char *vidmem = (char *)0xB8000000;
for(i=0;i<2999; i =i+2)
*(vidmem +i) = 'A';
The cast added removed the warning.
Edit: Probably I don't have to write about it, but you need to check if you're writing to the right address, that you have the right hardware memory etc mapped at that address and so on.