├── README.md
├── boot.asm
├── kernel.c
├── link.ld
└── run.sh


/README.md:
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1 | # beginOS
2 | A hello world OS from scratch for absolute beginners.
3 | 
4 | #
5 | 
6 | Run it by executing the run.sh script in Linux terminal. 
7 | 
8 | You need to have the programs seen inside the script file.
9 | 


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/boot.asm:
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 1 | bits	32
 2 | section	.text
 3 | 	align	4
 4 | 	dd	0x1BADB002
 5 | 	dd	0x00
 6 | 	dd	- (0x1BADB002+0x00)
 7 | 
 8 | global	start
 9 | extern	kernel_main
10 | start:
11 | 	cli		; clears the interrupts
12 | 	call kernel_main
13 | 	hlt


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/kernel.c:
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 1 | char* vgaBuffer = (char*) 0xb8000; // screen buffer (VGA Text Mode)
 2 | 
 3 | #define VGA_HEIGHT 80 // screen height
 4 | #define VGA_WIDTH 25 // screen width
 5 | 
 6 | // We will come to this later
 7 | void terminal_clear_screen(void);
 8 | 
 9 | kernel_main()
10 | {
11 | 	terminal_clear_screen();
12 | 	
13 | 	/*
14 | 	Each VGA character is 16 bits. First 8 bits determine the ASCII character
15 | 	and last 8 bits determine the color of that character. We used an 
16 | 	8 bit char pointer for avoiding complexity when writing to each element
17 | 	in VGA buffer. Therefore each 2 indexes here make up a single VGA character.
18 | 	
19 | 	(See Wikipedia "VGA Text Mode")
20 | 	*/	 
21 | 
22 | 	vgaBuffer[0]  = 'H'; // First 8 bits: character
23 | 	vgaBuffer[1]  =  79; // Last  8 bits: color
24 | 
25 | 	vgaBuffer[2]  = 'e'; // First 8 bits: character
26 | 	vgaBuffer[3]  =  15; // Last  8 bits: color
27 | 
28 | 	vgaBuffer[4]  = 'l'; // and so on...
29 | 	vgaBuffer[5]  =  10; 
30 | 
31 | 	vgaBuffer[6]  = 'l'; 
32 | 	vgaBuffer[7]  =   4; 
33 | 
34 | 	vgaBuffer[8]  = 'o'; 
35 | 	vgaBuffer[9]  = 160; 
36 | 
37 | 	vgaBuffer[10] = '!';
38 | 	vgaBuffer[11] =  64;
39 | 	
40 | 	int vgaBufferPos = 12;
41 | 	const char *message = " world!";
42 | 	// Prints the message
43 | 	int i = 0;
44 | 	while (message[i] != '\0') {
45 | 		vgaBuffer[vgaBufferPos] = message[i];
46 | 		vgaBuffer[vgaBufferPos+1] = 15;
47 | 		
48 | 		++i;
49 | 		vgaBufferPos = vgaBufferPos + 2;
50 | 	}
51 | 	
52 | 	// Printing text in a practical way complicates the code a fair bit 
53 | 	// so I have deleted those parts
54 | }
55 | 
56 | // We have to clear the leftover text from boot screen
57 | void terminal_clear_screen(void)
58 | {
59 | 	/*
60 | 	We multiply width by 2 because we have an 8 bit pointer instead of a 
61 | 	16 bit. Therefore we have to increment twice on horizontal axis in 
62 | 	order to reach to the next VGA character
63 | 	*/	
64 | 	int limit = (VGA_WIDTH * 2) * (VGA_HEIGHT); 
65 | 	for (int i = 0; i < limit; ++i) {
66 | 		vgaBuffer[i] = 0;
67 | 	}
68 | }
69 | 


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/link.ld:
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 1 | OUTPUT_FORMAT(elf32-i386)
 2 | ENTRY(start)
 3 | SECTIONS
 4 | 	{
 5 | 		. = 0x100000;
 6 | 		.text : { *(.text) }
 7 | 		.data :{ *(.data) }
 8 | 		.bss : { *(.bss) }
 9 | 	}
10 | 


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/run.sh:
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1 | nasm -f elf32 boot.asm -o boot.o
2 | gcc -m32 -c kernel.c -o kernel.o
3 | ld -m elf_i386 -T link.ld -o kernel.bin boot.o kernel.o
4 | qemu-system-i386 -kernel kernel.bin
5 | 


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