1 Title: INTRODUCTION TO MODE X (XINTRO.TXT)
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5 Author: Robert Schmidt <robert@stud.unit.no>
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7 Copyright: (C) 1993 of Ztiff Zox Softwear - refer to Status below.
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9 Last revision: 25-Nov-93
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11 Figures: 1. M13ORG - memory organization in mode 13h
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12 2. MXORG - memory organization in unchained modes
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14 The figures are available both as 640x480x16 bitmaps
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15 (in GIF format), and as 7-bit ASCII text (ASC) files.
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17 C sources: 1. LIB.C v1.2 - simple graphics library for planar,
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18 256-color modes - optionally self-testing.
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20 Excerpts from the source(s) appear in this article.
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21 Whenever there are conflicts, the external source file(s)
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22 are correct (or, at least, newest), _not_ the excerpts
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25 Status: This article, its associated figures and source listings
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26 named above, are all donated to the public domain.
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27 Do with it whatever you like, but give credit where
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28 credit is due. I would prefer it if this archive was
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29 distributed in its entirety, including the files
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32 The standard disclaimer applies.
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35 1. INTRODUCTION TO THE VGA AND ITS 256-COLOR MODE
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36 2. GETTING MORE PAGES AND PUTTING YOUR FIRST PIXEL
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37 3. THE ROAD FROM HERE
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38 4. BOOKS ON THE SUBJECT
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44 This text gives a fairly basic, yet technical, explanation to what, why
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45 and how Mode X is. It first tries to explain the layout of the VGA
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46 memory and the shortcomings of the standard 320x200 256-color mode,
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47 then gives instructions on how one can progress from mode 13h to a
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48 multipage, planar 320x200 256-color mode, and from there to the
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49 quasi-standard 320x240 mode, known as Mode X.
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51 A little experience in programming the standard VGA mode 13h
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52 (320x200 in 256 colors) is assumed. Likewise a good understanding of
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53 hexadecimal notation and the concepts of segments and I/O ports is
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54 assumed. Keep a VGA reference handy, which at least should have
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55 definitions of the VGA registers at bit level.
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57 Throughout the article, a simple graphics library for unchained (planar)
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58 256-color modes is developed. The library supports the 320x200 and
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59 320x240 modes, active and visible pages, and writing and reading
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63 1. INTRODUCTION TO THE VGA AND ITS 256-COLOR MODE
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65 Since its first appearance on the motherboards of the IBM PS/2 50, 60
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66 and 80 models in 1987, the Video Graphics Array has been the de facto
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67 standard piece of graphics hardware for IBM and compatible personal
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68 computers. The abbreviation, VGA, was to most people synonymous with
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69 acceptable resolution (640x480 pixels), and a stunning rainbow of colors
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70 (256 from a palette of 262,144), at least compared to the rather gory
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73 Sadly, to use 256 colors, the VGA BIOS limited the users to 320x200
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74 pixels, i.e. the well-known mode 13h. This mode has one good and one
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75 bad asset. The good one is that each one of the 64,000 pixels is easily
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76 addressable in the 64 Kb video memory segment at 0A000h. Simply calculate
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77 the offset using this formula:
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79 offset = (y * 320) + x;
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81 Set the byte at this address (0A000h:offset) to the color you want, and
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82 the pixel is there. Reading a pixel is just as simple: just read the
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83 corresponding byte. This was heaven, compared to the havoc of planes and
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84 masking registers needed in 16-color modes. Suddenly, the distance from a
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85 graphics algorithm on paper to an implemented graphics routine in assembly
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86 was cut down to a fraction. The results were impressively fast, too!
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88 The bad asset is that mode 13h is also limited to only one page, i.e.
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89 the VGA can hold only one screenful at any one time (plus 1536 pixels, or
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90 about four lines). Most 16-color modes let the VGA hold more than one page,
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91 and this enables you to show one of the pages to the user, while drawing on
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92 another page in the meantime. Page flipping is an important concept in making
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93 flicker free animations. Nice looking and smooth scrolling is also almost
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94 impossible in mode 13h using plain VGA hardware.
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96 Now, the alert reader might say: "Hold on a minute! If mode 13h enables
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97 only one page, this means that there is memory for only one page. But I
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98 know for a fact that all VGAs have at least 256 Kb RAM, and one 320x200
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99 256-color page should consume only 320*200=64000 bytes, which is less
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100 than 64 Kb. A standard VGA should room a little more than four 320x200
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101 pages!" Quite correct, and to see how the BIOS puts this limitation on
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102 mode 13h, I'll elaborate a little on the memory organization of the VGA.
