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Text File | 1994-05-10 | 40.1 KB | 807 lines

Programming the VGA Registers by Boone (boone@ucsd.edu), March '94 The IBM PC has long been slammed by owners of other computers which come with superior graphics capabilities built right into hardware. The PC is a strange beast to program in general, and when it comes to graphics the programmer doesn't get much help from the video hardware. However, there are quite a few neat tricks you can do using the VGA registers, as I'm sure you're aware. The trick is knowing just which registers to use and how to use them to achieve the desired results. In particular, precise timing is necessary to avoid screen flicker and/or "snow". The registers on your video card are necessary for just about any communication with the VGA besides basic reading/writing of pixels. Some of the registers are standard, which are the ones we will be discussing here. Most SVGA chipsets have their own special functions associated with different registers for things such as bank switching, which is part of what makes trying to write SVGA programs so difficult. The registers are also used to set the various attributes of each video mode: horizontal and vertical resolution, color depth, refresh rate, chain-4 mode, and so on. Luckily, BIOS handles all this for us and since we only need to set the video mode once at program start-up and once at exit, you should need to mess with these particular functions too much, unless you are using a special mode, such as mode X. (See the mode X section for more info on all this.) If you want to experiment with the video mode registers, ftp yourself a file called TWEAK*.* (my version is TWEAK10.ZIP). For now we'll just assume the video mode has already been set to whatever mode you wish. One of the most common techniques used by game programmers is fade in/out. A clean fade is simple but very effective. Suprisingly, even big-budget games like Ultima VII often have a lot of screen noise during their fades. With a little effort you can easily write your own noise-free fade routines. There's nothing like giving a professional first impression on your intro screen, since the fade-in is likely to be the very first thing they see of your program. BIOS is much to slow for this timing-critical opperation, so we'll have to get down and dirty with our VGA card. Fading is a fairly simple process. As you should know, the VGA palette consists of 256 colors with 3 attributes for each color: red, green and blue. Every cycle of the fade, we have to go through all 768 attributes and if it is larger than 0 subtract one. We'll use regsiters 3C8h and 3C9h for palette opperations. The operation for sending a palette to the card is straight-forward: send a 0 to port 3C8h and then your 768 byte buffer to port 3C9h. This is good enough for setting the palette at the start of your program, but of course it has to go in a loop for the fade, since you'll have to do this 256 times, subtracting one from each non-zero member of the buffer. The pseudo-code looks something like this: constant PALSIZE = 256*3; unsigned character buffer[PALSIZE]; boolean done; counter i,j; for j = 255 to 0 { for i = 0 to PALSIZE-1 if buffer[i] > 0 buffer[i] = buffer[i] - 1; output 0 to port 3C8h; for i = 0 to PALSIZE-1 output buffer[i] to port 3C9h; } Easy enough, right? If you convert this to the language of your choice it should run fine. (Make sure you have the buffer pre-loaded with the correct palette, however, or you will get very strange results...) But you'll notice the "snow" mentioned earlier. Depending on your video card, this could mean that you see no noise at all to fuzz covering your entire screen. Even if it look fine on your system, however, we want to make sure it will be smooth on *all* setups it could potentially be run on. For that we're going to have to ask the video card when it's safe to send the palette buffer to the card, and for that we'll need the retrace register. Putting aside palette concerns for a moment, I'll briefly cover the retrace on your video card. (See the next section of this article for a more in-depth discussion of this.) Bascially the vertical retrace is a short time in which the screen is not being updated (from video memory to your monitor) and you can safely do writes to your video memory or palette without worrying about getting snow, flicker, tearing, or other unwanted side-effects. This is a pretty quick period (retrace occurs 60 to 70 times a second) so you can't do too much at once. Returning to our fade: we want to update the palette during the vertical retrace. The value we want is bit 3 of register 3DAh. While that bit is zero we're safe to write. The best practice in this case is to wait for the bit to change to one (screen is being traced) and then the instant it changes to 0, blast all our new video info to the card. It won't be necessary in this case since all we are doing is fading the palette and then waiting for the next retrace, but if you're doing animation or playing music at the same time you'll want to include this extra bit of code as a safety net. Otherwise you might detect the 0 in the refresh bit at the very last instant of the retrace and end up writing while the screen is being traced. The pseudo-code now goes like this: for j = 255 to 0 { for i = 0 to PALSIZE-1 if buffer[i] > 0 buffer[i] = buffer[i] - 1; while bit 3 of port 3DAh is 0 no opperation; while bit 3 of port 3DAh is 1 no opperation; output 0 to port 3C8h; for i = 0 to PALSIZE-1 output buffer[i] to port 3C9h; } That's it! All that's left is for you to implement it in your favorite language. However, I can hear the cries right now: "Code! Give us some real assembly code we can use!" I'm reluctant to provided it as this is the exact sort of thing that is easy to cut and paste into your program without knowing how it works. However, I'll give you the unoptimized main loop in 80x86 assembly as this may be clearer to you that my explanation or pseudo-code. Two things to remember about this code: it is optimized enough to be smooth on any video card (or any that I've seen, anyway) assuming that the fade is the _only_ thing going on. There's some other things you may want to change if you plan to say, play music during this process. Secondly, you'll need to have the current palette loaded into the buffer beforehand. You could read it from the VGA card using either registers or BIOS, but this is both slow and (in my oppinion) sloppy coding. You should *never* ask the video card about anything (excluding retrace) that you could keep track of yourself. In the case of the palette, you probably already loaded it from disk anyway, or if you are using the default palette <cough, gag, choke> just read the values once and store them in your executable or in a resource file. palbuf DB 768 DUP (?) fadecnt DW 040h ; At this point, you should: ; 1) have the video mode set ; 2) have palbuf loaded with the current palette ; 3) have something on the screen to fade! fadeloop: xor al,al ; used for comparisons and port 3D8h mov cx,768 ; loop counter mov si,offset palbuf ; save palette buffer in si decloop: mov dl,[si] ; put next pal reg in dx cmp al,dl ; is it 0? je next ; nope... dec dl ; yes, so subtract one mov [si],dl ; put it back into palette buffer next: dec cx ; decrement counter inc si ; increment our buffer cmp cx,0 jne decloop ; not done yet, so loop around mov cx,768 ; reset for palette output sub si,768 ; reset palbuf pointer mov dx,03c8h out dx,al ; inform VGA of palette change inc dx ; DX = 3C8h + 1 = 3C9h mov ch,02h ; do outter loop 2 times mov dx,03dah ; prepare refresh register mov bx,03c9h ; prepare palette reg (for quick loading) cli ; disable interrupts! outloop: mov cl,80h ; do inner loop 128 times in al,dx ; wait for current retrace to end test al,08h jnz $-5 in al,dx ; wait for current screen trace to end test al,08h jz $-5 mov dx,bx ; load up the palette change register innerloop: mov al,[si] ; load next byte of palbuf out dx,al ; send it to the VGA card dec cl ; decrement counter inc si ; increment palbuf pointer cmp cl,0 jne innerloop ; loop while not done dec ch ; decrement outer loop counter cmp ch,0 jne outloop ; loop while not done sti ; restore interrupts mov ax,fadecnt ; entire palette has been sent dec ax ; so check fade loop mov fadecnt,ax cmp ax,0 ; ready to quit? jne fadeloop ; nope, keep fading! I should add a few comments about this code segment. First of all, it assumes you want to fade every color all the way down. You may only want to fade certain sections of the palette (if your screen was only using a certain number of colors) or maybe your palette is low-intensity so you don't need to go the full 256 loops to get every color down to 0. It also goes by ones, so if you want a faster fade you can have it subtract two from each attribute. If you want to fade to a certain color other than black (for instance, fade to red such as the "getting hit" effect in Doom), you'll need to check if each attribute is above or below your target color and increment or decrement accordingly. Also, you may have noticed something in the code absent from the pseudo-code: it only sends 128 colors to the card each retrace! This is because if you use all 256 the next retrace may start before you get all colors sent to the video card, thanks to the unoptimized code. Some recommend as little as 64 colors per retrace, however I've found 128 to be okay and certainly much faster. The above code works for any VGA-equiped machine, regardless of processor, but you'll