Q11785: Getting the DTA Address Using INT86

Article: Q11785
Product(s): See article
Version(s): 2.00 2.01 3.00 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | B_BasicCom | mspl13_basic
Last Modified: 28-DEC-1989

Problem:

I would like to search for files with a given extension, save them in
an array, and display them on the screen for user selection using
QuickBASIC Version 2.00 and the DOS interrupt routine INT86. I have
written a sample program that calls function 4E and 4F to find the
desired files. They appear to be working correctly, as the flag
returned in the low byte of register AX is showing 0 for each
successful find and 18 when no files exist or when the last file has
been found. However, I do not know how to read the DTA (Disk Transfer
Address) after I successfully find a file, thus I cannot store the
filename for later display.

Response:

In QuickBASIC Versions 2.00, 2.01, and 3.00, you can perform DOS
Interrupts by calling the external routine INT86. In QuickBASIC
Versions 4.00, 4.00b, and 4.50, you can call the external routines
INT86OLD INT86XOLD INTERRUPT and INTERRUPTX.

It is difficult to find the current DTA of a program (and its offset
is subject to change without notice); therefore, it is better to ask
DOS (via INT 21, function Hex 1A "Set DTA") to reassign the DTA to a
location over which you have control. For the application indicated,
you should try the following:

1. Issue the DOS function call to change the DTA address (INT 21,
   function Hex 1A) to an address of a structure in your code space
   (this way you will know where to find it).

2. Perform your "Find first" (INT 21, function Hex 4E).

3. Perform your loop of "Continue file search" (INT 21, function Hex
   4F).

Within your loop you may check your variables in the structure as
desired. The "Microsoft MS-DOS Programmer's Reference" manual includes
an example of how to set up the structure for the DTA in assembler.

The book "Advanced MS-DOS," by Ray Duncan (Microsoft Press, 1986) also
is a very helpful reference for using DOS function calls.

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