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VGA TEXT ATTRIBUTES AND SPECIAL EFFECTS by Chris Dunford While working on a project recently, I realized that I didn't know very much about how VGA text attributes work. I knew that there wasn't a simple one-to-one relationship between attribute numbers and colors, but that's about it. So, I did a little research and fooling around, and found that lots of interesting effects are possible. You can draw people's attention to important screen information in ways more (and less!) subtle than simply flashing it on and off or using intensified colors. For example, you can have colors that gradually shade from normal to high intensity and back, "pulsing" on the screen; you can display text that changes from one color to another (gradually shading from blue to red and back, for example); you can soften blinking by having the color "fade" rather than just disappear. And, there are some things you can do under software control that can't be done with the usual hardware attributes; for example, you can use flashing backgrounds, or you can using flashing foregrounds without sacrificing high intensity backgrounds. These can be done very efficiently, without the need to change the attributes of individual characters. This document is an attempt to share some of what I learned; it explains how VGA text attributes work, and how these effects are possible. The archive that contains this document should also contain a demo program and a linkable object module (with source) that allow you to use the effects in your own programs. You might want to do a quick run of DEMO1 to see some of the possible effects in action. What's different with the VGA? ------------------------------ The CGA implemented a direct relationship between a 4-bit attribute and a color. Each attribute mapped to one color, and the color could not be changed. That changed with the EGA (which had palette registers to play with), and changed again, radically, with the VGA. The attribute number specifies the displayed color very indirectly; it is combined with bits and pieces of 5 different internal VGA registers before it appears on the screen as one of the 262,144 possible colors. There are therefore several ways to change a character's color other than by changing its attribute. And, of course, if you change the way an attribute is displayed, all characters with that attribute are affected. Thus, it is possible to "design" attributes with interesting effects, and to alter the way entire screen fields are displayed very efficiently. Mapping attributes to colors ---------------------------- Before we start, let me note that we are working here with 4-bit attributes. The 8-bit attribute that's associated with a character is, of course, two 4-bit attributes: one for the foreground, and one for the background. This document discusses how these 4-bit attributes become colors; everything here applies equally to foreground and background attributes (thus it is possible to use flashing backgrounds, etc.) Screen colors in text modes are controlled by the VGA's attribute controller. In one of its two modes (the one that the PS/2 BIOS sets by default), here is how a color is selected by an attribute: AND attribute |-------| color plane enable register | | selects | palette register 0-15 color select register | | | | | bits 0-5 bits 6-7 | +------------+------------+ | | AND +-------+--------video DAC mask register | | selects | video DAC color register 0-255 This appears complex, but it can be simplified. First, notice the two logical AND operations. Since both the color plane enable register and the video DAC mask register normally contain all ones, they tend to drop out of the picture, resulting in: attribute 0-15 | | selects | palette register 0-15 color select register | | | | | bits 0-5 bits 6-7 | +------------+------------+ | | selects | video DAC color register 0-255 The 4-bit attribute (0-15) selects one of the 16 palette registers. The palette register contains a number. This number is combined with the contents of the color select register to form another number. For example, suppose: attribute = 13 palreg[13] = 1Bh CSR = 3 Attribute 13 selects palette register 13, which contains 1BH. The color select register contains 3. We put the contents of the color select register in bits 6-7 of the final number, and the contents of palreg[13] in bits 0-5: 3 1B - -- 11 011011 The resulting number is 11011011 binary, 219 decimal. This number selects one of the 256 video DAC color registers. A C-like formula for the video DAC color resgister selection is: r = (p[a] & 0x3F) | ((c << 6) & 0xC0) where r is the DAC color register number, p[] is the array of 16 palette registers, a is the attribute number (0..15), and c is the current contents of the color select register. The color that will be displayed is the color defined by the selected video DAC color register (DAC stands for Digital-to-Analog Converter). Each video DAC register is an 18-bit register that contains three 6-bit values: one for each of the red, green, and blue primary colors. Each value specifies the intensity of that primary color on the screen. A zero for a primary means that it's not included in the color; 63 is maximum intensity. Suppose, in our example, that video DAC register 219 contains the numbers 0, 3FH, and 3FH (it's convenient to think of the register as three 6-bit values rather than one 18-bit value). This means that red will be off, and green and blue will be at maximum intensity. The color