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104 The memory is separated into four bit planes. The reason for this stems
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105 from the EGA, where graphics modes were 16-color. Using bit planes, the
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106 designers chose to let each pixel on screen be addressable by a single
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107 bit in a single byte in the video segment. Assuming the palette has
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108 not been modified from the default, each plane represent one of the EGA
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109 primary colors: red, green, blue and intensity. When modifying the bit
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110 representing a pixel, the Write Plane Enable register is set to the
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111 wanted color. Reading is more complex and slower, since you can
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112 only read from a single plane at a time, by setting the Read Plane
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113 Select register. Now, since each address in the video segment can
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114 access 8 pixels, and there are 64 Kb addresses, 8 * 65,536 = 524,288
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115 16-color pixels can be accessed. In a 320x200 16-color mode, this makes
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116 for about 8 (524,288/(320*200)) pages, in 640x480 you get nearly 2
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117 (524,288/(640*480)) pages.
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119 In a 256-color mode, the picture changes subtly. The designers decided
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120 to fix the number of bit planes to 4, so extending the logic above to 8
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121 planes and 256 colors does not work. Instead, one of their goals was to
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122 make the 256-color mode as easily accessible as possible. Comparing the
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123 8 pixels/address in 16-color modes to the 1-to-1 correspondence of
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124 pixels and addresses of mode 13h, one can say that they have
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125 succeeded, but at a certain cost. For reasons I am not aware of, the
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126 designers came up with the following effective, but memory-wasting
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129 The address space of mode 13h is divided evenly across the four bit
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130 planes. When an 8-bit color value is written to a 16-bit address in the
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131 VGA segment, a bit plane is automatically selected by the 2 least
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132 significant bits of the address. Then all 8 bits of the data is written
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133 to the byte at the 16-bit address in the selected bitplane (have a look at
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134 figure 1). Reading works exactly the same way. Since the bit planes are so
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135 closely tied to the address, only every fourth byte in the video memory is
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136 accessible, and 192 Kb of a 256 Kb VGA go to waste. Eliminating the
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137 need to bother about planes sure is convenient and beneficial, but to
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138 most people the loss of 3/4 of the total VGA memory sounds just hilarious.
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140 To accomodate this new method of accessing video memory, the VGA
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141 designers introduced a new configuration bit called Chain-4, which
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142 resides as bit number 3 in index 4 of the Sequencer. In 16-color modes,
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143 the default state for this bit is off (zero), and the VGA operates as
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144 described earlier. In the VGA's standard 256-color mode, mode 13h, this
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145 bit is turned on (set to one), and this turns the tieing of bit
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146 planes and memory address on.
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148 In this state, the bit planes are said to be chained together, thus mode
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149 13h is often called a _chained mode_.
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151 Note that Chain-4 in itself is not enough to set a 256-color mode -
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152 there are other registers which deals with the other subtle changes in
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153 nature from 16 to 256 colors. But, as we now will base our work with
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154 mode X on mode 13h, which already is 256-color, we won't bother about
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159 2. GETTING MORE PAGES AND PUTTING YOUR FIRST PIXEL
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161 The observant reader might at this time suggest that clearing the
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162 Chain-4 bit after setting mode 13h will give us access to all 256 Kb of
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163 video memory, as the two least significant bits of the byte address
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164 won't be `wasted' on selecting a bit plane. This is correct. You might
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165 also start feeling a little uneasy, because something tells you that
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166 you'll instantly loose the simple addressing scheme of mode 13h. Sadly,
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167 that is also correct.
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169 At the moment Chain-4 is cleared, each byte offset addresses *four*
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170 sequential pixels, corresponding to the four planes addressed in 16-color
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171 modes. Every fourth pixel belong in the same plane. Before writing to a byte
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172 offset in the video segment, you should make sure that the 4-bit mask in the
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173 Write Plane Enable register is set correctly, according to which of the four
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174 addressable pixels you want to modify. In essence, it works like a 16-color
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175 mode with a twist. See figure 2.
