No acess to the video memory????

This is a discussion on No acess to the video memory???? within the Game Programming forums, part of the General Programming Boards category; Good morning Srs. I have check by a vesa card then i have fill a srtuct ModeInfo vbeGetModeInfo(0x101,&ModeInfo) and vbeSetMode(0x101); ...

  1. #1
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    No acess to the video memory????

    Good morning Srs.



    I have check by a vesa card
    then i have fill a srtuct ModeInfo
    vbeGetModeInfo(0x101,&ModeInfo)
    and vbeSetMode(0x101);
    now i set some bytes in video memory lyke
    _fmemset(far*ptr,val,len)

    but i just see the screen all black

    and i dont figure out why

    can some one give me a idea??

    why i dont have acess to the video memory?
    all the tests return sucess and that mode is suported by the card
    and i have VESA ver 3.0

    thanks good day to you

    Code:
    void main(){
    	  short oldmode=0;
    	  vbeInfo info;
    	  vbeModeInfo modeInfo;
    	  if(vbeGetInfo(&info)){
    		  if(memcmp(info.vesa,"VESA",4)==0){
    				 vbePrintInfo(&info);
    				 getch();
    				 if(vbeGetModeInfo(0x101,&modeInfo)){
    						vbeGetMode(&oldmode);
    						vbeSetMode(0x101);
    						_fmemset((unsigned char far*)MK_FP(0xa000,0),0x15,20);
    				 }
    		  }else printf("Not Vesa\n");
    	  }else printf("Error\n");
    		getch();
    		if(oldmode)vbeSetMode(oldmode);
    }
    Last edited by joseCarlos; 10-27-2009 at 09:29 AM.

  2. #2
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    Are you actually programming on the DOS operating system?
    bit∙hub [bit-huhb] n. A source and destination for information.

  3. #3
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    yes i am, my fryend
    and im shure i am in 0x101 mode width out problems

  4. #4
    Registered User Codeplug's Avatar
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    What version of DOS?
    What compiler?
    What [video] hardware?
    Where are the "vbeXXX" functions from?

    gg

  5. #5
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    sorry!!

    OS- Dos 5.1
    video Card - Intel(r) 82945GM VESA 3.0
    that information i can get from my code, and this mode(0x101) is available
    on that card
    i can see all the information on vbeModeInfo structure
    compiler- Borland 4.5

    the vbe functions comes from some exemples
    i got in web

    [CODE]

    int
    vbeSetMode(int16 mode){
    union REGS reg;
    struct SREGS sreg;
    reg.x.ax = 0x4f02;
    reg.x.bx = mode;
    int86x(0x10, &reg, &reg, &sreg);
    return (reg.x.ax == 0x004f);
    }
    [\CODE]

    thanks for your time
    Last edited by joseCarlos; 10-27-2009 at 11:15 AM.

  6. #6
    Registered User Codeplug's Avatar
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    Is the OS running in a virtual machine or directly on the HW?

    gg

  7. #7
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    is directly on HW
    im just learning to comunicate width VESA card

    and i can work width vga modes also width mode 12h
    but when i go to svga modes, the screen is black

  8. #8
    Registered User Codeplug's Avatar
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    You are not checking for errors when calling vbe[Get/Set]Mode(). Does it return success?

    gg

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    yes it return 1 ... ax=0x004f

    i have a lapptop ASUS, is that important?

    if i put cout<<.. or printf(..)
    nathing appears

    to see samthing i need old the data, and
    after in vga modes print that data
    Last edited by joseCarlos; 10-27-2009 at 12:43 PM.

  10. #10
    Super Moderator VirtualAce's Avatar
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    It's been a loooong time since I've done any linear frame buffer code but I'm sure Ralph Brown's Interrupt List would be a good place to start learning about VESA.

    You must get a far pointer to the LFB and then you can access the LFB just like an array. However to access the LFB I believe you will need a DOS extender. You will be able to set the mode but you will not be able to write to it. If you do not have an extender the max you can memcpy at any one time will be limited to 64KB. Also if you do not have an extender there is no way to map the LFB into your address space.

  11. #11
    l'Anziano DavidP's Avatar
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    _fmemset((unsigned char far*)MK_FP(0xa000,0),0x15,20);
    So this is the line where you actually set pixels.

    I can't find any documentation on mode 0x101, so I don't know what palette is available and what your resolution is, but if you look here you will notice that color 0x15 on the 256-color palette is a fairly dark gray color. Since you are only writing 20 bytes (4 pixels per byte using 256-color palette), you are only writing 5 pixels.

