os/src/drivers/vga.cpp

#include "../kernel/io.h"
#include "vga.h"

int VGA::getOffset(int col, int row) {
    return 2 * (row * VGA_MAX_COLS + col);
}

int VGA::getOffsetRow(int offset) {
    return offset / (2 * VGA_MAX_COLS);
}

int VGA::getOffsetCol(int offset) {
    return (offset - (getOffsetRow(offset) * 2 * VGA_MAX_COLS)) / 2;
}

int VGA::getCursorOffset() {
    io_byte_out(REG_VGA_CTL, 14);
    int offset = io_byte_in(REG_VGA_DATA) << 8;  // High byte

    io_byte_out(REG_VGA_CTL, 15);
    offset += io_byte_in(REG_VGA_DATA);  // Low byte

    return offset * 2;  // Character cell is 2 bytes, so * position by 2
}

void VGA::setCursorOffset(int offset) {
    offset /= 2;  // Char cell is 2 bytes, so divide the memory offset by 2 to get position
    io_byte_out(REG_VGA_CTL, 14);  // write the high-byte
    io_byte_out(REG_VGA_DATA, (unsigned char)(offset >> 8));  // get only the high-byte
    io_byte_out(REG_VGA_CTL, 15);  // write the low-byte
    io_byte_out(REG_VGA_DATA, (unsigned char)(offset & 0xff));  // get only the low-byte
}

int VGA::printChar(char character, int col, int row, char attrs) {
    // Create a char pointer to the start of video memory
    unsigned char* vidmem = (unsigned char*) VGA_ADDRESS;

    // If the attributes are nullish, then assume the default
    if ( !attrs ) {
        attrs = VGA_WHITE_ON_BLACK;
    }

    // Calculate the video memory offset for the current screen location
    int offset;

    // If col & row are non-negative, use those
    if ( col >= 0 && row >= 0 ) {
        offset = getOffset(col, row);
    } else {
        // Otherwise, use the current location of the cursor.
        offset = getCursorOffset();
    }

    // If we see a newline character, advance offset to the end of the current row,
    // so it will be advanced to the first column of the next row.
    if ( character == '\n' ) {
        int current_row = getOffsetRow(offset);
        offset = getOffset(VGA_MAX_COLS - 1, current_row);
    } else {
        // Otherwise, write the char and its attr byte to the specified vidmem offset
        vidmem[offset] = character;
        vidmem[offset + 1] = attrs;
    }

    // Handle scrolling, when we reach the bottom of the screen
    offset = adjustScrolling(offset);

    // Update the offset to the next cell, 2 bytes ahead of the current one
    offset += 2;

    // Update the cursor position on the screen device
    setCursorOffset(offset);

    return offset;
}

void VGA::clearScreen() {
    int screen_size = VGA_MAX_COLS * VGA_MAX_ROWS;
    unsigned char* vidmem = (unsigned char*) VGA_ADDRESS;

    for ( int i = 0; i < screen_size; i += 1 ) {
        vidmem[i * 2] = ' ';
        vidmem[(i * 2) + 1] = VGA_WHITE_ON_BLACK;
    }

    setCursorOffset(getOffset(0, 0));
}

int VGA::adjustScrolling(int offset) {
    int row = getOffsetRow(offset);
    int col = getOffsetCol(offset);

    if ( row >= (VGA_MAX_ROWS - 1) && col >= (VGA_MAX_COLS - 2) ) {
        advanceScrolling();
        return getOffset(col, row - 1);
    }

    return offset;
}

void VGA::advanceScrolling() {
    int screen_size = VGA_MAX_COLS * VGA_MAX_ROWS;
    unsigned char* vidmem = (unsigned char*) VGA_ADDRESS;

    // Loop over every character cell on the screen
    for ( int i = 0; i < screen_size; i += 1 ) {
        // Compute the existing offset & row/column
        int offset = i * 2;
        int col = getOffsetCol(offset);
        int row = getOffsetRow(offset);

        if ( row > 0 ) {
            // If this is not the top (scrolled-off) row, copy the char
            // to the same column in the row above it.
            int new_offset = getOffset(col, row - 1);
            vidmem[new_offset] = vidmem[offset];
            vidmem[new_offset + 1] = vidmem[offset + 1];
        }

        if ( row == (VGA_MAX_ROWS - 1) ) {
            // If this is the bottom row, clear the cells.
            vidmem[offset] = ' ';
            vidmem[offset + 1] = VGA_WHITE_ON_BLACK;
        }
    }
}

void VGA::printAt(char* string, int col, int row) {
    // Update the cursor if col & row nonnegative
    if ( col >= 0 && row >= 0 ) {
        setCursorOffset(getOffset(col, row));
    }

    // Loop through each char of the string and print it
    int i = 0;
    int offset;
    while ( string[i] != 0 ) {
        offset = printChar(string[i], col, row, VGA_WHITE_ON_BLACK);

        // Compute row/col for next iteration
        i += 1;
        col = getOffsetCol(offset);
        row = getOffsetRow(offset);
    }
}

void VGA::print(char* string) {
    printAt(string, -1, -1);
}