How can I write directly to the screen?

Question

I\'m a teenager who has become very interested in assembly language. I\'m trying to write a small operating system in Intel x86 assembler, and I was wondering how to write direc

Answer 1

To write directly to the screen, you should probably write to the VGA Text Mode area. This is a block of memory which is a buffer for text mode.

The text-mode screen consists of 80x25 characters; each character is 16 bits wide. If the first bit is set the character will blink on-screen. The next 3 bits then detail the background color; the final 4 bits of the first byte are the foreground (or the text character)'s color. The next 8 bits are the value of the character. This is usually code-page 737 or 437, but it could vary from system to system.

Here is a Wikipedia page detailing this buffer, and here is a link to codepage 437

Almost all BIOSes will set the mode to text mode before your system is booted, but some laptop BIOSes will not boot into text mode. If you are not already in text mode, you can set it with int10h very simply:

xor ah, ah
mov al, 0x03
int 0x10

(The above code uses BIOS interrupts, so it has to be run in Real Mode. I suggest putting this in your bootsector.)

Finally, here is a set of routines I wrote for writing strings in protected mode.

unsigned int terminalX;
unsigned int terminalY;
uint8_t terminalColor;
volatile uint16_t *terminalBuffer;

unsigned int strlen(const char* str) {
    int len;
    int i = 0;
    while(str[i] != '\0') {
        len++;
        i++;
    }
    return len;
}

void initTerminal() {
    terminalColor = 0x07;
    terminalBuffer = (uint16_t *)0xB8000;
    terminalX = 0;
    terminalY = 0;

    for(int y = 0; y < 25; y++) {
        for(int x = 0; x < 80; x++) {
            terminalBuffer[y * 80 + x] = (uint16_t)terminalColor << 8 | ' ';
        }
    }
}

void setTerminalColor(uint8_t color) {
    terminalColor = color;
}

void putCharAt(int x, int y, char c) {
    unsigned int index = y * 80 + x;
    if(c == '\r') {
        terminalX = 0;  
    } else if(c == '\n') {
        terminalX = 0;
        terminalY++;
    } else if(c == '\t') {
        terminalX = (terminalX + 8) & ~(7);
    } else {
        terminalBuffer[index] = (uint16_t)terminalColor << 8 | c;
        terminalX++;
        if(terminalX == 80) {
            terminalX = 0;
            terminalY++;
        }
    }
}

void writeString(const char *data) {
    for(int i = 0; data[i] != '\0'; i++) {
        putCharAt(terminalX, terminalY, data[i]);       
    }           
}

You can read up about this on this page.