: ds:dx- Address of zero terminated pathname cx- File attribute Exit parameters: If the carry is set ax contains one of the following error codes: 3- Path not found 4- Too many open files 5- Access denied If the carry is clear ax is returned containing the file handle
Create opens a new file for output. As with the OPEN
ds:dx points at a zero terminated string containing the filename.
Since this call creates a new file
DOS assumes that you're opening the file for writing
only. Another parameter
is the initial file attribute
settings. The L.O. six bits of
cx contain the following values:
Bit Meaning if equal to one 0 File is a Read-Only file 1 File is a hidden file 2 File is a system file 3 File is a volume label name 4 File is a subdirectory 5 File has been archived
you shouldn't set any of these bits. Most
normal files should be created with a file attribute of zero. Therefore
should be loaded with zero before calling the create function.
Upon exit the carry flag is set if an error occurs. The "Path not found" error requires some additional explanation. This error is generated not if the file isn't found (which would be most of the time since this command is typically used to create a new file) but if a subdirectory in the pathname cannot be found.
If the carry flag is clear when DOS returns to your
then the file has been properly opened for output and the
contains the file handle for this file.
188.8.131.52 Close File
Function (ah): 3Eh Entry parameters: bx- File Handle Exit parameters: If the carry flag is set ax contains 6 the only possible error which is an invalid handle error.
This call is used to close a file opened with the Open or
Create commands above. It is passed the file handle in the
bx register and
assuming the file handle is valid
closes the specified file.
You should close all files your program uses as soon as you're through with them to avoid disk file corruption in the event the user powers the system down or resets the machine while your files are left open.
Note that quitting to DOS (or aborting to DOS by pressing control-C or control-break) automatically closes all open files. However you should never rely on this feature since doing so is an extremely poor programming practice.
184.108.40.206 Read From a File
Function (ah): 3Fh Entry parameters: bx- File handle cx- Number of bytes to read ds:dx- Array large enough to hold bytes read Exit parameters: If the carry flag is set ax contains one of the following error codes 5- Access denied 6- Invalid handle If the carry flag is clear ax contains the number of bytes actually read from the file.
The read function is used to read some number of bytes from
a file. The actual number of bytes is specified by the
cx register upon entry
into DOS. The file handle
which specifies the file from which the bytes are to be read
is passed in the
bx register. The
ds:dx register contains the
address of a buffer into which the bytes read from the file are to be stored.
if there wasn't an error
contains the number of bytes actually read. Unless the end of file (EOF) was reached
number will match the value passed to DOS in the
cx register. If the end of
file has been reached
the value return in
ax will be somewhere between zero
and the value passed to DOS in the
cx register. This is the only test for the
Example: This example opens a file and reads it to the EOF
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 putc ;Print it jmp LP ;Read next byte EOF: mov bx FHndl mov ah 3eh ;Close file int 21h jc CloseError
This code segment will read the entire file whose
(zero-terminated) filename is found at address "Filename" in the current data
segment and write each character in the file to the standard output device using the UCR
putc routine. Be forewarned that one-character-at-a-time I/O such as
this is extremely slow. We'll discuss better ways to quickly read a file a little later in
220.127.116.11 Write to a File
Function (ah): 40h Entry parameters: bx- File handle cx- Number of bytes to write ds:dx- Address of buffer containing data to write Exit parameters: If the carry is set ax contains one of the following error codes 5- Accessed denied 6- Invalid handle If the carry is clear on return ax contains the number of bytes actually written to the file.
This call is almost the converse of the read command
presented earlier. It writes the specified number of bytes at
ds:dx to the
file rather than reading them. On return
if the number of bytes written to the file is
not equal to the number originally specified in the
the disk is
full and this should be treated as an error.
cx contains zero when this function is
DOS will truncate the file to the current file position (i.e.
all data following
the current position in the file will be deleted).
18.104.22.168 Seek (Move File Pointer)
Function (ah): 42h Entry parameters: al- Method of moving 0- Offset specified is from the beginning of the file. 1- Offset specified is distance from the current file pointer. 2- The pointer is moved to the end of the file minus the specified offset. bx- File handle. cx:dx- Distance to move in bytes. Exit parameters: If the carry is set ax contains one of the following error codes 1- Invalid function 6- Invalid handle If the carry is clear dx:ax contains the new file position
This command is used to move the file pointer around in a
random access file. There are three methods of moving the file pointer
distance within the file (if
some positive distance from the current
file position (if
or some distance from the end of the file (if
If AL doesn't contain 0
DOS will return an invalid function error. If this call
is successfully completed
the next byte read or written will occur at the specified
Note that DOS treats
cx:dx as an unsigned
a single seek command cannot be used to move backwards in the file.
method #0 must be used to position the file pointer at some absolute position in
the file. If you don't know where you currently are and you want to move back 256 bytes
you can use the following code:
mov ah 42h ;Seek command mov al 1 ;Move from current location xor cx cx ;Zero out CX and DX so we xor dx dx ; stay right here mov bx FileHandle int 21h jc SeekError sub ax 256 ;DX:AX now contains the sbb dx 0 ; current file position so mov cx dx ; compute a location 256 mov dx ax ; bytes back. mov ah 42h mov al 0 ;Absolute file position int 21h ;BX still contains handle.
