redrawing characters in command prompt

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k, this is gonna sound pretty silly, but just bear with me for a sec. what i'm looking for here is some function, or weird method, of rapidly redrawing the characters on the screen in a dos prompt. I intend to make a silly little ascii game for my high school computer information science class where we have a computer formated with dos 7.0, and i ran into this little problem. i would like to redraw the text instantly, kinda like how direct x draws 60 frames per second, but simply with redrawing text. at such a low resolution, i assume it should be very simple, but when i do something like this, it's nasty blinky!

while(1)
{
    cout << "test string here";
    system("cls");
}

an old software doesn't make the computer old hardware, why is the graphics adapter not able to keep up with something so simple? So i guess my question is, is there a way to repaint it more rapidly? or as a bypass to said blinkiness, is there a way to redraw only individual characters on the screen so that, say, if a person hits the right arrow, and only the x in the middle of the screen moves one character space to the rigth, i don't have to redraw the entire screen, just redraw that x?

EDIT: btw, i'm not writing the program specifically for dos yet, i'm just writing it for the command prompt in windows.

Posted 13-Oct-11 17:17pm

FatalCatharsis

Updated 13-Oct-11 19:05pm

Pravin Patil, Mumbai

v3

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Philippe Mori 14-Oct-11 8:23am

Done correctly, it could be pretty fast on hardware 100 time less powerfull than today machines. It was already pretty fast in the mid-1990's when writting directly to the video buffer on top-of-the art hardware at that time.

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Sergey Alexandrovich Kryukov · Accepted Answer · 2011-10-13T18:45:00

Solution 1

Nobody did such things on DOS!

Instead, people would go directly to video card RAM, figure out the address of text screen buffer, depending on card type, and would write directly to it. Don't even play with the idea of using console -- it is not designed for such things. Now, the buffer of color mode text screen consists of pair of bytes: one byte is the character to output, another one is background and foreground color, one half-byte for foreground, another half-byte for background, so you have 16 colors for foreground and 16 for background. I hope I remember this correctly.

Therefore, you have an array of 16-bit words which you can write in arbitrary order. The horizontal line is usually 80 characters, so you can address the array like an array of rank 2. Changing any byte in the array changes output in any position on screen.

This is easy enough. We even used to draw all kind of pictures on screen, using pseudo-graphics characters. You also can modify fonts for text mode which are also stored in the video card RAM. This way, you can draw a very special graphics which looks almost like graphical (pixel-level) mode.

Finally, there is one more trick: reloading font for some characters dynamically. It runs in a very smooth way, so there were a popular programs addressing a mouse cursor on a pixel level (in a pure text mode!). It was done via reading of the buffer characters around mouse location, modifying character bytes and bitmap of the font for these characters dynamically.

What else? Yes, you can get a hardware interrupt triggered by the video card at the moment of diagonal motion of the electron beam when the monitor moves the beam from the position of button right pixel to the position of top left one. People install interrupt handler on this IRQ to trigger changes in the video card buffer. This is an analog of "double buffering": instead of costly double buffering, all changes were written to the card when the image update was paused to the back move of the electron beam.

[EDIT]

For rudimentary reference material, use this: http://en.wikipedia.org/wiki/VGA_compatible_text_mode[^]. When you learn terminology but miss some information, search fore more detail in the Web using basic terms and concepts.

And here is the reference for keyboard interrupt usage: http://webpages.charter.net/danrollins/techhelp/0106.HTM[^]. Look at the whole site, it looks promising.

—SA

Posted 13-Oct-11 18:45pm

Sergey Alexandrovich Kryukov

Updated 20-Oct-11 7:25am

v4

Comments

FatalCatharsis 14-Oct-11 1:17am

i understood it fairly well, until that last paragraph :P, don't have any idea what an interrupt is, or what a handler is for, but that can wait for a different discussion. On to the questions:

1) how does one figure out the address of the text screen buffer, and store it in a pointer, to write to?

2) how do i do the same for the font data, and then what is the format it's in and how do i edit it?

3) just another thought that occurred to me, what you described was a single byte character with color data, does dos support the use of unicode (wide) characters?

