This section presents SNAP, a memory-resident program that demonstrates most of the techniques discussed in the chapter. SNAP takes a snapshot of the current screen and copies the text to a specified file. SNAP accommodates screens with various column and line counts, such as CGA's 40-column mode or VGA's 50-line mode. The program ignores graphics screens.
Once installed, SNAP occupies approximately 7.5K (kilobytes) of memory. When it detects the ALT+LEFT SHIFT+S key combination, SNAP displays a prompt for a file specification. The user can type a new file name, accept the previous file name by pressing ENTER, or press ESC to cancel the request.
SNAP reads text directly from the video buffer and copies it to the specified file. The program sets the file pointer to the end of the file so that text is appended without overwriting previous data. SNAP copies each line only to the last character, ignoring trailing spaces. The program adds a carriage return–linefeed sequence (0D0Ah) to the end of each line. This makes the file accessible to any text editor that can read ASCII files.
To demonstrate how a program accesses resident data through the Multiplex interrupt, SNAP can reset the display attribute of its prompt box. After installing SNAP, run the main program with the /C option to change box colors:
SNAP /Cxx
The argument xx specifies the desired attribute as a two-digit hexadecimal number—for example, 7C for red on white, or 0F for monochrome high intensity. For a list of color and monochrome display attributes, refer to a description of Basic's COLOR command or to the “Tables” section of the Macro Assembler Reference.
SNAP can deinstall itself, provided another TSR has not been loaded after it. Deinstall SNAP by executing the main program with the /D option:
SNAP /D
If SNAP successfully deinstalls, it displays the following message:
TSR deinstalled