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177 So, is this mode X? Not quite. We need to elaborate to the VGA how to
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178 fetch data for refreshing the monitor image. Explaining the logic
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179 behind this is beyond the scope of this getting-you-started text, and it
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180 wouldn't be very interesting anyway. Also, mode 13h has only 200 lines,
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181 while I promised 240 lines. I'll fix that later below. Here is the minimum
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182 snippet of code to initiate the 4 page variant of mode 13h (320x200), written
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183 in plain C, using some DOS specific features (see header for a note about the
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186 ----8<-------cut begin------
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188 /* width and height should specify the mode dimensions. widthBytes
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189 specify the width of a line in addressable bytes. */
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191 int width, height, widthBytes;
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193 /* actStart specifies the start of the page being accessed by
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194 drawing operations. visStart specifies the contents of the Screen
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195 Start register, i.e. the start of the visible page */
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197 unsigned actStart, visStart;
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200 * set320x200x256_X()
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201 * sets mode 13h, then turns it into an unchained (planar), 4-page
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202 * 320x200x256 mode.
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210 /* Set VGA BIOS mode 13h: */
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213 int86(0x10, &r, &r);
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215 /* Turn off the Chain-4 bit (bit 3 at index 4, port 0x3c4): */
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217 outport(SEQU_ADDR, 0x0604);
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219 /* Turn off word mode, by setting the Mode Control register
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220 of the CRT Controller (index 0x17, port 0x3d4): */
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222 outport(CRTC_ADDR, 0xE317);
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224 /* Turn off doubleword mode, by setting the Underline Location
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225 register (index 0x14, port 0x3d4): */
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227 outport(CRTC_ADDR, 0x0014);
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229 /* Clear entire video memory, by selecting all four planes, then
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230 writing 0 to the entire segment. */
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232 outport(SEQU_ADDR, 0x0F02);
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233 memset(vga+1, 0, 0xffff); /* stupid size_t exactly 1 too small */
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236 /* Update the global variables to reflect the dimensions of this
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237 mode. This is needed by most future drawing operations. */
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242 /* Each byte addresses four pixels, so the width of a scan line
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243 in *bytes* is one fourth of the number of pixels on a line. */
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245 widthBytes = width / 4;
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247 /* By default we want screen refreshing and drawing operations
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248 to be based at offset 0 in the video segment. */
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250 actStart = visStart = 0;
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254 ----8<-------cut end------
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256 As you can see, I've already provided some of the mechanics needed to
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257 support multiple pages, by providing the actStart and visStart variables.
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258 Selecting pages can be done in one of two contexts:
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260 1) selecting the visible page, i.e. which page is visible on
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263 2) selecting the active page, i.e. which page is accessed by
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266 Selecting the active page is just a matter of offsetting our graphics
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267 operations by the address of the start of the page, as demonstrated in
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268 the put pixel routine below. Selecting the visual page must be passed
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269 in to the VGA, by setting the Screen Start register. Sadly enough, the
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270 resolution of this register is limited to one addressable byte, which
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271 means four pixels in unchained 256-color modes. Some further trickery is
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272 needed for 1-pixel smooth, horizontal scrolling, but I'll make that a subject
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273 for later. The setXXXStart() functions provided here accept byte
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274 offsets as parameters, so they'll work in any mode. If widthBytes and
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275 height are set correctly, so will the setXXXPage() functions.
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277 ----8<-------cut begin------
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280 * setActiveStart() tells our graphics operations which address in video
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281 * memory should be considered the top left corner.
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284 setActiveStart(unsigned offset)
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290 * setVisibleStart() tells the VGA from which byte to fetch the first
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291 * pixel when starting refresh at the top of the screen. This version
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292 * won't look very well in time critical situations (games for
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293 * instance) as the register outputs are not synchronized with the
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294 * screen refresh. This refresh might start when the high byte is
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295 * set, but before the low byte is set, which produces a bad flicker.
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296 * I won't bother with this now.
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299 setVisibleStart(unsigned offset)
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302 outport(CRTC_ADDR, 0x0C); /* set high byte */
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303 outport(CRTC_ADDR+1, visStart >> 8);
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304 outport(CRTC_ADDR, 0x0D); /* set low byte */
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305 outport(CRTC_ADDR+1, visStart & 0xff);
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309 * setXXXPage() sets the specified page by multiplying the page number
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310 * with the size of one page at the current resolution, then handing the
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311 * resulting offset value over to the corresponding setXXXStart()
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312 * function. The first page number is 0.
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315 setActivePage(int page)
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317 setActiveStart(page * widthBytes * height);
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320 setVisiblePage(int page)
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322 setVisibleStart(page * widthBytes * height);
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325 ----8<-------cut end------
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327 Due to the use of bit planes, the graphics routines tend to get more
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328 complex than in mode 13h, and your first versions will generally tend to
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329 be a little slower than mode 13h algorithms. Here's a put pixel routine
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330 for any unchained 256-color mode (it assumes that the 'width' variable
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331 from the above code is set correctly). Optimizing is left as an exercise
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332 to you, the reader. This will be the only drawing operation I'll cover
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333 in this article, but all general primitives like lines and circles can be
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334 based on this routine. (You'll probably not want to do that though, due
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335 to the inefficiency.)