probably want to compress all the IN and OUT loops into REP INSB/OUTSB, REP INSW/OUTSW, or REP INSD/OUTSD instructions depending upon the minimum processor requirement for your game/demo. I won't describe fading in since it's the same sort of thing, and I'm sure you can figure it out once you know how to use the registers themselves. It's a little more complicated since you need a second buffer of target values for your attributes, but otherwise quite similar. Next up is vertical retrace. This is simply one of many read registers on your VGA, but it happens to be one of the most useful for animation and palette fades (as shown above). Here's a quick rundown of what exactly the vertical retrace is, and why it's useful. There's an electron gun in the back of your monitor that keeps the pixels "refreshed" with their correct values every 1/60th of a second or so. It fires electrons at each pixel, row by row. The horizontal retrace is the time it takes it to return from the right side of the screen after it has traced a row. This is a very short time and I wouldn't worry about that too much right now, as it is only useful for very specialized (and quite tricky) hardware effects. More useful, however, is the vertical retrace which occurs when the electron gun reaches the bottom of the screen (one entire screen traced) and it returns diagonally to the upper-right hand corner of the screen. During this time you are free to update anything you like having to do with video with no noise or interference (since nothing on the screen is being updated). This is a fairly short amount of time, though, so whatever you want to do you better do it _quickly_. For animation, you'll usually want to keep a second buffer in main memory (remember that video RAM is quite slow compared to main RAM) which you can use to write your animations to. When the vertical retrace occurs, you'll want to blast the entire thing to the VGA as quickly as possible, using a memory copy instruction. You can find more on this in articles which cover animation. Lastly I'll briefly describe the VGA mode-set registers. There are quite a number of them and for the most part they're pretty boring. By sending different values to these registers you can achieve the various video modes that your card is capable of. These registers set values such as horizontal and vertical resolution, retrace timing, addressing modes, color depth, timing, and other fun stuff. The truth is that it's easier and just as effective to let the BIOS (gasp!) handle setting the screen mode for you, particularly since most games use standard modes such as 320x200 anyway. At the very least you can let BIOS set the mode to begin with and then just modify the registers to "tweak" the mode the way you want it. Any of these non-BIOS modes are generally refered to as mode X. I don't want to go deep into detail on the setting and usage of mode X because there is already so much info availible on the topic. Check out the Mode X Faq (regularly posted in comp.sys.ibm.pc.demos and rec.games.programmer), Micheal Abrash's collumn in Dr. Dobb's and his X-sharp library, or the section on mode X in the PC-GPE. One mode register I'll cover quickly is the chain-4 enable/disable. A lot of programmers seem to have trouble visualizing what this thing does exactly. Bit 3 of port 3C4h (index 4) controls chain-4 mode. Normally it is on. This allows fast linear addressing of the bytes in video memory, which is the way you are probably used to addressing them. For example, to change the second pixel on the screen to a certain color, you simply write the value to address A000:0001. With chain-4 disabled (the main feature of mode X besides better resolution) A000:0000 refers to the first pixel in the upper-left corner of your screen, A000:0001 refers to the fourth pixel, A000:0002 to the eight pixel and so on. The odd pixels are accessed by changing the write plane. Since there are four planes, you effectively get an extra two bits of addressing space, boosting the total bit width for your pixel addressing from 16 to 18. Standard chain-4 four only allows access to 64K of memory (2^16) while disabling this feature gives you the full 256K (2^18) of memory to work with. The disadvantage, of course, is that pixel writes are slower due to the port writes required to access odd pixels. How can this be an advantage? For one thing, you can write four pixels at a time as long as they are all the same color - handy for single-color polygons, as in flight simulators. Secondly, you get four times as much memory. This allows you to have higher resolutions without bank switching, or scroll the screen using hardware scrolling, or do page flipping for smooth animation. And since you can change the resolution, you can give yourself a sqaure