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 Complete Instructions to BLOAD and BSAVE EGA and VGA Screens - Q45699
 
 This article describes how to BLOAD and BSAVE EGA and VGA SCREEN modes
 (7, 8, 9, 10, 11, 12, and 13) in Microsoft Visual Basic version 1.00
 for MS-DOS.
 
 More Information:
 
 SCREEN modes 11, 12, and 13 require a VGA card, and SCREEN modes 7, 8,
 9, and 10 require an EGA or VGA card.
 
 Using Visual Basic for MS-DOS, the BLOAD and BSAVE technique
 required for EGA and VGA screens is more involved than the technique
 used for CGA and Hercules screen modes. EGA and VGA screen modes 7, 8,
 9, 10, 11, and 12 are stored differently in video memory than other
 screen modes. EGA and VGA memory (except SCREEN mode 13) is broken up
 into four different bit planes: blue, green, red, and intensity.
 
 Each bit plane is addressed in parallel; that is, the planes are all
 in one location, stacked on top of one another. When manipulating a
 particular address in video memory, the address refers to 4 bytes, not
 just 1 (1 byte in each bit plane). To read or write EGA/VGA screens,
 we need to send or retrieve information to or from each of the four
 bit planes separately. Each plane can be accessed by placing
 information in registers located on the Graphics Controller.
 
 Programming EGA and VGA graphics depends heavily on the Graphics
 Controller. The Graphics Controller manages the CPU and video memory
 when executing EGA/VGA read and write operations. To access the
 Graphics Controller, sending output to a set of ports is necessary.
 Two registers are important when reading and writing EGA/VGA screens:
 map mask and read map select. These registers are located on the
 Graphics Controller.
 
 Accessing the map mask and read map registers requires writing to two
 ports. Writing to the first port (the address register) is necessary
 to select the desired register, and the second (the data register) to
 send information to the selected register. The second port is used by
 all the registers. To select the map mask register for writing (i.e.,
 BLOADing) EGA/VGA screens, send a 2 (the index for the map mask
 register) to port &H3C4 (the sequencer address register). Then, put
 the bit plane number (0 to 3) you want to write to in port &H3C5 (the
 data register). To select the read map register for reading (i.e.,
 BSAVEing) EGA/VGA screens, send a 4 (the index for the read map
 register) to port &H3CE (the graphics address register). Then, put the
 bit plane number you want to read in port &H3CF (the data register).
 This procedure is demonstrated in the subprogram, GRAPHSUB.BAS, shown
 below.
 
 Different screen modes have different resolutions. Therefore, variable
 amounts of EGA/VGA memory must be BLOADed or BSAVEd. The formula for
 calculating the number of bytes to be BLOADed or BSAVEd (with the
 exception of SCREEN mode 13) is:
 
    Number of bytes = (Maximum y-coordinate)*(Maximum x-coordinate)/8
 
 For example, SCREEN mode 9 has a resolution of 640 x 350. The number
 of pixels in this screen mode is 640 * 350 or 224,000 pixels/bits. To
 obtain the number of bytes to save or load, divide this number by 8.
 Therefore, the number of bytes to save or load is 28,000.
 
 In SCREEN mode 13 (VGA medium resolution), the Graphics Controller is
 not used. As in other non-EGA/VGA modes, a contiguous block of memory
 is BLOADed or BSAVEd. SCREEN mode 13 has a resolution of 320 x 200
 with 256K colors (1 byte per pixel); therefore, 64,000 bytes need to
 be saved or loaded.
 
 References
 ----------
 
 The following are excellent resources for detailed information
 concerning EGA and VGA screen modes and how to program for them:
 
    "Programmer's Guide to PC and PS/2 Video Systems"
    by Richard Wilton, published by Microsoft Press (1987)
 
    "The New Peter Norton Programmer's Guide to the IBM PC and PS/2"
    by Peter Norton, published by Microsoft Press (1988)
 
    "The Waite Group's Microsoft C Programming for the PC Revised
    Edition"
    by Robert Lafore, published by The Waite Group, Inc. (1989)
 
    "The Programmer's PC Sourcebook"
    by Thom Hogan, published by Microsoft Press (1988)
 
 Illustrated in the example below is a BASIC program, MAIN.BAS, which is a
 driver program that CALLs the BLOAD and BSAVE EGA/VGA subprogram EgaVgaSub
 further below. The EgaVgaSub subprogram module can be easily pasted into
 your own programs.
 
    ◄See Example►