displayed by attribute 13 will be bright cyan. Notice that you could change the color of a character with FG attribute N in four different ways: 1. Change the attribute 2. Change the contents of palreg[N] 3. Change the contents of the video DAC register specified by palreg[N] and the color select register 4. Change the contents of the color select register These all have different effects on the total screen display. Changing an attribute affects a single character; changing a palette register affects all characters with that attribute; changing a video DAC register affects any character whose attribute, when combined with color select, selects that register--this could be several attributes; changing the color select register could very well affect everything on the screen. Another way to visualize color selection ---------------------------------------- There is a simpler way to look at the process. The two bits used from the color select register always end up in the high two bits of the video DAC color register number. Thus, the selected DAC color register will always be one of: CSR=0: 00xx xxxx (register range 0-63) CSR=1: 01xx xxxx (64-127) CSR=2: 10xx xxxx (128-191) CSR=3: 11xx xxxx (192-255) The remaining 6 bits come directly from the selected palette register. If we break the 256 video DAC color registers into four palettes of 64 colors each, as follows: DAC registers Palette 0-63 0 64-127 1 128-191 2 192-255 3 then the color select register selects one of the four 64-color palettes, and the palette register (which is selected by the attribute) selects one of the 64 colors of the CSR-selected palette. Thus, it's simply stated: the attribute selects one of the 64 colors available from the palette selected by the color select register. Mode 1 ------ I mentioned that the above represents one of the two modes supported by the attribute controller; call it "mode 0". The other mode (mode 1) is very similar; the only difference is in which bits are combined from the palette register and the color select register. The simplified diagram is as follows: attribute 0-15 | | selects | palette register 0-15 color select register | | | | | bits 0-3 bits 4-7 | +------------+------------+ | | selects | video DAC color register 0-255 The difference is that there are four bits each from the palette register and the color select register (instead of 6 and 2 under the mode 0). A C-like formula for the video DAC color resgister selection in mode 1: r = (p[a] & 0x0F) | ((c << 4) & 0xF0) The effect of mode 1 is that the 256 video DAC color registers are broken down into sixteen 16-color palettes instead of four 64-color palettes: DAC registers Palette 0-15 0 16-31 1 32-47 2 ... 240-255 15 The palette register (still selected by the attribute) selects one of the 16 colors available in the palette selected by the current color select register. The more complex effects (graded color changes, pulsing, etc.) of the demo program DEMO1 are generated under mode 1. This is because all 256 colors defined by the video DAC color registers can be accessed by simply changing the contents of the color select register, which is very efficient (it takes about 1/100th of a second). Under mode 0, only 75% of the available colors can be accessed by just changing the color select register (this is because there are 64 colors per palette, but only 16 attributes). To get to the remaining colors, you have to change the palette registers. But there are 16 palette registers; if you need to alter more than one displayed color, this is less efficient than using the color select register. A mode 1 technique ------------------ As mentioned, there are a number of ways to alter displayed colors. I will concentrate on one technique: creating a series of palettes and then switching palettes by systematically changing the contents of the color select register. This document concentrates on mode 1; some of the same effects (such as periodic intensification and simulated blinking) can be obtained in mode 0. Note that the use of mode 1 largely dictates that you must reinitialize the palette registers. The VGA BIOS initializes some of these registers with values greater than 15; these will be masked in mode 1 to the low 4 bits, yielding values in the range 0..15. These values will probably not result in the colors you intend (two attributes will yield red foreground, for example). The simplest solution is to just fill the palette registers sequentially with numbers from 0..15; this largely causes the palregs to drop out of the picture: attribute N will map directly to color N within the palette selected by the color select register. Assuming that the palette registers are set as described, create a "base" palette in palette 0. That is, fill palette 0 (video DAC color registers 0-15) with 16 base color definitions. It makes sense to set the standard color combinations for the 16 CGA-like colors that people will expect to see (color intensity values in hex): DAC regs Attr R G B Color 0-2 0 0 0 0 Black 3-5 1 0 0 2A Blue 6-8 2 0 2A 0 Green 9-11 3 0 2A 2A Cyan 12-14 4 2A 0 0 Red 15-17 5 2A 0 2A Magenta 18-20 6 2A 2A 0 Brown 21-23 7 2A 2A 2A White 24-26 8 0 0 15 "Gray" (not really) 27-29 9 0 0 3F Bright blue 30-32 10 0 3F 0 Bright green 33-35 11 0 3F 3F Bright cyan 36-38 12 3F 0 0 Bright red 39-41 13 3F 0 3F Bright magenta 42-44 14 3F 3F 0 Yellow 45-47 15 3F 3F 3F Intense white (In practice, I use 33h in place of 3Fh. This leaves some room to flash even the bright colors by intensification.) The second step is to