22.214.171.124 Set Disk Transfer Address (DTA)
Function (ah): 1Ah Entry parameters: ds:dx- Pointer to DTA Exit parameters: None
This command is called "Set Disk Transfer Address" because it was (is) used with the original DOS v1.0 file functions. We wouldn't normally consider this function except for the fact that it is also used by functions 4Eh and 4Fh (described next) to set up a pointer to a 43-byte buffer area. If this function isn't executed before executing functions 4Eh or 4Fh DOS will use the default buffer space at PSP:80h.
126.96.36.199 Find First File
Function (ah): 4Eh Entry parameters: cx- Attributes ds:dx- Pointer to filename Exit parameters: If carry is set ax contains one of the following error codes 2- File not found 18- No more files
The Find First File and Find Next File (described next) functions are used to search for files specified using ambiguous file references. An ambiguous file reference is any filename containing the "*" and "?" wildcard characters. The Find First File function is used to locate the first such filename within a specified directory the Find Next File function is used to find successive entries in the directory.
Generally when an ambiguous file reference is provided the Find First File command is issued to locate the first occurrence of the file and then a loop is used calling Find Next File to locate all other occurrences of the file within that loop until there are no more files (error #18). Whenever Find First File is called it sets up the following information at the DTA:
Offset Description 0 Reserved for use by Find Next File 21 Attribute of file found 22 Time stamp of file 24 Date stamp of file 26 File size in bytes 30 Filename and extension (zero terminated)
(The offsets are decimal)
Assuming Find First File doesn't return some sort of error the name of the first file matching the ambiguous file description will appear at offset 30 in the DTA.
Note: if the specified pathname doesn't contain any wildcard characters then Find First File will return the exact filename specified if it exists. Any subsequent call to Find Next File will return an error.
cx register contains the search attributes
for the file. Normally
cx should contain zero. If non-zero
Find First File
(and Find Next File) will include file names which have the specified attributes as well
as all normal file names.
188.8.131.52 Find Next File
Function (ah): 4Fh Entry parameters: none Exit parameters: If the carry is set then there aren't any more files and ax will be returned containing 18.
The Find Next File function is used to search for additional file names matching an ambiguous file reference after a call to Find First File. The DTA must point at a data record set up by the Find First File function.
Example: The following code lists the names of all the files in the current directory that end with ".EXE". Presumably the variable "DTA" is in the current data segment:
mov ah 1Ah ;Set DTA lea dx DTA int 21h xor cx cx ;No attributes. lea dx FileName mov ah 4Eh ;Find First File int 21h jc NoMoreFiles ;If error we're done DirLoop: lea si DTA+30 ;Address of filename cld PrtName: lodsb test al al ;Zero byte? jz NextEntry putc ;Print this character jmp PrtName NextEntry: mov ah 4Fh ;Find Next File int 21h jnc DirLoop ;Print this name
184.108.40.206 Delete File
Function (ah): 41h Entry parameters: ds:dx- Address of pathname to delete Exit parameters: If carry set ax contains one of the following error codes 2- File not found 5- Access denied
This function will delete the specified file from the directory. The filename must be an unambiguous filename (i.e. it cannot contain any wildcard characters).
220.127.116.11 Rename File
Function (ah): 56h Entry parameters: ds:dx- Pointer to pathname of existing file es:di- Pointer to new pathname Exit parameters: If carry set ax contains one of the following error codes 2- File not found 5- Access denied 17- Not the same device
This command serves two purposes: it allows you to rename one file to another and it allows you to move a file from one directory to another (as long as the two subdirectories are on the same disk).
