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When to Use Write Mode 2 and When to Use Set/Reset

As indicated earlier, write mode 2 and set/reset are functionally interchangeable. Write mode 2 lends itself to more efficient implementations when the drawing color changes frequently, as in Listing 27.2.

Set/reset tends to be superior when many pixels in succession are drawn in the same color, since with set/reset enabled for all planes the Set/Reset register provides the color data and as a result the CPU is free to draw whatever byte value it wishes. For example, the CPU can execute an OR instruction to display memory when set/reset is enabled for all planes, thus both loading the latches and writing the color value with a single instruction, secure in the knowledge that the value it writes is ignored in favor of the set/reset color.

Set/reset is also the mode of choice whenever it is necessary to force the value written to some planes to a fixed value while allowing the CPU byte to modify other planes. This is the mode of operation when set/reset is enabled for some but not all planes.

Mode 13H—320×200 with 256 Colors

I’m going to take a minute—and I do mean a minute—to discuss the programming model for mode 13H, the VGA’s 320×200 256-color mode. Frankly, there’s just not much to it, especially compared to the convoluted 16-color model that we’ve explored over the last five chapters. Mode 13H offers the simplest programming model in the history of PC graphics: A linear bitmap starting at A000:0000, consisting of 64,000 bytes, each controlling one pixel. The byte at offset 0 controls the upper left pixel on the screen, the byte at offset 319 controls the upper right pixel on the screen, the byte at offset 320 controls the second pixel down at the left of the screen, and the byte at offset 63,999 controls the lower right pixel on the screen. That’s all there is to it; it’s so simple that I’m not going to spend any time on a demo program, especially given that some of the listings later in this book, such as the antialiasing code in Chapter F on the companion CD-ROM, use mode 13H.

Flipping Pages from Text to Graphics and Back

A while back, I got an interesting letter from Phil Coleman, of La Jolla, who wrote:

“Suppose I have the EGA in mode 10H (640×350 16-color graphics). I would like to preserve some or all of the image while I temporarily switch to text mode 3 to give my user a ‘Help’ screen. Naturally memory is scarce so I’d rather not make a copy of the video buffer at A000H to ‘remember’ the image while I digress to the Help text. The EGA BIOS says that the screen memory will not be cleared on a mode set if bit 7 of AL is set. Yet if I try that, it is clear that writing text into the B800H buffer trashes much more than the 4K bytes of a text page; when I switch back to mode 10H, “ghosts” appear in the form of bands of colored dots. (When in text mode, I do make a copy of the 4K buffer at B800H before showing the help; and I restore the 4K before switching back to mode 10H.) Is there a way to preserve the graphics image while I switch to text mode?’’

“A corollary to this question is: Where does the 64/128/256K of EGA memory ‘hide’ when the EGA is in text mode? Some I guess is used to store character sets, but what happens to the rest? Or rather, how can I protect it?”

Those are good questions. Alas, answering them in full would require extensive explanation that would have little general application, so I’m not going to do that. However, the issue of how to go to text mode and back without losing the graphics image certainly rates a short discussion, complete with some working code. That’s especially true given that both the discussion and the code apply just as well to the VGA as to the EGA (with a few differences in mode 12H, the VGA’s highmode, as noted below).

Phil is indeed correct in his observation that setting bit 7 of AL instructs the BIOS not to clear display memory on mode sets, and he is also correct in surmising that a font is loaded when going to text mode. The normal mode 10H bitmap occupies the first 28,000 bytes of each of the VGA’s four planes. (The mode 12H bitmap takes up the first 38,400 bytes of each plane.) The normal mode 3 character/attribute memory map resides in the first 4000 bytes of planes 0 and 1 (the blue and green planes in mode 10H). The standard font in mode 3 is stored in the first 8K of plane 2 (the red plane in mode 10H). Neither mode 3 nor any other text mode makes use of plane 3 (the intensity plane in mode 10H); if necessary, plane 3 could be used as scratch memory in text mode.

Consequently, you can get away with saving a total of just under 16K bytes—the first 4000 bytes of planes 0 and 1 and the first 8K bytes of plane 2—when going from mode 10H or mode 12H to mode 3, to be restored on returning to graphics mode.

That’s hardly all there is to the matter of going from text to graphics and back without bitmap corruption, though. One interesting point is that the mode 10H bitmap can be relocated to A000:8000 simply by doing a mode set to mode 10H and setting the start address (programmed at CRT Controller registers 0CH and 0DH) to 8000H. You can then access display memory starting at A800:8000 instead of the normal A000:0000, with the resultant display exactly like that of normal mode 10H. There are BIOS issues, since the BIOS doesn’t automatically access display memory at the new start address, but if your program does all its drawing directly without the help of the BIOS, that’s no problem.

The mode 12H bitmap can’t start at A000:8000, because it’s so long that it would run off the end of display memory. However, the mode 12H bitmap can be relocated to, say, A000:6000, where it would fit without conflicting with the default font or the normal text mode memory map, although it would overlap two of the upper pages available for use (but rarely used) by text-mode programs.

At any rate, once the graphics mode bitmap is relocated, flipping to text mode and back becomes painless. The memory used by mode 3 doesn’t overlap the relocated mode 10H bitmap at all (unless additional portions of font memory are loaded), so all you need do is set bit 7 of AL on mode sets in order to flip back and forth between the two modes.


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Graphics Programming Black Book © 2001 Michael Abrash