duplicate palette 0 into palettes 1-15, and then vary the color definitions for attributes of interest in some systematic way. For example, let's specify that attribute 1 is not going to be "blue"; it's going to be a green that continuously cycles from normal intensity to high intensity and back. The effect is similar to flashing text, but more subtle. To do this, set the colors for attribute 1 in each of the palettes as follows: Palette 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 R 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 G 2A 2C 2E 31 33 35 36 37 38 39 3A 3B 3C 3D 3E 3F B 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Note that the intensity of the green primary slowly increases from normal (2A) in palette 0 to maximum intensity (3F) in palette 1. Also note that attribute 1 no longer has anything to do with blue--it's green. (See below for information on how to set a video DAC color register definition.) If these values are loaded into the video DAC color registers, and you then sequence through the palettes in a cyclical pattern (0->15->0), characters with attribute 1 will slowly "pulse" from green to bright green. The effect is more subtle--and readable--than simply switching the characters off and on. The palette change is easily and efficiently accomplished by setting the color select register--say, in a timer tick intercept routine. Using a timer intercept is an ideal way to accomplish this sort of special effect. Unlike hardware flashing, the special effects described in this document must be accomplished by your software. Using an externally triggered routine allows this to occur in background; you simply set up the attributes the way you want them and then go about your work. The special effects will continue to be generated even when you're in DOS or BIOS, say, waiting for a keystroke. The skeleton for a typical timer routine might be (assume that DELTA, COUNT, and RESET are all initialized to 1): count = count - 1 if count = 0 then palette = palette+delta if palette > 15 then palette = 15 delta = -delta else if palette < 0 then palette = 0 delta = -delta end set color select register to palette count = reset end This is both efficient and flexible: the blink rate can be adjusted by altering the values of either DELTA and RESET (or both). Increasing DELTA causes the blink rate to speed up by skipping palettes; increasing RESET slows the blinking by skipping clock ticks. Be sure that PALETTE and DELTA are treated as signed numbers. (Note that FLASH1 uses a slughtly fancier technique.) Effects ------- Many "special effects" are possible using this technique. "Pulsing" has already been discussed; here are some others: PERIODIC INTENSIFICATION Pulsing is a gradual change from a low intensity color to a high intensity color. The effect can be hardened by using the normal color for half of the palettes and an intnsified version for the other half. For example, the following palettes flash cyan to bright cyan: Palette 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 R 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 G 2A 2A 2A 2A 2A 2A 2A 2A 3F 3F 3F 3F 3F 3F 3F 3F B 2A 2A 2A 2A 2A 2A 2A 2A 3F 3F 3F 3F 3F 3F 3F 3F The effect can be varied by altering the ratio of low intensity palettes to high intensity palettes ("beacons", below, are simply extreme cases of periodic intensification). COLOR CHANGES Another effect is to flash text from one color to another. This set of palettes flashes bright green text to bright red. It's quite an eye catcher: Palette 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 R 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 G 3F 3F 3F 3F 3F 3F 3F 3F 0 0 0 0 0 0 0 0 B 0 0 0 0 0 0 0 0 3F 3F 3F 3F 3F 3F 3F 3F GRADED COLOR CHANGES An interesting effect is to "grade" one color into another. Rather than simply flashing green to red, change it gradually by fading out the green primary and fading in the red: Palette 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 R 0 2 4 6 9 0C 0F 12 15 18 1B 1E 21 24 27 2A G 2A 27 24 21 1E 1B 18 15 12 0F 0C 9 6 4 2 0 B 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 This attribute will still change from green to red, but the change is not instantaneous; it slowly fades from one color to the other, passing through various other unnamed colors (all combinations of red and green) on the way. SIMULATED FLASHING Regular hardware flashing can be simulated via a variant of the above scheme; simply use the background color for half of the palettes (or the foreground color, if you want to flash the background): Palette 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 R 3F 3F 3F 3F 3F 3F 3F 3F 0 0 0 0 0 0 0 0 G 3F 3F 3F 3F 3F 3F 3F 3F 0 0 0 0 0 0 0 0 B 0 0 0 0 0 0 0 0 2A 2A 2A 2A 2A 2A 2A 2A The example flashes yellow text if displayed on a blue background. SOFTENED FLASHING By placing the background attribute in half of the palettes (as for flashing) and fading out the foreground in the other half, a softer version of flashing is possible: Palette 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 R 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 G 2A 24 1E 19 14 0F 0A 5 0 0 0 0 0 0 0 0 B 2A 24 1E 19 14 0F 0A 5 0 0 0 0 0 0 0 0 The example shows softened flashing of cyan text on a black background. If any of the background primaries are also components of the foreground color, do not fade those: Palette 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 R 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 G 2A 24 1E 19 14 0F 0A 5 