screen.c

#include "screen.h"
#include "io.h"

unsigned char *vidptr; // Video memory lives here
char attr = 0x0F;

// Software cursor
int cursorX = 0;
int cursorY = 0;

#define SCREEN_WIDTH 80
#define SCREEN_HEIGHT 25

/**
 * Clear the screen
 */
void cls() {
    for (int i = 0; i < (SCREEN_WIDTH * SCREEN_HEIGHT * 2); i += 2) {
        vidptr[i] = ' ';
        vidptr[i + 1] = attr;
    }
}

/**
 * Print a string to the screen
 */
void puts(char str[]) {
    int i = 0;

    while(str[i] != '\0') {
        putch(str[i]);
        i++;
    }
}

/**
 * Print a character to the screen using the current attributes
 *
 * TODO: Handle tab character
 */
void putch(unsigned char c) {
    unsigned int location;

    // Backspace
    if (c == '\b') {
        if (cursorX != 0) cursorX--;
    }

    // Carriage return
    else if (c == '\r') {
        cursorX = 0;
    }

  // Newline
    else if (c == '\n') {
        cursorX = 0;
        cursorY++;
    }

    // Non-special characters
    else if (c >= ' ') {
        location = ((cursorY * SCREEN_WIDTH) + cursorX) * 2;
        vidptr[location] = c;
        vidptr[location + 1] = attr;
        cursorX++;
    }

  // Move cursor to next line if it's beyond the right edge of the screen
    if (cursorX >= SCREEN_WIDTH) {
        cursorX = 0;
        cursorY++;
    }

  // Scroll the screen up if necessary
    if (cursorY >= SCREEN_HEIGHT) {
        cursorY = SCREEN_HEIGHT - 1;
        scroll_up();
    }

    move_hw_cursor(cursorX, cursorY);
}

/**
 * Scroll the screen up by one row
 */
void scroll_up() {
    int last_row_start = (SCREEN_WIDTH * (SCREEN_HEIGHT - 1) * 2);

    // Shift characters and their attributes up a single row
    for (int pos = SCREEN_WIDTH; pos < (SCREEN_WIDTH * SCREEN_HEIGHT * 2); pos++) {
        vidptr[pos - SCREEN_WIDTH * 2] = vidptr[pos];
    }

    // Clear the last row
    for (int pos = last_row_start; pos < (SCREEN_WIDTH * SCREEN_HEIGHT * 2); pos += 2) {
        vidptr[pos] = ' ';
    }
}

/**
 * Move hardware cursor to x, y coords
 */
void move_hw_cursor(int x, int y) {
    // @STODO - Move hardware cursor to software cursor position
    unsigned short position = (y * 80) + x;

    // High byte
    write_port(0x3D4, 0x0F);
    write_port(0x3D5, (unsigned char) (position & 0xFF));
    
    // Low byte
    write_port(0x3D4, 0x0E);
    write_port(0x3D5, (unsigned char) ((position >> 8) & 0xFF));
}

void init_video() {
    vidptr = (unsigned char*) 0xB8000;
    cls();
}