aspect ration (320x240) which is better for bitmap rotations and the like. But remember that it can be slower for bitmapped graphics because you have to do at least four writes to the card (to change planes) in order to copy bitmaps from main RAM to video memory. Don't use mode X just because you think it's "cool"; make sure you have a good reason for wanting to use it in your program, or otherwise you're wasting a lot of effort for no reason. Now, I'm sure you want me to continue until I divulge all the secrets of the VGA register to you - but, I only have some much time and space. Besides, I still haven't uncovered all of their mysteries and capabilities myself. However, below is a list of the registers which you may want to play with for various effects. The following list was posted on rec.games.programmer by Andrew Bromage (bromage@mundil.cs.mu.OZ.AU), so thanks to him for posting in to begin with. That's it for this article and I hope it helped you understand your VGA card a little better. If not, re-read it, and try writing your own programs which use the registers. The only way to really understand it (as with most things) is to get some hands-on experience. If you've got any questions, comments, flames, or corrections related to this document or game programming/design in general, feel free to post an article in rec.games.programmer (in case you haven't noticed by now, I hang out there regularly) or send mail to boone@ucsd.edu. Here's the list. Have fun... Documentation Over the I/O Registers for Standard VGA Cards Documentated by Shaggy of The Yellow One Email: D91-SJD@TEKN.HJ.SE Feel free to spread this to whoever wants it..... ------------------------------------------------------------ Port-Index: - Port: Write/03c2h Read/03cch usage: d7 Vertical sync polarity d6 Horizontal sunc polarity d5 Odd /even page d4 Disable video d3 Clock select 1 d2 Clock select 0 d1 Enable/Disable display RAM d0 I/O address select Description: Sync polarity: Bits are set as below for VGA displays that use sync polarity to determine screen resolution. Many newer multiple frequency displays are insensitive to sync polarity d7 d6 Resolution 0 0 Invalid 0 1 400 lines 1 0 350 lines 1 1 480 lines I/O address select: When set to zero, selects the monochrome I/O address space (3bx). When set to one, it selects the color I/O address space (3dx) ------------------------------------------------------------ Port-Index: - Port: 03c2h ; read only usage: d7 Vertical Retrace Interrupt pendling d6 Feature connector bit 1 d5 Feature connector bit 0 d4 Switch sense d0-d3 Unused Description: d7 uses IRQ2 ------------------------------------------------------------ Port-Index: - Port: 03bah,03dah ; read only usage: d3 Vertical retrace d0 Horizontal retrace ------------------------------------------------------------ Port-Index: - Port: 03c3h,46e8h usage: d7-d1 Reserved d0 VGA enable/disable (03c3h only) Description: Disables access to display memmory and the other VGA's ports ------------------------------------------------------------ Port-Index: 00h Port: 03d4h, 03b4h usage: Horizontal total Description: Total number of characters in horizontal scan minus five ( including blanked and border characters) ------------------------------------------------------------ Port-Index: 01h Port: 03d4h, 03b4h usage: Horizontal display enable Description: Total number of characters displayed in horizontal scan minus one. ------------------------------------------------------------ Port-Index: 02h Port: 03d4h, 03b4h usage: Start horizontal blanking Description: Character at which blanking starts ------------------------------------------------------------ Port-Index: 03h Port: 03d4h, 03b4h usage: End horizontal blanking d7 Test d6 Skew control d5 Skew control d0-d4 End blanking Description: End blanking: is five LSB bits of six-bit value, which define the character at which blanking stops. The MSB bit of this value is in register index 5. ------------------------------------------------------------ Port-Index: 04h Port: 03d4h, 03b4h usage: Start horizontal retrace Description: Character at which horizontal retrace starts ------------------------------------------------------------ Port-Index: 05h Port: 03d4h, 03b4h usage: End horizontal retrace d7 End horizontal blanking bit 5 d6 Horizontal retrace delay d5 Horizontal retrace delay d0-d4 End horizontal retrace Description: End horizontal retrace: defines the character at which horizontal retrace ends ------------------------------------------------------------ Port-Index: 06h Port: 03d4h, 03b4h usage: Vertical total Description: Total number of horizontal scan lines minus two (including blanked and border characters). MSB bits of this value are in register index 7 ------------------------------------------------------------ Port-Index: 07h Port: 03d4h, 03b4h usage: Overflow register d7 Vertical retrace start (bit 9) d6 Vertical display enable end (bit 9) d5 Vertical total (bit 9) d4 Line compare (bit 8) d3 Start vertical blank (bit 8) d2 Vertical retrace start (bit 8) d1 Vertical display enable end (bit 8) d0 Vertical total (bit 8) ------------------------------------------------------------ Port-Index: 08h Port: 03d4h, 03b4h usage: Preset row scan d7 Unused d6 Byte panning control d5 Byte panning control d0-d4 Preset row scan Description: Byte panning control: is used to control byte panning. This register together with attribute controller register 13h allows for up to 31 pixels of panning in double word modes Preset row scan: Which character scan line is the first to be displayed ------------------------------------------------------------ Port-Index: 09h Port: 03d4h, 03b4h usage: Maximum scan line/Character height d7 double scan d6 bit d9 of line compare register d5 bit d9 of start vertical blank register d0-d4 Maximum scan line Description: d0-d5=Character height-1, only in textmodes ------------------------------------------------------------ Port-Index: 0ah Port: 03d4h, 03b4h usage: Cursor start d7,d6 Reserved (0) d5 Cursor off d4-d0 Cursor start Description: ------------------------------------------------------------ Port-Index: 0bh Port: 03d4h, 03b4h usage: Cursor end d7 reserved d6,d5 Cursor skew d4-d0 Cursor end Description: ------------------------------------------------------------ Port-Index: 0ch Port: 03d4h, 03b4h usage: Start address high ------------------------------------------------------------ Port-Index: 0dh Port: 03d4h, 03b4h usage: Start address low Description: Determine the offset in display memory to be displayed on the upper-left corner on the screen ------------------------------------------------------------ Port-Index: 0eh Port: 03d4h, 03b4h usage: Cursor location (high byte) ------------------------------------------------------------ Port-Index: 0fh Port: 03d4h, 03b4h usage: Cursor location (low byte) Description: Where the cursor is displayed on screen ------------------------------------------------------------ Port-Index: 10h Port: 03d4h, 03b4h usage: Vertical retrace start Description: 8 bits out of 10 ------------------------------------------------------------ Port-Index: 11h Port: 03d4h, 03b4h usage: Vertical retrace end d7 Write protect CRTC register 0 to 7 d6 refresh cycle select d5 enable vertical interrupt (when 0) d4 Clear vertical interrupt (when 0) d0-d3 Vertical retrace end ------------------------------------------------------------ Port-Index: 12h Port: 03d4h, 03b4h usage: Vertical display enable end Description: eight LSB bits out of ten-bit value which define scan line minus one at which the display ends. The other two are in CRTC register index 7 ------------------------------------------------------------ Port-Index: 13h Port: 03d4h, 03b4h usage: Offset / Logical screen width Description: Logical screen width between successive scan lines ------------------------------------------------------------ Port-Index: 14h Port: 03d4h, 03b4h usage: Underline location register d7 Reserved d6 Double word mode d5 count by 4 d0-d4 Underline location Description: Underline location: Monochrome textmode only ------------------------------------------------------------ Port-Index: 15h Port: 03d4h, 03b4h usage: Start vertical blanking Description: eight LSB bits of ten-bit value minus one which define at which scan line the vertical blanking starts. The other two bits are in CRTC registers index 7 and 9 ------------------------------------------------------------ Port-Index: 16h Port: 03d4h, 03b4h usage: End vertical blanking Description: eight LSB bits of a value which determine the scan line after which vertical blanking ends. ------------------------------------------------------------ Port-Index: 17h Port: 03d4h, 03b4h usage: Mode control register d7 Enable vertical and hoizontal retrace d6 Byte mode (1), word mode (0) d5 Address wrap d4 Reserved d3 count by 2 d2 multiple vertical by 2 (use half in CRTC (8,10,12,14,18) d1 Select row scan counter (not used) d0 compatibilty mode support (enable interleave) ------------------------------------------------------------ Port-Index: 18h Port: 03d4h, 03b4h usage: Line compare register Description: Split screen, 8 bit value out of a ten-bit value ------------------------------------------------------------ Port-Index: 00h Port: 03c4h usage: Reset register d7-d2 Reserved d1 Synchronous reset d0 Asynchronous reset Description: Synchr. when set to zero, will halt and reset the sequencer at the end of its current cycle Asyncht. when set to zero, will immediatly halt and reset the sequencer. Data can be loss. ------------------------------------------------------------ Port-Index: 01h Port: 03c4h usage: Clock mode register d7,d6 Reserved d5 display off d4 Allow 32-bit Fetch (not used in standard modes) d3 Divide dot clock by 2 (used in some 320*200 modes) d2 Allow 16-bit fetch (used in mon graphics modes) d1 Reserved d0 Enable (0) 9 dot characters (mono text and 400-line) Description: Display off: Will blank screen and give the cpu uninterrupted access the display memory. ------------------------------------------------------------ Port-Index: 02h Port: 03c4h usage: Color plane write enable register d7,d6 Reserved d3 Plane 3 Write enable d2 Plane 2 Write enable d1 Plane 1 Write enable d0 Plane 0 Write enable Description: ------------------------------------------------------------ Port-Index: 03h Port: 03c4h usage: Character generator select register d7,d6 Reserved d5 Character generator table select A (MSB) d4 Character generator table select B (MSB) d3,d2 Character generator table select A d1,d0 Character generator table select B Description: This register is only of interest if your software will be using multiple character sets. Either one or two character sets can be active. Table A selects the charcater with attribute d3 set to zero and Table B is the one with d3 set to one. ------------------------------------------------------------ Port-Index: 04h Port: 03c4h usage: Memory mode register d4-d7 Reserved d3 Chain 4 (address bits 0&1 to select plan, mode 13h) d2 Odd/even (address bit 0 to select plane 0&2 or 1&3 text modes) d1 Extended memory (disable 64k modes) d0 Reserved Description: ------------------------------------------------------------ Port-Index: 00h Port: 03ceh usage: Set / Reset register d7-d4 Reserved (0) d3 Fill data for plane 3 d2 Fill data for plane 2 d1 Fill data for plane 1 d0 Fill data for plane 0 ------------------------------------------------------------ Port-Index: 01h Port: 03ceh usage: Set / Reset enable register d7-d4 Reserved (0) d3 enable set/reset for plane 3 (1 = enable) d2 enable set/reset for plane 2 (1 = enable) d1 enable set/reset for plane 1 (1 = enable) d0 enable set/reset for plane 0 (1 = enable) Description: Set/Reset enable defines which memory planes will receive fill data from set/reset register. Any plane that is disable for set/reset will be written with normal processor output data ------------------------------------------------------------ Port-Index: 02h Port: 03ceh usage: Color compare register d7-d4 Reserved d3 Color compare value for plane 3 d2 Color compare value for plane 2 d1 Color compare value for plane 1 d0 Color compare value for plane 0 Description: one indicate that color is the same ------------------------------------------------------------ Port-Index: 03h Port: 03ceh usage: Data rotate / Function select register d7-d5 Resrved (0) d4,d3 Function select d2-d0 Rotate count d4 d3 Function 0 0 Write data unmodified 0 1 Write data ANDed with processor latches 1 0 Write data ORed with processor latches 1 1 Write data XORed with processor latches Description: Rotation is made before writing data ------------------------------------------------------------ Port-Index: 04h Port: 03ceh usage: Read plane select register d7-d2 Reserved (0) d1,d0 Defines color plane for reading (0-3) Description: Doesnt matter in color compare mode ------------------------------------------------------------ Port-Index: 05h Port: 03ceh usage: Mode register d7 Reserved (0) d6 256-colour mode d5 Shift register mode d4 Odd / Even mode d3 Color compare mode enable (1 = enable) d2 Reserved (0) d1,d0 Write mode d1 d0 Write mode 0 0 Direct write (data rotate, set/reset may apply) 0 1 Use processor latches as write data 1 0 Color plane n (0-3) is filled with the value of bit n in the write data 1 1 Use (rotated) write data ANDed with Bit mask as bit mask. Use set/reset as if set/reset was enable for all planes Description: ------------------------------------------------------------ Port-Index: 06h Port: 03ceh usage: Miscellaneous register d7-d4 Reserved d3-d2 Memory map 00 = A000h for 128k 01 = A000h for 64k 10 = B000h for 32k 11 = B800h for 32k d1 Odd/even enable (used in text modes) d0 Graphics mode enable Description: Memory map defines the location and size of the host window ------------------------------------------------------------ Port-Index: 07h Port: 03ceh usage: Color don't care register d7-d4 Reserved (0) d3 Plane 3 don't care d2 Plane 2 don't care d1 Plane 1 don't care d0 Plane 0 don't care Description: Color don't care is used in conjunction with color compare mode. This register masks particular planes from being tested during color compare cycles. ------------------------------------------------------------ Port-Index: 08h Port: 03ceh usage: Bitmask register Description: The bitmask register is used to mask