0 0 0 0 0 0 0 0 B 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A The example shows softened flashing of cyan on blue. FADING This is an even softer version of flashing text. It fades the foregound over the full 16 palettes by grading the foregound color into the background color. Palette 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 R 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 G 2A 27 24 21 1E 1B 18 15 12 0F 0C 9 6 4 2 0 B 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 The example fades a green foreground into a black background. If the background is a color other than black, just use a graded color change from the FG color to the BG color. STROBES AND BEACONS These are two final sample effects. A strobe is a color that's invisible most of the time (BG=FG) but flashes briefly to maximum intensity. The follow palettes strobe yellow when used with a blue background: Palette 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 R 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 3F G 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 3F B 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A 2A 0 A beacon is a low intensity color that briefly flashes to maximum intensity. The following palletes create a blue beacon: Palette 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 R 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 G 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 B 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 3F Note to programmers: if you use the routines provided in FLASH1.ASM, note that only palettes that are a multiple of DELTA are ever used. E.g., if DELTA is 2, palette 15 will not be used; put the strobe or beacon color in palette 14 instead. Advantages/disadvantages ------------------------ Creating special effects in this manner has several advantages over using hardware effects (such as flashing), and some disdvantages. Among the advantages are: - MANY more effects are possible; I have mentioned only a few of the possibilities. - Transition rates are under your control; the hardware blink rate is fixed. - Effects can be mixed on one screen (one attribute could be a strobe, another a graded color change). - Any effect can be applied to background as well as foreground. Among the disadvantages are: - Additional programming effort is required, along with increased code and data storage. - A background timer routine, if used, has some effect on system efficiency (but it's small). - Effects are associated with attribute numbers, not with bit positions. The last one bears some explanation. Hardware blinking is associated with a single bit (bit 7) of the FG/BG attribute byte. Thus, you have 16 FG colors, all of which can be flashed. Software-controlled special effects, however, are associated with attributes. If you set, say, attribute 6 to be pulsed green, you no longer have a brown attribute available. Thus, the number of effective "base" colors is reduced for each special effect attribute you create. Mode 0 ------ I have covered attribute control mode 1 in some detail, but some of the same effects can be accomplished in mode 0. The primary difference is that you have only four palettes to work with. The simpler effects such as color changes, periodic intensification, and simulated blink can easily be accomplished in mode one by putting the second color in palette 1, and then alternating between palettes 0 and 1. More complex effects would require periodic alterations to the palette registers and/or color plane enable register. Programming ----------- The programming required to accomplish these special effects is relatively straightforward. The VGA BIOS provides all necessary services; no register-level progamming is required (but see the next section). Useful BIOS services are described below. The only element that might need clarification is how to alter a color definition in a video DAC color register. BIOS provides services to set a single color (BIOS video function 10h, subfunction 10h) and to set a whole block of colors (function 10h, subfunction 12h). If you are altering a few colors only, setting them individually is efficient, but it's more efficient to update larger numbers of colors by reloading the entire set of registers. Video function 10h, subfunction 17h reads a block of DAC color registers into memory allocated by your program. Each 18-bit register is split into three bytes, one for each of the red, green, and blue primaries (in that order). If you read all 256 registers, 3*256 or 768 bytes of storage will be required. To obtain the video DAC register number for a particular color in a particular palette, the formula is: r = 64*p + pr[a] (mode 0) r = 16*p + pr[a] (mode 1) where r=register number, p=palette number, pr[] is the array of 16 palette registers, and a=attribute number. The array of palette registers can be read and set via video function 10, subfunctions 9 and 2 respectively. Note that if you have set the palette registers sequentially (pr[0]=0, pr[1]=1,...,pr[15]=15), then they drop out of the picture for calculation purposes, and the formula becomes: r = 64*p + a (mode 0) r = 16*p + a (mode 1) To locate the offset of the 3-byte color definition for a specific video DAC color register within the table described above: o = r * 3 where r is the register number described above. A combined formula that locates an absolute address of the definition of a particular attribute/palette combination: address = base + 64*p + pr[a] (mode 0) address = base + 16*p + pr[a] (mode 1) where base is the base address of the table. The function GET_DAC_PTR in FLASH1.ASM performs this calculation. To alter the displayed color for a palette/register combination, calculate the address of its definition, alter the R/G/B bytes as desired, and use video BIOS function 10h, subfunction 12h. Because this sunfunction can reload the entire set of DAC color registers, you can alter as many registers as you want in one shot. To alter the definitions for a particular attribute in all 16 palettes, just locate the first one (palette 0). The definitions for subsequent palettes will follow at intervals of 16*3 or 48 (30h) bytes in mode 1, or at intervals of 64*3 bytes in mode 0. For example, if the mode 1 definition for attribute 8 in palette 0 is at 1000h, attribute 8 in palette 1 will be at 1030h, in palette 2 at 1060h, etc. Changing the current palette (color select register) ---------------------------------------------------- A palette is selected by setting the contents of the color select register to the desired palette number. This can be done by using a BIOS video service as described below (int 10H, function 10H, subfunction 13h, BL=1). This takes about 1/100th of a second and can easily be done from within a timer intercept routine without any real problems. However, you may want to set the color select register using register level programming, for one reason: programs such as screen blankers may be watching for BIOS video activity. If you use BIOS services to change the palette 18 times a second, your user's screen will never blank. The technique is shown in the timer routine in FLASH1.ASM. It is assembled only if the equate USE_BIOS is zero during assembly. If USE_BIOS is nonzero, FLASH1 is assembled to use the BIOS service for palette switching. There is no significant difference in efficiency between using BIOS and programming the registers yourself; most of the time is spent in waiting for a video retrace (and you thought you were through with that when you got rid of your CGA, didn't you). The demo (DEMO1.ASM) was assembled with USE_BIOS off. The linkable object module -------------------------- FLASH1.OBJ(ASM) provides a linkable object module that can be used to generate all of the special effects described above, and others. The module is organized for COM files; adjusting for other models should be relatively painless. The primary assumption is that DS=ES; there is no assumption that CS=DS (except in the timer intercept), or any explicit use of the stack. The module prologue describes the services available. Be sure to call FLASH_INIT when you start and FLASH_TERM when you exit. The demo program (DEMO1.ASM) demonstrates use of the module. BIOS services -------------- The VGA BIOS supports all necessary manipulation of VGA registers, including the ability to read register contents (for state restoration when you are done). All of the following are accessed through the standard video BIOS service call (int 10h), function 10h. To use video BIOS: ; Set reg AL and others as needed mov ah,10h int 10h Most of these functions are reasonably efficient. An operation such as setting the color select register can easily be accomplished within an interrupt service routine. PALETTE REGISTERS Read one palette register Entry: AL=7 BL=palette register number (0-15) Return: BH=palette register value Set one palette register Entry: AL=0 BH=color value BL=palette register number (0-15) Return: none Read all palette registers Entry: AL=9 ES:DX -> 17-byte buffer Return: ES:DX[0..15] = current palette regs ES:DX[16] = current overscan (border) color value Set all palette registers Entry: AL=2 ES:DX -> 17-byte buffer (formatted as for function 9) Return: None VIDEO DAC REGISTERS Read one video DAC register Entry: AL=15h BX=register number (0..255) Return: CH=green intensity (0..63) CL=blue intensity (0..63) DH=red intensity (0..63) Set one video DAC register Entry: AL=10h BX=register number (0..255) CH=green intensity (0..63) CL=blue intensity (0..63) DH=red intensity (0..63) Return: none Read a block of video DAC registers Entry: AL=17H BX=first register (0..255) CX=number of registers ES:DX -> buffer Return: On return, the buffer is filled with video DAC color values. The first 3 bytes contain the R/G/B values for the first specified register, etc. The buffer should be of size 3*CX or more. Ensure that BX+CX <= 256. Seta block of video DAC color registers Entry: AL=12H BX=first register (0..255) CX=number of registers ES:DX -> buffer The buffer is formatted as for function 12H. Ensure that BX+CX <= 256. Return: None ATTRIBUTE CONTROL MODE/COLOR SELECT REGISTER Read attribute control mode and color select register Entry: AL=1AH Return BL=current attribute control mode (0/1) BH=color select register contents Set attribute control mode Entry: AL=13H BL=0 BH=attribute control mode (0/1) Return: None Set color select register Entry: AL=13H BL=1 BH=value for color select register (0..3 for attribute control mode 0, 0..255 for mode 1). Return: None Author and reference -------------------- This document and the DEMO1 and FLASH1 modules are by: Chris Dunford The Cove Software Group PO Box 1072 Columbia MD 21044 301/992-9371 CompuServe 76703,2002 The document is copyright (C) 1989 by the author. Permission is granted to distribute freely by electronic or other means, but it may not be republished without permission. Reference (and a darn good one, too): Programmer's Guide to IBM PC and PS/2 Video Systems; Richard Wilton, Microsoft Press 1987. 10/09/89