I have a function called kmain(), which just has 1 variable: vidmem. This is a char* type variable (one character), but i want it to be a string.
This is my code at the moment:
kmain()
{
char* vidmem=(char*)0xb8000;
vidmem[0] = 'A';
vidmem[1] = 0x04;
}
As explained before, this variable is just one character: A. How can I make this a string, i.e. Hello, World!?
Others explained to you how to get a string, which was your question. However, this is not your problem. You use the x86 and want to write to the text buffer at 0xB8000 to show a text on the screen. This is not normal, since most other program on x86 run inside an OS.
To get what you want, you have to set every second byte to the character you want and every other byte to the color you want. A string will not work for your problem.
Means you will need something like this:
#define COLOR 0x04 //red text on black background
vidmem[0] = 'A';
vidmem[1] = COLOR;
vidmem[2] = 'B';
vidmem[3] = COLOR;
vidmem[4] = 'C';
vidmem[5] = COLOR;
Of course it would make more sense to do this in a for or while loop and don't do it by hand, but i want to explain what you need to do in order to print some text.
You can directly accessing individual positions on the screens. vidmem is an array of 25 lines, each line is an array of positions, each position has a byte for the symbol and one for the color. You can change your vidmem declaration so you see at a glance what you access.
#define COLOR 0x04
void kmain(void) //also, use a proper declaration, with return type.
{
//vidmem is a pointer to an array with 50 elements of an array of 2 elements with type char
//The array [50][2] is exaclty the same as an array of [100], but
//with [50][2] we tell the compiler how to access it.
//The memory pointed to by vidmem has a special hardware function, so it should be volatile.
volatile char (*vidmem)[50][2]=(void*)0xb8000;
// ^----- 50 columns per line
// ^- 2 bytes per character
// V--------------- Line
// V------------ Column
// V--------- Symbol (0) or color (1)
vidmem[0][0][0] = 'A'; //symbol at position 0,0 is a 'A'
vidmem[0][0][1] = COLOR; //Set color for position 0,0
vidmem[0][1][0] = 'B'; //symbol at position 0,1 is a 'B'
vidmem[0][1][1] = COLOR; //Set color for position 0,1
//you can also directly access any other position
vidmem[1][0][0] = 'C'; //symbol at position 1,0 (second line) is a 'C'
vidmem[1][0][1] = COLOR; //Set color for position 1,0
}
Strings in C are null-terminated sequences of bytes. A char * points to the beginning of this sequence. Such sequence, as I've said, must end with a \0 symbol, the NUL symbol. So if you want to extend your code to make vidmem contain Hello World, you can do something like:
char vidmem[64]; // stack memory allocation (64 bytes)
vidmem[0] = 'H';
vidmem[1] = 'e';
vidmem[2] = 'l';
...
vidmem[11] = 'l';
vidmem[12] = 'd';
vidmem[13] = '!';
vidmem[14] = '\0'; // very important
And now you can
printf("%s", vidmem);
to observe
Hello, world!
A string is a contiguous sequence of characters terminated by a null character, for purposes of C standard library functions, per clause 7.1.1 paragraph 1 of the 2018 C standard. To make a string with the characters 'A' and 0x04, simply put a zero byte after them.
That said, char* vidmem(char*)0xb8000; is not correct C syntax. Perhaps you meant char *vidmem = (char *) 0xb8000;. This is a definition that might be used in special circumstances (different from general-purpose multi-user environments) to define a pointer to a designated place in memory. However, the name vidmem suggests this might be video memory. A character string is an unusual thing to have in video memory. This code may not be doing what you intend.
