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Given the above assumptions, drawing text is easy; we simply copy each byte of each character to the appropriate location in display memory, and voila, we’re done. Text copying is done in write mode 0, in which the byte written to display memory is copied to all four planes at once; hence, 1-bits turn into white (color value 0FH, with 1-bits in all four planes), and 0-bits turn into black (color value 0). This is faster than using write mode 3 because write mode 3 requires a read/write of display memory (or at least preloading the latches with the background color), while the write mode 0 approach requires only a write to display memory.

Is write mode 0 always the best way to do text? Not at all. The write mode 0 approach described above draws both foreground and background pixels within the character box, forcing the background pixels to black at the same time that it forces the foreground pixels to white. If you want to draw transparent text (that is, draw only the character pixels, not the surrounding background box), write mode 3 is ideal. Also, matters get far more complicated if characters that aren’t 8 pixels wide are drawn, or if characters are drawn starting at arbitrary pixel locations, without the multiple-of-8 column restriction, so that rotation and masking are required. Lastly, the Map Mask register can be used to draw text in colors other than white—but only if the background is black. Otherwise, the data remaining in the planes protected by the Map Mask will remain and can interfere with the colors of the text being drawn.

I’m not going to delve any deeper into the considerable issues of drawing VGA text; I just want to sensitize you to the existence of approaches other than the ones used in Listings 44.1 and 44.2. On the VGA, the rule is: If there’s something you want to do, there probably are 10 ways to do it, each with unique strengths and weaknesses. Your mission, should you decide to accept it, is to figure out which one is best for your particular application.

Page Flipping

Now that we know how to update the screen reasonably quickly, it’s time to get on to the fun stuff. Page flipping answers the second requirement for animation, by keeping bitmap changes off the screen until they’re complete. In other words, page flipping guarantees that partially updated bitmaps are never seen.

How is it possible to update a bitmap without seeing the changes as they’re made? Easy—with page flipping, there are two bitmaps; the program shows you one bitmap while it updates the other. Conceptually, it’s that simple. In practice, unfortunately, it’s not so simple, because of the design of the VGA. To understand why that is, we must look at how the VGA turns bytes in display memory into pixels on the screen.

The VGA bitmap is a linear 64 K block of memory. (True, most adapters nowadays are SuperVGAs with more than 256 K of display memory, but every make of SuperVGA has its own way of letting you access that extra memory, so going beyond standard VGA is a daunting and difficult task. Also, it’s hard to manipulate the large frame buffers of SuperVGA modes fast enough for real-time animation.) Normally, the VGA picks up the first byte of memory (the byte at offset 0) and displays the corresponding 8 pixels on the screen, then picks up the byte at offset 1 and displays the next 8 pixels, and so on to the end of the screen. However, the offset of the first byte of display memory picked up during each frame is not fixed at 0, but is rather programmable by way of the Start Address High and Low registers, which together store the 16-bit offset in display memory at which the bitmap to be displayed during the next frame starts. So, for example, in mode 10H (640×350, 16 colors), a large enough bitmap to store a complete screen of information can be stored at display memory offsets 0 through 27,999, and another full bitmap could be stored at offsets 28,000 through 55,999, as shown in Figure 44.1. (I’m discussing 640×350 mode at the moment for good reason; we’ll get to 640×480 shortly.) When the Start Address registers are set to 0, the first bitmap (or page) is displayed; when they are set to 28,000, the second bitmap is displayed. Page flipped animation can be performed by displaying page 0 and drawing to page 1, then setting the start address to page 1 to display that page and drawing to page 0, and so on ad infinitum.


Figure 44.1
  Memory allocation for mode 10h page flipping.


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Graphics Programming Black Book © 2001 Michael Abrash