OSDev.org

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bits 16
global _start
extern kmain

section .text

_start:
	cli
	mov ax, 07C0h		; Set up 4K stack space after this bootloader
	add ax, 288		; (4096 + 512) / 16 bytes per paragraph
	mov ss, ax
	mov sp, 4096

	mov ax, 07C0h		; Set data segment to where we're loaded
	mov ds, ax
	sti

        call kmain

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void kmain(void) {
	const char* string = "stage1 loaded\0";

	volatile char *video = (volatile char*)0xB8000;
	while( *string != 0 )
    	{
        	*video++ = *string++;
        	*video++ = 0x07;
    	}
}

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ENTRY(_start)
STARTUP(boot.o)
INPUT(kernel.o)

SECTIONS
{
   .text :
   {
	*(.text);
   }
}
OUTPUT_FORMAT(binary)
OUTPUT(myos.bin)

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nasm -f elf -o boot.o boot.asm
i686-elf-gcc -c kernel.c -o kernel.o -std=gnu99 -ffreestanding -O2 -Wall -Wextra
i686-elf-gcc -T kernel.ld -o myos.bin -ffreestanding -O2 -nostdlib -lgcc

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dd if=/dev/zero of=floppy.img bs=512 count=2880
dd if=myos.bin of=floppy.img

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i686-elf-gcc -m16 -c kernel.c -o kernel.o -std=gnu99 -ffreestanding -O2 -Wall -Wextra

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asm(".code16gcc\n");

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void kmain(void) __attribute__((section(".text")));

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----------------
IN: 
# kmain(void)
0x00007c43:  call   0x7e00

----------------
IN: 
0x00007e00:  add    %al,(%bx,%si)
0x00007e02:  add    %al,(%bx,%si)
...