Simple Kernel in C and Assembly

Hello, world ! Today I'm going to show you how to write a kernel in C and a little bit of assembly. This is a simple kernel written in C and Assembly which could be loaded with the GRUB bootloader on an x86 system. This kernel will display a message on the screen and then hang. All the source code is available on my github repository.

Tools

Before writing the kernel, make sure that the following tools are available in your system.

• An x86 computer (of course)
• Linux
• NASM assembler
• gcc
• ld (GNU Linker)
• grub

Let's start coding

We like to write everything in C, but we cannot avoid a little bit of assembly. We will write a small file in x86 assembly-language that serves as the starting point for our kernel.

Here is our kernel.asm file.

;;kernel.asm
bits 32		;nasm directive
section .text
	;multiboot spec
	align 4
	dd 0x1BADB002			;magic
	dd 0x00				;flags
	dd - (0x1BADB002 + 0x00)	;checksum. m+f+c should be zero

global start
extern kmain	;kmain is defined in the c file

start:
	cli	;block interrupts
	call kmain
	hlt	;halt the CPU

In the kernel.asm we make a call to kmain. So our execution starts at kmain() in the main C file kernel.c.

/*
 *
 * kernel.c - version 1.0.2
 * 
 */


#define WHITE_TXT 0x07 /* light gray on black text */

void k_clear_screen();
unsigned int k_printf(char *message, unsigned int line);

/* simple kernel written in C */
void k_main() 
{
	k_clear_screen();
	k_printf("Hello, world! Welcome to my kernel.", 0);
};

/* k_clear_screen : to clear the entire text screen */
void k_clear_screen()
{
	char *vidmem = (char *) 0xb8000;
	unsigned int i=0;
	while(i < (80*25*2))
	{
		vidmem[i]=' ';
		i++;
		vidmem[i]=WHITE_TXT;
		i++;
	};
};

/* k_printf : the message and the line # */
unsigned int k_printf(char *message, unsigned int line)
{
	char *vidmem = (char *) 0xb8000;
	unsigned int i=0;

	i=(line*80*2);

	while(*message!=0)
	{
		if(*message=='\n') // check for a new line
		{
			line++;
			i=(line*80*2);
			*message++;
		} else {
			vidmem[i]=*message;
			*message++;
			i++;
			vidmem[i]=WHITE_TXT;
			i++;
		};
	};

	return(1);
}

All our kernel will do is clear the screen and write to it the string "Hello, world! Welcome to my kernel."

Now the linker.ld script.

/*
 * link.ld
 */

OUTPUT_FORMAT(elf32-i386)
ENTRY(start)
SECTIONS
{
	. = 0x100000;
	.text : {*(.text)}
	.data : {*(.data)}
	.bss  : {*(.bss)}
}

That's it. All done.

##Building the kernel We will now create object files from kernel.asm and kernel.c and then link it using our linker script.

nasm -f elf32 kernel.asm -o kasm.o

Now we will run the assembler to create the object file kasm.o in ELF-32 bit format.

gcc -m32 -c kernel.c -o kc.o

Now the linking part,

ld -m elf_i386 -T link.ld -o kernel kasm.o kc.o

##Now run your kernel We will now run the kernel on the qemu emulator.

qemu-system-i386 -kernel kernel

That's it.

License

All the files in this repository are GPL 3.0 licensed.

Author

Debashis Barman (http://www.debashisbarman.in)