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337 ----8<-------cut begin------
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339 putPixel_X(int x, int y, char color)
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342 /* Each address accesses four neighboring pixels, so set
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343 Write Plane Enable according to which pixel we want
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344 to modify. The plane is determined by the two least
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345 significant bits of the x-coordinate: */
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347 outportb(0x3c4, 0x02);
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348 outportb(0x3c5, 0x01 << (x & 3));
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350 /* The offset of the pixel into the video segment is
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351 offset = (width * y + x) / 4, and write the given
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352 color to the plane we selected above. Heed the active
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353 page start selection. */
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355 vga[(unsigned)(widthBytes * y) + (x / 4) + actStart] = color;
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359 char getPixel_X(int x, int y)
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362 /* Select the plane from which we must read the pixel color: */
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364 outport(GRAC_ADDR, 0x04);
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365 outport(GRAC_ADDR+1, x & 3);
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367 return vga[(unsigned)(widthBytes * y) + (x / 4) + actStart];
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371 ----8<-------cut end------
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374 However, by now you should be aware of that the Write Plane Enable
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375 register isn't limited to selecting just one bit plane, like the
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376 Read Plane Select register is. You can enable any combination of all
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377 four to be written. This ability to access 4 pixels with one
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378 instruction helps quadrupling the speed in certain respects, especially when
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379 drawing horizontal lines and filling polygons of a constant color. Also, most
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380 block algorithms can be optimized in various ways so that they need only
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381 a constant number of OUTs (typically four) to the Write Plane Enable
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382 register. OUT is a relatively slow instruction.
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384 The gained ability to access the full 256 Kb of memory on a standard
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385 VGA enables you to do paging and all the goodies following from that:
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386 smooth scrolling over large maps, page flipping for flicker free
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387 animation... and I'll leave something for your own imagination.
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389 In short, the stuff gained from unchaining mode 13h more than
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390 upweighs the additional complexity of using a planar mode.
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392 Now, the resolution of the mode is of little interest in this
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393 context. Nearly any 256-color resolution from (about) 80x8 to 400x300
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394 is available for most VGAs. I'll dwell particularly by 320x240, as this
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395 is the mode that Michael Abrash introduced as 'Mode X' in his DDJ
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396 articles. It is also the resolution that most people refer to when
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399 The good thing about the 320x240 mode is that the aspect ratio is
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400 1:1, which means that each pixel is 'perfectly' square, i.e. not
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401 rectangular like in 320x200. An ellipse drawn with the same number of
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402 pixels along both main axes will look like a perfect circle in 320x240,
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403 but like a subtly tall ellipse in 320x200.
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405 Here's a function which sets the 320x240 mode. You'll notice that
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406 it depends on the first piece of code above:
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408 ----8<-------cut begin------
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413 /* Set the unchained version of mode 13h: */
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415 set320x200x256_X();
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417 /* Modify the vertical sync polarity bits in the Misc. Output
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418 Register to achieve square aspect ratio: */
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420 outportb(0x3C2, 0xE3);
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422 /* Modify the vertical timing registers to reflect the increased
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423 vertical resolution, and to center the image as good as
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426 outport(0x3D4, 0x2C11); /* turn off write protect */
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427 outport(0x3D4, 0x0D06); /* vertical total */
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428 outport(0x3D4, 0x3E07); /* overflow register */
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429 outport(0x3D4, 0xEA10); /* vertical retrace start */
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430 outport(0x3D4, 0xAC11); /* vertical retrace end AND wr.prot */
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431 outport(0x3D4, 0xDF12); /* vertical display enable end */
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432 outport(0x3D4, 0xE715); /* start vertical blanking */
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433 outport(0x3D4, 0x0616); /* end vertical blanking */
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435 /* Update mode info, so future operations are aware of the
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442 ----8<-------cut end------
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445 As you've figured out, this mode will be completely compatible with the
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446 utility functions presented earlier, thanks to the global variable
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447 'height'. Boy, am I foreseeing or what!
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449 Other resolutions are achieved through giving other values to the sync
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450 timing registers of the VGA, but this is quite a large and complex
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451 subject, so I'll postpone this to later, if ever.
