CLOSE

At this point we have loaded kernel.asm which does the entering into protected mode by setting up gdt tables, enabling protected mode bit (PE bit).

Now we proceed a step further by linking C with asm and calling C function which would be our kernel.

products/kernel.bin: products/kernel_entry.o products/kernel.o
	ld -o $@ -Ttext 0x1000 $^ --oformat binary
// kernel.c
#include <stddef.h>
#include <stdint.h>

// VGA text buffer constants
static const size_t VGA_WIDTH = 80;
static const size_t VGA_HEIGHT = 25;
static uint16_t* const VGA_BUFFER = (uint16_t*) 0xB8000;

// VGA color constants
enum vga_color {
    VGA_COLOR_BLACK = 0,
    VGA_COLOR_BLUE = 1,
    VGA_COLOR_GREEN = 2,
    VGA_COLOR_CYAN = 3,
    VGA_COLOR_RED = 4,
    VGA_COLOR_MAGENTA = 5,
    VGA_COLOR_BROWN = 6,
    VGA_COLOR_LIGHT_GREY = 7,
    VGA_COLOR_DARK_GREY = 8,
    VGA_COLOR_LIGHT_BLUE = 9,
    VGA_COLOR_LIGHT_GREEN = 10,
    VGA_COLOR_LIGHT_CYAN = 11,
    VGA_COLOR_LIGHT_RED = 12,
    VGA_COLOR_LIGHT_MAGENTA = 13,
    VGA_COLOR_LIGHT_BROWN = 14,
    VGA_COLOR_WHITE = 15,
};

// Function to create a VGA color attribute byte
static inline uint8_t vga_entry_color(enum vga_color fg, enum vga_color bg) {
    return fg | bg << 4;
}

// Function to create a VGA text entry
static inline uint16_t vga_entry(unsigned char uc, uint8_t color) {
    return (uint16_t) uc | (uint16_t) color << 8;
}

// Kernel main function
void kernel_main() {
    uint8_t color = vga_entry_color(VGA_COLOR_LIGHT_GREY, VGA_COLOR_BLACK);
    VGA_BUFFER[0] = vga_entry('A', color);  // Print 'A' at the top-left corner
}

 

enable_a20:
    in al, 0x92
    or al, 2
    out 0x92, al
    ret

 

 

print_string:
    ; Print a string pointed by DS:BX
    mov ah, 0x0E        ; Teletype output function
.next_char:
    lodsb               ; Load next character from DS:SI into AL
    cmp al, 0
    je .done
    int 0x10            ; BIOS video interrupt
    jmp .next_char
.done:
    ret
    
    
    msg_error db 'Error loading kernel', 0

 

load_error:
    ; Print error message (simplified, could be extended)
    mov bx, msg_error
    call print_string

 

// kernel.c
void kmain(void) {
    char *video_memory = (char *)0xB8000;
    *video_memory = 'j';
    *(video_memory + 1) = 0x07;  // Light grey on black background

    // Infinite loop to prevent exiting
    while (1) {}
}
dd if=/dev/zero of=floppy.img bs=512 count=2880
dd if=stage1boot.bin of=floppy.img bs=512 count=1 conv=notrunc
dd if=stage2boot.bin of=floppy.img bs=512 seek=1 count=1 conv=notrunc
dd if=kernel.bin of=floppy.img bs=512 seek=2 conv=notrunc