Sergey Alexandrovich Kryukov 14-Oct-11 1:53am

How can you be so naive? Unicode? what to advise you to read on history?.. well, this is called anachronism... Not Unicode at that time.
--SA

Sergey Alexandrovich Kryukov 14-Oct-11 1:59am

I wish I remembered... Addresses are line B8000, C4000(?). I used to have a couple of reference books... You have very few text modes, and few types of cards: EGA, VGA, Hercules. There is documentation on each card and each mode -- addresses of all buffers are fixed.

Formats for font data depend on text modes. Just a black-and-white bitmaps like 8x8, 8x14, 9x16, a pixel per bit, a byte is arranges horizontally left to right (lo-bit on left). A card is controlled by calling a software interrupts putting parameters in registers. (Can you call interrupts? Its in assembly, usually in-line one, with "pushf" and "retf" instead of "ret"?). DOS/BIOS programming is all based on interrupts.

--SA

Sergey Alexandrovich Kryukov 14-Oct-11 2:06am

I added an article for reference, it's good for starters. Please see after [EDIT].
--SA

Sergey Alexandrovich Kryukov 14-Oct-11 2:11am

Referenced one more article...
--SA

FatalCatharsis 14-Oct-11 1:23am

also, while you're here, i have another dos related question that i was wondering earlier. how do you check if a key has been pressed? not check what string has been entered, but actively, in a sort of message loop like i had earlier, check to see if the state of a key has changed?

Sergey Alexandrovich Kryukov 14-Oct-11 2:02am

You need to install interrupt procedure on key interrupt and read status from register, then chain existing handler, remember previous interrupt procedure address to restore it on the exit of your program.
--SA

FatalCatharsis 14-Oct-11 9:49am

i apologize, but which are you saying i'm naive over? that i don't know what interupts are (i'll take a stab at it and say, it's an event triggered by certain events from the system, which changes the state of a memory address, which you monitor and handle when it changes? I'll give that webpage you suggest a look over when i have a bit more time), or whether vga does or does not support unicode, because from what i've found, vga can hold only 512 glyphs at max. also, "character anachronism" yields no relevant information on google.

EDIT: After trawling through google, i finally found some information on vga addressing specifications (more sparse then it should be?) and found the vga memory is mapped between 0xA0000 - 0xBFFFF , where the graphical data is between 0xA0000 - 0xAFFFF , monochrome text data is between 0xB0000 - 0xB7FFF, and color text data is between 0xB8000 - 0xBFFFF , does that sound about right to you? so if i write a word of data at 0xB8000, that would display a character on the screen in text only mode? or is there some more to it? ( i don't have dos available to me now, nor do i wish to be directly changing the value of memory locations, without being sure that that where i'm writing isn't vital to the performance of the system :/ )

Sergey Alexandrovich Kryukov 14-Oct-11 12:23pm

I said "naive" just about your suggestion of Unicode, which probably did not exist at that time, sorry if I made you feel not very good about it, I really did not mean it. There is no such term "character anachronism", that's just "anachronism" http://en.wikipedia.org/wiki/Anachronism, a general "cultural" notion. This is if someone make a movie of World War II where soldiers would talk to each other using iPhone :-). Same thing about Unicode.

Again, you can write one byte to write a character on screen and another byte to change background color (including blink which is one bit of the color) and foreground color, according to the bit map explained here: http://en.wikipedia.org/wiki/VGA_compatible_text_mode#Text_buffer. Yes, there is nothing else you should do about output on screen in this mode. In principle, you can change the color set by color changing palette, if you want it. The addresses you found sound right.

--SA

FatalCatharsis 15-Oct-11 15:03pm

okay, once i again, i went scouring the internet through very loosely documented information on vga, and i have not been able to find anywhere that tells the memory address of where the fonts are stored at, nor the format that they are stored in. I did however learn that it can hold 8 x 256 character sets, and that, in most 80x25 and 80x50 fonts that they are 8 x 8 pixels. which is cool, but now i need to know where to write it :/ . you wouldn't just happen to have a long lost reference book on this would you?

Sergey Alexandrovich Kryukov 20-Oct-11 11:54am

Maybe, but it will take time to look for those. I had everything about it and routinely used font reloading (once I even used it to draw pseudo-graphics with many custom elements like rounded lined and junctions, including diagonal). Don't hope much, I'm not sure I kept it, as I thought DOS had gone. What do you use? EGA, VGA, what modes?
--SA