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453 Anyway, I hope I've helped getting you started using mode X. As far as
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454 I know, the two modes I've used above should work on *any* VGA and Super
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455 VGA available, so this is pretty stable stuff. Let me know of any
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461 3. THE ROAD FROM HERE
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463 I'm providing information on various libraries and archives which relate
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464 to what this article deals with. If you want me to add anything to this
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465 list (for future articles), let me know, although I can't promise anything.
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466 I am assuming you have ftp access.
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469 wuarchive.wustl.edu:/pub/MSDOS_UPLOADS/programming/xlib06.zip
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471 This is the current de facto C/assembler library for programming
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472 unchained modes (do not confuse with a X Windows library). All sources
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473 are included, and the library is totally free. It has functions for
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474 pixels, lines, circles, bezier curves, mouse handling, sprites (bitmaps),
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475 compiled bitmaps, and supports a number of resolutions. The version number
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476 ('06') is current as of November 1993.
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481 Michael Abrash' articles in Doctor Dobbs Journal is always mentioned
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482 with awe. In this 350 Kb archive, most of his interesting stuff has
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483 been gathered. Read about Mode X development and techniques from month
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484 to month. Included is also all the individual source code snippets from
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485 each article, and also the full XSHARP library providing linedrawing,
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486 polygons, bitmaps, solid 3D projection and speedy rendering, and even an
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487 implementation of 2D texture mapping (can be used for quasi-3D texture
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488 mapping), plus an article on assembly optimization on the i86 processor
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489 family. Definitely recommended.
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492 oak.oakland.edu:/pub/msdos/vga/vgadoc2.zip
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494 This is a bare bones VGA register reference. It also contains register
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495 references for the CGA, EGA and Hercules cards, in addition to dozens of
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496 SuperVGAs. Check out the BOOKS section for some decent VGA references
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497 though - you don't want to start tweaking without a real one.
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500 wuarchive.wustl.edu:/pub/MSDOS_UPLOADS/programming/tweak15b.zip
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502 TWEAK might be of interest to the more adventurous reader. TWEAK lets you
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503 play around with the registers of the VGA in an interactive manner.
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504 Various testing screens for viewing your newmade modes are applied at
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505 the press of a key. Version 1.5 adds a test screen which autodetects your
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506 graphics mode and displays various information about resolutions etc.
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507 Keep a VGA reference handy. Don't try it if this is the first time you've
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508 heard of 'registers' or 'mode X' or 'tweaking'. I was planning a version
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509 based on the Turbo Vision interface, but time has been short. Maybe later!
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514 4. BOOKS ON THE SUBJECT
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516 Extremely little has been published in written form about using
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517 'Mode X'-style modes. Below are some books which cover VGA programming
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518 at varying degrees of technical level, but the only one to mention
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519 unchained modes and Mode X, is Michael Abrash'. I'd get one of the VGA
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520 references first, though.
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522 o George Sutty & Steve Blair : "Advanced Pogrammer's Guide to the
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523 EGA/VGA" from Brady. A bit old perhaps, but covers all *standard*
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524 EGA/VGA registers, and discusses most BIOS functions and other
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525 operations. Contains disk with C/Pascal/assembler source code.
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526 There's a sequel out for SuperVGAs, which I haven't seen.
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528 o Michael Abrash : "Power Graphics Programming" from QUE/Programmer's
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529 Journal. Collections of (old) articles from Programmer's Journal on
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530 EGA/VGA, read modes and write modes, animation, tweaking (320x400
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531 and 360x480). His newer ravings in DDJ covers fast 256-color
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532 bitmaps, compiled bitmaps, polygons, 3D graphics, texture mapping
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535 o Richard F. Ferraro : "Programmer's Guide to the EGA and VGA video
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536 cards including Super VGA". I don't have this one, but heard it's
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537 nice. Detailed coverage of all EGA/VGA registers. The Super VGA
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538 reference makes it attractive.
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540 o Richard Wilton : "Programmer's Guide to PC & PS/2 Video Systems"
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541 Less technical, more application/algorithm oriented. Nice enough,
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542 even though it is a bit outdated, in that he discusses CGA and
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543 Hercules cards just as much as EGA/VGA.
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550 I am considering writing a text describing in more detail the process of
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551 using TWEAK to achieve the VGA resolution you want or need. However, I
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552 thought I'd let this document go first, and see if I get any reactions.
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553 If I don't, I'll stop. Feel free to forward any suggestions,
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554 criticisms, bombs and beers.
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556 I can be reached via:
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558 o e-mail: robert@stud.unit.no
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568 Nothing would encourage or please me more than a postcard from where you
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