I’m writing a simple OS. I’m having trouble with the output string in qemu screen, the screen just shows something like this (it only shows the output of the boot.asm file, but the output of the kernel.c file doesn’t show):

I have the following code:

boot.asm

[bits 16]
[org 0x7c00]

boot:
    ; Set up segments
    xor ax, ax
    mov ds, ax
    mov es, ax
    mov ss, ax
    mov sp, 0x7c00

    ; Print a message
    mov si, hello_msg
    call print_string

    ; Load the kernel
    mov bx, 0x1000  ; Load address
    mov dh, 2       ; Number of sectors to read
    mov dl, 0       ; Drive number (0 = floppy)
    mov ch, 0       ; Cylinder number
    mov cl, 2       ; Sector number (1-based, so start from 2)

    mov ah, 0x02    ; BIOS read sector function
    int 0x13        ; Call BIOS interrupt

    ; Jump to the kernel
    jmp 0x1000

print_string:
    lodsb
    or al, al
    jz done
    mov ah, 0x0e
    int 0x10
    jmp print_string
done:
    ret

hello_msg db 'Hello from bootloader!', 0x0D, 0x0A, 0

times 510-($-$$) db 0
dw 0xaa55

kernel.c

void kmain(void) {
    const char *str = "Hello from kernel!";
    char *vidptr = (char*)0xb8000;
    unsigned int i = 0;
    unsigned int j = 0;

    while(j < 80 * 25 * 2) {
        vidptr[j] = ' ';
        vidptr[j+1] = 0x07;
        j = j + 2;
    }

    j = 0;

    while(str[i] != '') {
        vidptr[j] = str[i];
        vidptr[j+1] = 0x07;
        ++j;
        ++j;
        ++i;
    }

    while(1);
}

linker.ld

ENTRY(kmain)
SECTIONS
{
    . = 0x1000;
    .text : { *(.text) }
    .data : { *(.data) }
    .bss  : { *(.bss)  }
}

Makefile

all: os-image

boot.bin: boot.asm
    nasm -f bin boot.asm -o boot.bin

kernel.o: kernel.c
    gcc -m32 -c kernel.c -o kernel.o -ffreestanding

kernel.bin: kernel.o
    ld -m elf_i386 -T linker.ld -o kernel.bin kernel.o

os-image: boot.bin kernel.bin
    cat boot.bin kernel.bin > os-image

run: os-image
    qemu-system-i386 -drive format=raw,file=os-image -monitor stdio

clean:
    rm -f *.bin *.o os-image

The output seems that the kernel.c file is not running, so how to solve this problem?