certain bit positons from being modified. ------------------------------------------------------------ Port-Index: - Port: 03c0h both index and data usage: d7,d6 Reserved d5 Palette address source 0 = palette can be modified, screen is blanked 1 = screen is enable, palette cannot be modified d4-d0 Palette register address Description: Palette register address selects which register of the attributes controller will be addres,sed by the next I/O write cycle ------------------------------------------------------------ Port-Index: 00h-0fh Port: 03c0h usage: Color palette register d6,d7 Reserved d5-d0 Color value Description: not used in 256 color modes ------------------------------------------------------------ Port-Index: 10h Port: 03c0h usage: Mode control register d7 p4,p5 source select d6 pixel width d5 Horizontal panning compatibility d4 Reserved d3 Background intensify / enable blinking d2 Line graphics enable (text modes only) d1 display type d0 graphics / text mode Description: p4,p5 source select: selects the source for video outputs p4 and p5 to the DACs. If set to zero, p4 and p5 are driven from the palette registers (normal operation). If set to one, p4 and p5 video outputs come from bits 0 and 1 of the color select register. pixel width: is set to one in mode 13h (256-color mode) horizontal panning compatibility: enhances the operation of the line compare register of the CRT controller, which allows one section of the screen to be scrolled while another section remains stationary. When this bit is set to one, the stationary section of the screen will also be immune to horizontal panning. ------------------------------------------------------------ Port-Index: 11h Port: 03c0h usage: Screen border color Description: In text modes, the screen border color register selects the color of the border that sorrounds the text display area on the screen. This is also referred to by IBM as overscan. Unfortunately, this feature does not work properly on EGA displays in 350-line modes. ------------------------------------------------------------ Port-Index: 12h Port: 03c0h usage: Color plane enable register d7,d6 Reserved d5,d4 Video status mux d3 Enable color plane 3 d2 Enable color plane 2 d1 Enable color plane 1 d0 Enable color plane 0 Description: The video status mux bits can be used in conjunction with the diagnostic bits of input status register 1 to read palette registers. For the EGA, this is the only means available for reading the palette registers. Enable color planes can be used to enable or disable color planes at the input to the color lockup table. A zero in any of these bit positions will mask the data from that color plane. The effect on the display will be the same as if that color plane were cleared to all zeros. ------------------------------------------------------------ Port-Index: 13h Port: 03c0h usage: Horizontal panning register d7-d4 reserved d3-d0 Horizontal pan Description: Horizontal pan allows the display to be shifted horizontally one pixel at a time. d3-d0 Number of pixels shifted to the left 0+,1+,2+ 13h Other modes 3+,7,7+ 0 1 0 0 1 2 1 - 2 3 2 1 3 4 3 - 4 5 4 2 5 6 5 - 6 7 6 3 7 8 7 - 8 9 - - ------------------------------------------------------------ Port-Index: 14h Port: 03c0h usage: Color select register d7-d4 Reserved d3 color 7 d2 color 6 d1 color 5 d0 color 4 Description: Color 7 and color 6: are normally used as the high order bits of the eight-bit video color data from the attribute controller to the DACs. The only exceptions are 256-color modes Color 5 and color 4: can be used in place of the p5 and p6 outputs from the palette registers (see mode control register - index 10h). In 16-color modes, the color select register can be used to rapidly cycle between sets of colors in the video DAC. ------------------------------------------------------------ Port-Index: - Port: 03c6h usage: Pixel mask register Description: ??? ------------------------------------------------------------ Port-Index: - Port: 03c7h usage: DAC state register (read-only) Description: if d0 and d1 is set to zero it indicates that the lookup table is in a write mode ------------------------------------------------------------ Port-Index: - Port: 03c7h usage: Lookup table read index register (Write only) Description: Used when you want to read the palette (set color number) ------------------------------------------------------------ Port-Index: - Port: 03c8h usage: Lookup table write index register Description: Used when you want to change palette (set color number) ------------------------------------------------------------ Port-Index: - Port: 03c9h usage: Lookup table data register Description: Read color value (Red-Green-Blue) or write same data. ------------------------------------------------------------