Most printers have a much higher resolution than the EGA display. Thus, the first step in planning a printer graphics display is to determine how many dots are to be printed for each display pixel. You must compare the printer resolution (obtained from the printer documentation) with the color card resolution. The EGA display has 640 pixels horizontally by 350 pixels vertically, giving an aspect ratio of 4:3. However, the pixels on the EGA do not have the same size in both directions; a pixel's vertical size is 1.37 times its horizontal size. The CalComp Plotmaster printer, for example, has a graphics area of 2000 dots horizontally by 1600 dots vertically in the landscape mode. The dots on the printer are square with the same horizontal and vertical sizes.

With this hardware, three printed horizontal dots can be used to represent each horizontal pixel, resulting in a graphics area of 1920 horizontal dots (640 x 3) and ideally, 1440 vertical dots to keep the proper proportions. Printing four vertical dots for every one in the original display gives 1400 vertical dots, resulting in a display that is close to the proper proportions.

Alternatively, one might use wider margins and print two horizontal dots for each horizontal pixel, giving 1280 printed dots horizontally. To maintain the proper proportions, there should be 960 vertical dots. Unfortunately, the best manageable approximation is three vertical dots for each horizontal pixel, producing 1050 printed dots. Though this will give a larger vertical dimension than desired, it is acceptable in representing a text display: when trying to copy a graphics display, the printer may reproduce circles as ovals.

You can't replicate the shape (and possibly the color) of the screen display by just sending characters to the printer. Instead, the printer, operating in graphics mode, must draw a reproduction of the screen display. Thus the screendump program must convert each ASCII character in display memory to the eight by 14 pixel grid which is actually written on the screen. The BIOS contains 14 bytes of shape information for each character. The eight bits in each byte represent the eight character columns, and each represents the one of the 14 character lines.

Each active (light) pixel that is represented by a 1; each inactive pixel by a 0. The beginning address of this character array for the EGA and VGA display is found by issuing interrupt 10H (the video services interrupt), with register AH containing 11H, register AL containing 30H, and register BH containing 02H. The address of the beginning of the character array will be returned in registers ES and BP.

The information for the characters comprising one display row is stored in a buffer consisting of two bytes for each of the 80 characters. The first byte contains the ASCII value of the character, and the second determines whether the foreground and background colors should be printed. The values in the attribute byte and the color panel determine the value of the second byte. It is zero if neither foreground or background is to be printed, one if the foreground is to be printed but not the background, two if the background is to be printed but not the foreground, and three if both are to be printed.

Once the information for a row of characters has been deciphered and stored, print_row sends the information for the row to the printer. This section starts with a loop that repeats for each of the 14 lines of pixels that make up the character. Since the resolution of the printer is set to print three identical lines of dots for each line of pixels, a loop duplicates the process three times. For each printer line, print_row outputs the characters that specify the raster mode and the number of data bytes that follow. Two bytes of data are then sent for each character.

One of four possible situations are selected by a switch statement. If neither foreground or background is to be printed, two bytes of zeroes are sent to the printer. If only the foreground is to be printed, two bytes in which each pair of bits corresponds to a single bit in the character representation are sent. If only the background color is to be printed, two bytes are sent; a one represents a background pixel and a zero a foreground pixel. If both foreground and background are to be printed, two bytes of 0FFH are sent to the printer, resulting in all pixels being printed.

If the new function to be installed, Turbo C's setvect command puts the address of print_handler in the vector table for interrupt 5. The function print_handler is of type interrupt, a special Turbo C type which creates a function that saves all registers at entry and restores them on exit.

The registers are loaded and a geninterrupt command obtains the address of the 8x14 EGA character table from BIOS. This address is placed in the variable new_chars for future use when the interrupt is activated. The message "Text Screen Printing Routine for Plot-Master Installed" is then displayed on the screen.

The Turbo C instruction keep is used to terminate but keep the new print screen interrupt handler resident in memory. The second parameter passed to the keep function is the amount of memory, in paragraphs, reserved for the function being saved. The amount of memory specified should be as little as possible, but not so small as to cut off part of the function. To determine the minimum amount of memory, temporarily comment out the main program and then compile the remainder while generating a map file. (In Turbo C type an ALT-0 (Options), selecting Linker and then Map File Detailed. When compilation is completed, you can examine the map file to get an idea of how much memory the program requires. You should add some memory as a safety factor, convert the result to paragraphs and enter it in the keep statement. With the comment delimiters removed, the program can be recompiled using the tiny memory model. The compiled program should be converted from an .EXE program to a .COM program using the EXE2BIN utility.

MS-DOS is not reentrant, so you must take care in designing of a function to act as an interrupt handler. Portions of the program that are to remain resident and replace the original print screen function must not include any calls to MS-DOS or BIOS routines. If the functions makes calls to MS-DOS or BIOS routines, it may behave unexpectedly when the interrupt is called while another MS-DOS function is active since one stack overlays another. There are ways of including checks and tests to assure that everything is compatible, but the safest method is to write the function without MS-DOS calls.