Example: Rename "MYPGM.EXE" to "YOURPGM.EXE"
; Assume ES and DS both point at the current data segment ; containing the filenames. lea dx OldName lea di NewName mov ah 56h int 21h jc BadRename . . . OldName byte "MYPGM.EXE" 0 NewName byte "YOURPGM.EXE" 0
Example #2: Move a filename from one directory to another:
; Assume ES and DS both point at the current data segment ; containing the filenames. lea dx OldName lea di NewName mov ah 56h int 21h jc BadRename . . . OldName byte "\DIR1\MYPGM.EXE" 0 NewName byte "\DIR2\MYPGM.EXE" 0
18.104.22.168 Change/Get File Attributes
Function (ah): 43h Entry parameters: al- Subfunction code 0- Return file attributes in cx 1- Set file attributes to those in cx cx- Attribute to be set if AL=01 ds:dx- address of pathname Exit parameters: If carry set ax contains one of the following error codes: 1- Invalid function 3- Pathname not found 5- Access denied If the carry is clear and the subfunction was zero cx will contain the file's attributes.
This call is useful for setting/resetting and reading a file's attribute bits. It can be used to set a file to read-only set/clear the archive bit or otherwise mess around with the file attributes.
22.214.171.124 Get/Set File Date and Time
Function (ah): 57h Entry parameters: al- Subfunction code 0- Get date and time 1- Set date and time bx- File handle cx- Time to be set (if AL=01) dx- Date to be set (if AL=01) Exit parameters: If carry set ax contains one of the following error codes 1- Invalid subfunction 6- Invalid handle If the carry is clear cx/dx is set to the time/date if al=00
This call sets the "last-write" date/time for the specified file. The file must be open (using open or create) before using this function. The date will not be recorded until the file is closed.
126.96.36.199 Other DOS Calls
The following tables briefly list many of the other DOS calls. For more information on the use of these DOS functions consult the Microsoft MS-DOS Programmer's Reference or the MS-DOS Technical Reference.
||-||Create Directory: Creates a new directory with the specified name.|
||-||Remove Directory: Deletes the directory with the specified pathname. Error if directory is not empty or the specified directory is the current directory.|
||-||Change Directory: Changes the default directory to the specified pathname.|
||Duplicate File Handle: creates a copy of a file handle so a program can access a file using two separate file variables. This allows the program to close the file with one handle and continue accessing it with the other.|
|Force Duplicate File Handle: Like function 45h above except you specify which handle (in cx) you want to refer to the existing file (specified by bx).|
||-||Get Current Directory: Stores a string containing the current pathname (terminated with a zero) starting at location ds:si. These registers must point at a buffer containing at least 64 bytes. The dl register specifies the drive number (0=default 1=A 2=B 3=C etc.).|
||Create Temporary File: Creates a file with a unique name in the directory specified by the zero terminated string at which ds:dx points. There must be at least 13 zero bytes beyond the end of the pathname because this function will store the generated filename at the end of the pathname. The attributes are the same as for the Create call.|
||Create New File: Like the create call but this call insists that the file not exist. It returns an error if the file exists (rather than deleting the old file).|
||-||Set Maximum Handle Count: This function sets the maximum number of handles a program can use at any one given time.|
||-||Commit File: Flushes all data to a file without closing it ensuring that the file's data is current and consistent.|
|Input Parameters||Output Parameters||Description|
|Set Interrupt Vector: Stores the specified address in ds:dx into the interrupt vector table at the entry specified by the al register.|
||Get Version Number: Returns the current version number of DOS (or value set by SETVER).|
||Get Break Flag: Returns the status of the DOS break flag. If on MS-DOS checks for ctrl-C when processing any DOS command; if off MS-DOS only checks on functions 1-0Ch.|
|Set Break Flag: Sets the MS-DOS break flag according to the value in dl (see function above for details).|
||Get MS-DOS Version: Returns the "real" version number not the one set by the SETVER command. Bits three and four of the version flags are one if DOS is in ROM or DOS is in high memory respectively.|
||Get InDOS Flag Address: Returns the address of the InDOS flag. This flag helps prevent reentrancy in TSR applications|
||Get Interrupt Vector: Returns a pointer to the interrupt service routine for the specified interrupt number. See function 25h above for more details.|
|Device Control: This is a whole family of additional DOS commands to control various devices. See the DOS programmer's reference manual for more details.|
||Get Child Program Return Value: Returns the last result code from a child program in al. The ah register contains the termination method which is one of the following values: 0-normal 1-ctrl-C 2-critical device error 3-terminate and stay resident.|
|Set PSP Address: Set DOS' current PSP address to the value specified in the bx register.|
||Get PSP Address: Returns a pointer to the current PSP in the bx register.|
||Get Extended Error: Returns additional information when an error occurs on a DOS call. See the DOS programmer's guide for more details on these errors and how to handle them.|
|Set Extended Error: copies the data from the extended error structure to DOS' internal record.|
In addition to the above commands there are several additional DOS calls that deal with networks and international character sets. See the MS-DOS reference for more details.
Chapter Thirteen: MS-DOS
File I/O (Part 6)
28 SEP 1996