Explain how the following program works?

Question

Here is the program in C language: (I removed some portion of the code which simply initializes VGA_COLOUR variables)

static inline uint8_t vga_entry_color(enum vga_color fg, 
enum vga_color bg) {
    return fg | bg << 4;
}

static inline uint16_t vga_entry(unsigned char uc, uint8_t color) {
    return (uint16_t) uc | (uint16_t) color << 8;
}

size_t strlen(const char* str) {
size_t len = 0;
while (str[len])
    len++;
return len;
}

static const size_t VGA_WIDTH = 80;
static const size_t VGA_HEIGHT = 25;

size_t terminal_row;
size_t terminal_column;
uint8_t terminal_color;
uint16_t* terminal_buffer;

void terminal_initialize(void) {
    terminal_row = 0;
    terminal_column = 0;
    terminal_color = 
    vga_entry_color(VGA_COLOR_LIGHT_GREY, 
    VGA_COLOR_BLACK);
    terminal_buffer = (uint16_t*) 0xB8000;
    for (size_t y = 0; y < VGA_HEIGHT; y++) {
        for (size_t x = 0; x < VGA_WIDTH; x++) {
            const size_t index = y * VGA_WIDTH + x;
            terminal_buffer[index] = vga_entry(' ', terminal_color);
        }
    }
}

void terminal_setcolor(uint8_t color) {
    terminal_color = color;
}

void terminal_putentryat(char c, uint8_t color, size_t x, 
size_t y) {
    const size_t index = y * VGA_WIDTH + x;
    terminal_buffer[index] = vga_entry(c, color);
}

void terminal_putchar(char c) {
    terminal_putentryat(c, terminal_color, terminal_column, terminal_row);
    if (++terminal_column == VGA_WIDTH) {
        terminal_column = 0;
        if (++terminal_row == VGA_HEIGHT)
            terminal_row = 0;
    }
}

void terminal_write(const char* data, size_t size) {
    for (size_t i = 0; i < size; i++)
        terminal_putchar(data[i]);
}

void terminal_writestring(const char* data) {
    terminal_write(data, strlen(data));
}

#if defined(__cplusplus)
extern "C" /* Use C linkage for kernel_main. */
#endif
void kernel_main(void) {
    /* Initialize terminal interface */
    terminal_initialize();

    /* Newline support is left as an exercise. */
    terminal_writestring("Hello, kernel World!\n");
}

It is from OSdev wiki, and although I do understand some parts of this program - actually kernel.c - I have problem with functions "terminal_initilialize", "terminal_putentryat", "terminal_putchar". Any explanation is welcome.

Answer 1

I hope that this explanations can give you some hints. By analyzing the code functions you should be able to figure out the rest.

VGA_WIDTH - width of the screen; VGA_HEIGHT - height of the screen

x - horizontal position of the character on the screen

y - vertical position of the character on the screen

Since VGA card keeps all characters in a linear memory it has to use x, y,
VGA_WIDTH and VGA_WIDTH to calculate an index for its internal buffer to store a character at the proper memory location.

void terminal_initialize - this function initializes your terminal. It just puts spaces on the entire screen.

void terminal_putentryat(char c, uint8_t color, size_t x, size_t y)

puts a character c, on the screen at the location (x,y).

void terminal_putchar(char c) - it puts a character c, at the screen.

This function has terminal_column and terminal_row variables to track the current terminal column and row.