Problems writing to video memory

[justin]

I am new to OS programming and I am having some problems writing to the video memory. The following code functions correctly. It detects a 1.44mb floppy drive and prints "drivelocated" (with the attribute 0xC).

Code: Select all

int main()
{ 
   const char * msg1  = "drivelocated";
   clrscr();
   if(Exist144Floppy())
      printc(msg1,0xC);
   for(;;);
   return 0;
}

However, if I change msg1 to "drivefound", it fails and resets my computer. It also fails if I add a space and make it "drive located". I believe I have my problem narrowed down to the letter "u" in "drivefound". Can someone explain to me why certain strings work just fine and others reset the computer?

Thank you,
Justin

[Solar]

No way to tell without seeing the source of printc...

[kerim]

none of us is visionary, so post the printc code

[justin]

Sorry 'bout that. Here is the code for printc:

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void printc(const char * message,const char color)
{
   unsigned long i;
   unsigned char *vidmem = (unsigned char *)0xB8000;
   for (i=0;message[i] != 0;i++) {
      *vidmem = message[i];
      vidmem++;
      *vidmem = color;
      vidmem++;
   }
}

I shaved it down to this and it still doesn't work.

[beyondsociety]

What settings do you use when you link your kernel? Because on my kernel project it works fine if I change msg1 = drivelocated and print it.

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const char *drivefound = "drivelocated";
printc(drivefound,0xC);

Two ways of doing the basic printc function:

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Your way, just smaller
void printc(const char *message, const char color)
{
       unsigned long i;
       unsigned char *vidmem = (unsigned char *) 0xB8000;

       for(i = 0; message[i] != 0; i++) 
       {
???      *vidmem++ = message[i];
???      *vidmem++ = color;
       }
}

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Or the way I do it
void printc(const char *message, const char color)
{
       unsigned long i;
       unsigned char *vidmem = (unsigned char *) 0xb8000;

       while(*message != 0)
      {
???       vidmem[i++] = *message++;
???       vidmem[i++] = color;
      }
}

Hope this helps.

[pini]

Urg... you'd better initialize your 'i' counter in the second solution, our you'll get bad bad results , or use this instead :

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void printc(const char *message, const char color)
{
      unsigned char *vidmem = (unsigned char *) 0xb8000;

      while(*message != 0)
      {
         *vidmem++ = *message++;
         *vidmem++ = color;
      }
}

(though I'm not sure if this works... I'm better at ASM than at C ;D)