    5 pixels of a dark gray color could be difficult to see on a high res screen (assuming mode 0x101 is high res). Try mode 0x13 and see what happens.

    Then try changing the color you are using to something else, say...0x30 (which according to the wikipedia page is a bright lime green).
    My Website

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  12. #12
    Registered User Codeplug's Avatar
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    0x13 is "OEM" (non-VESA) mode - http://www.delorie.com/djgpp/doc/rbinter/it/10/0.html
    0x101 is 640x480@256 - http://www.delorie.com/djgpp/doc/rbinter/it/83/0.html
    Older VBE 1.2 spec found here - http://docs.ruudkoot.nl/vesasp12.txt

    Definitely been a loooong time. Good catch on only 20 bytes being set

    Found op's code here: http://www.gameprogrammer.com/2-frust.html, which was originally implemented for use with a DOS extender. However it still works with 64K chunks of video memory via function 4F05h - http://www.delorie.com/djgpp/doc/rbinter/id/83/2.html

    gg

  13. #13
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    Talking

    I feel like an idiot

    I think you are right, I chose a very dark color
    Thank you for your time
    one can laugh at will,
    happens to those who have little experience

  14. #14
    l'Anziano DavidP's Avatar
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    I think you are right, I chose a very dark color
    Thank you for your time
    It was the first thing I suspected because I remember when I played around with low-level video modes like that several years back, I had encountered something very similar.

    Always keep in mind the palette and colors you are using. Keep in mind that you are using a graphics mode in which you specify a color from 0 to 256, and that color is initially defined based upon the palette shown on the wikipedia page I referenced earlier.

    You actually can specify colors based on RGB values in this mode, but you have to use the palette as an intermediary way of doing things, and hence even though you can specify RGB values of colors you want, you can only have 256 different colors displayed on the screen at one time.

    To change your palette and get different colors based upon RGB values you want to work with, try the following functions:

    Code:
    void writepal(int color, unsigned char r, unsigned char g, unsigned char b)
    {
    	outp(0x3C6, 0xFF);
    	outp(0x3C8, color);
    	outp(0x3C9, r);
    	outp(0x3C9, g);
    	outp(0x3C9, b);
    }
    
    void getpal(int color, unsigned char &r, unsigned char &g, unsigned char &b)
    {
    	outp(0x3C6, 0xFF);
    	outp(0x03C7, color);
    	r = inp(0x03C9);
    	g = inp(0x03C9);
    	b = inp(0x03C9);
    }
    I just dug this up from some old code I wrote. I don't guarantee it works. I don't really even remember what the function outp does

    [EDIT]

    Page about outp:
    http://developers.cogentrts.com/coge...s/re-outp.html

    Page about inp:
    http://developers.cogentrts.com/coge...cs/re-inp.html

    [/EDIT]
    Last edited by DavidP; 10-28-2009 at 01:32 PM.
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  15. #15
    Super Moderator VirtualAce's Avatar
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    AFAIK you cannot use the LFB without a DOS extender. You can however use the old bank switching either via an interrupt or a far pointer to the bank switch function on the video card. I'm not sure if modern video cards are still using this or even support it but I would imagine the underlying core rasterization of video has probably not changed much through the years. It is my guess that the nitty gritty, while still there, is now handled and masked by the drivers that both OGL and D3D interact with.

    Remember that you cannot just memcpy() in 640x480 bit modes or in 1024x768 8 bit modes b/c of the bank switch. In higher color modes the 64kb limit will split one of the pixels in two where 2 bytes will be on the first bank and the next 2 bytes will fall on the second bank. As long as you align everything in those modes on a 4 byte boundary you will be fine and your code will send the right data to the right banks.

    I highly recommend using Delorie DJGPP as it comes with a DOS extender and will eliminate the need for the bank switch. It is so much easier to use the near pointer hack from DOS 6.0/6.2 days to get a pointer to the LFB than it is to mess with bank switching.

    I also recommend you write your own memcpy() to do the blitting to the screen since the CRT memcpy() is meant to be generic and may not be optimal for what you want to use it for.

    @DavidP:
    Wow it's been a long time since I changed palettes that way. But ya know even with all the Direct3D and shaders I mess with now something way down deep inside of me still sorta misses all that low level stuff. Maybe I'm just sick.

    It's amazing that any of us remember this stuff and our willingness to contribute so much info is a testament to our love for low level graphics. My how far we have come from those days. The newcomers to graphics don't know how easy they have it and how much has been taken off of their shoulders.
    Last edited by VirtualAce; 10-28-2009 at 10:17 PM.

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