Hello again dear coders and welcome to the second trick in Abe's bag of tricks. The demoschool have been closed for the summer because I've been working, but now I'm back in school. That means more demoschool parts and more tricks in the bag. Isn't it wonderful to study and have all this time left over to code...
I have received a lot of letters about the SVGA putpixel routine. It only works on some cards. The problem is the bankswitch routine. The last time I said that you should put the banknumber*16 in dx before calling the bankswitch interrupt. That is not true for all cards. You should put banknumber*(winsize/granularity) in dx. On my card the granularity happens to be 4k and the winsize is always 64k for 256 color modes. That makes (winsize/granularity) = 16. Other cards have granularity 64k, then (winsize/granularity) is 1, and you should only put the banknumber in dx.
To make the same code work on ALL vesa compatible cards you have to get information about the winsize and granularity. VESA has provided an interrupt call that gives this info and much more about all available modes. Actually it's 2 interrupt calls. One call gives info about if VESA is available and if it is, which modes are available. The other call gives info about one of the available modes, for example 640*480*256 (mode 101h).
Before making the interrupt calls you have to define structs to hold the information. The struct for the first call looks like this:
typedef struct {
char signature[4]; //VESA
short version; //VESA version
char far *oem; //Name of the card
long capabilities;
unsigned far *videomodes; //pointer to available modes
short totalmemory;
char reserved[236];
} vesaInfo;
and to fill a vesaInfo struct with information use this function:
int getVesaInfo(vesaInfo far *vesainfo)
{
asm mov ax,04f00h //ax = 0x4f00
asm les di, [vesainfo] //es:di -> vesainfo
asm int 10h //call interrupt and get vesainfo
asm cmp ax,4fh //if ax is 0x4f then vesa is detected
asm jz done
return(0); //if not, return 0 (no vesa card)
done:
return(1); //successful, return 1
}
Now to get info about the card just declare a vesaInfo struct and call the function:
vesaInfo vesainfo;
if(getVesaInfo(&vesainfo;)==1) printf("VESA detected\n");
else printf("Sorry, no VESA detected\n");
Now you have determined if VESA is available and filled the vesainfo struct. The most important information is in vesainfo.videomodes. If you look at the struct you see that it has the type unsigned far*. To step through all available video modes, first declare a pointer with the same type:
unsigned far *mode;
then loop through all available modes (they end with -1):
for( mode = vesainfo.videomodes ; *mode != (unsigned)-1 ; mode++ )
printf("Mode %Xh detected\n",*mode);
Now you have printed all available VESA modes on the screen. But we want more. Remember the bankswitch problem? We have to get more information about each mode. This is done with the getModeInfo VESA interrupt call. Before making the call you have to declare a modeInfo struct:
typedef struct {
unsigned short mode;
unsigned char wina;
unsigned char winb;
unsigned short granularity; //granularity and winsize are
unsigned short winsize; //used to calculate the bankshift
unsigned short winasegment;
unsigned short winbsegment;
void (far *bankSwitch)(void); //bankswitch function
unsigned short bytesperline;
unsigned short width; //width and
unsigned short height; //height of the mode
unsigned char charwidth;
unsigned char charheight;
unsigned char bitplanes;
unsigned char bitsperpixel; //if this is 8 then it's 256 colors
unsigned char banks;
unsigned char memorymodel;
unsigned char banksize;
unsigned char imagepages;
unsigned char reserved1;
unsigned char redmasksize;
unsigned char redfieldposition;
unsigned char greenmasksize;
unsigned char greenfieldposition;
unsigned char bluemasksize;
unsigned char bluefieldposition;
unsigned char rsvdmasksize;
unsigned char rsvdfieldposition;
unsigned char directcolormode;
unsigned char reserved2[216];
} modeInfo;
As you can see there is a lot of information about the modes. The most important fields are the winsize, granularity, width, height and bitsperpixel fields. Another interesting field is the bankSwitch field. You might not be familliar with function pointers but it is a pointer to a function that switches bank, perfect. Now we don't have to use the slow VESA bankswitch interrupt to switch bank. We can call the provided bankswitch routine directly.
To fill in a modeInfo struct, first decide wich mode to get info about and then call this function:
int getVesaModeInfo(int mode, modeInfo far *modeinfo)
{
asm mov ax,4f01h
asm mov cx,[mode] //mode in cx
asm les di,[modeinfo] //es:di -> modeinfo
asm int 10h //and get the info
return (modeinfo->mode & 1); //return 1 if succesful, else 0
}
If mode is one of the detected modes then this function should return 1 and the modeinfo struct is filled with info about the mode. Else, if a non supported mode is chosen, it returns 0.
Now we have all needed info about the mode. (winsize/granularity) is always a power of 2. That means that instead of multiplying you can use left shifts to multiply the banknumber. I made a global variable, bankshift, that contains the number of steps to shift dx. Banksift is calculated like this:
i = modeinfo.winsize/modeinfo.granularity;
switch(i)
{
case 1: bankshift=0; break;
case 2: bankshift=1; break;
case 4: bankshift=2; break;
case 8: bankshift=3; break;
case 16: bankshift=4; break;
case 32: bankshift=5; break;
case 64: bankshift=6; break;
default: bankshift=0; //this should not happen...
}
You can define a global bankswitch function pointer like this:
void (far *switchBank)(void);
Then after making the getModeInfo call, set the functionpointer:
switchBank = modeinfo.bankSwitch;
Now finally we're done. To switch bank: clear bx, put the desired banknumber in dx, multiply with (winsize/granularity) and call switchBank();
asm mov bx,0 //clear bx
asm mov dx,[bank] //move desired bank to dx
asm mov cx,[bankshift] //the number of bits to shift to cx
asm shl dx,cl //and shift
switchBank(); //call the provided bankswitch routine.
This really should work for ALL VESA compatible cards. It might seem like a lot of work to detect modes and to get modeinfo but you just have to put the code in an initialisation routine and then use the same init routine every time.
In my init-routine I also put in a putpixel function pointer. First there is a global putpixel pointer:
void (*putpixel)(int x,int y,char color);
Then I made 2 putpixel routines, one for mode 13h and one for 256 color vesa modes. If mode 13h is chosen then
putpixel = putpixel_13h;
else
putpixel = putpixel_SVGA_256;
There also are global variables that tells the screen width and screen height. The rest of the code is totally unaware of which mode is chosen. It only uses the putpixel routine and the screen width and height variables. It's the init routine that sets everything up.
There are 2 C-programs to show you how things work. The first is called detect.c and it only detects witch modes are available and then prints information about the available modes. The second is called vesabase.c and it lists all available 256-color modes (including mode 13h) and let you choose one of them. Then it draws a simple pattern over the screen in the selected mode. The program is only a base that you can build your own code upon. All 256-color modes work the same way with the colors so you can use the palette routines from the demoschool part 1 & 2 in any VESA mode: coolpal, fade in and out, rotate palette and so on.
I thank everybody who has sent me e-mail about the problems with the bankswitch routine and about the possible solutions. A mail from Maurice van der Pot finally gave me the working solution and then I also got a lot of help from the VESA library found HERE.
download bag2.zip
Happy VESA Hacking
September 1996 Abe Racadabra
s-mail: Albert Veli
Spisringsg. 9
SE-724 76 Väster;ås;
SWEDEN
mail:dat94avi@bilbo.mdh.se.