1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
|
# https://www.gnu.org/software/grub/manual/multiboot/multiboot.html#Boot-information-format
# http://wiki.osdev.org/Bare_Bones
# Declare constants used for creating a multiboot header.
.set ALIGN, 1<<0 # align loaded modules on page boundaries
.set MEMINFO, 1<<1 # provide memory map
.set GFXINFO, 1<<2 # provide gfx info
.set FLAGS, ALIGN | MEMINFO | GFXINFO # this is the Multiboot 'flag' field
.set MAGIC, 0x1BADB002 # 'magic number' lets bootloader find the header
.set CHECKSUM, -(MAGIC + FLAGS) # checksum of above, to prove we are multiboot
# Declare a header as in the Multiboot Standard. We put this into a special
# section so we can force the header to be in the start of the final program.
# You don't need to understand all these details as it is just magic values that
# is documented in the multiboot standard. The bootloader will search for this
# magic sequence and recognize us as a multiboot kernel.
.section .multiboot
.align 4
.long MAGIC
.long FLAGS
.long CHECKSUM
.long 0 # we dont need this for ELF
.long 0
.long 0
.long 0
.long 0
.long 1 #gfx_stuff
.long 800
.long 600
.long 24
# Currently the stack pointer register (esp) points at anything and using it may
# cause massive harm. Instead, we'll provide our own stack. We will allocate
# room for a small temporary stack by creating a symbol at the bottom of it,
# then allocating 16384 bytes for it, and finally creating a symbol at the top.
.section .bootstrap_stack, "aw", @nobits
stack_bottom:
.skip 16384 # 16 KiB
stack_top:
# The linker script specifies _start as the entry point to the kernel and the
# bootloader will jump to this position once the kernel has been loaded. It
# doesn't make sense to return from this function as the bootloader is gone.
.section .text
.global _start
.global stack_top
.global stack_bottom
.type _start, @function
_start:
# Welcome to kernel mode! We now have sufficient code for the bootloader to
# load and run our operating system. It doesn't do anything interesting yet.
# Perhaps we would like to call printf("Hello, World\n"). You should now
# realize one of the profound truths about kernel mode: There is nothing
# there unless you provide it yourself. There is no printf function. There
# is no <stdio.h> header. If you want a function, you will have to code it
# yourself. And that is one of the best things about kernel development:
# you get to make the entire system yourself. You have absolute and complete
# power over the machine, there are no security restrictions, no safe
# guards, no debugging mechanisms, there is nothing but what you build.
# By now, you are perhaps tired of assembly language. You realize some
# things simply cannot be done in C, such as making the multiboot header in
# the right section and setting up the stack. However, you would like to
# write the operating system in a higher level language, such as C or C++.
# To that end, the next task is preparing the processor for execution of
# such code. C doesn't expect much at this point and we only need to set up
# a stack. Note that the processor is not fully initialized yet and stuff
# such as floating point instructions are not available yet.
lgdt gdt_descriptor #load descriptor table!
# To set up a stack, we simply set the esp register to point to the top of
# our stack (as it grows downwards).
movl $stack_top, %esp
# We are now ready to actually execute C code. We cannot embed that in an
# assembly file, so we'll create a kernel.c file in a moment. In that file,
# we'll create a C entry point called kernel_main and call it here.
push %ebx #pass address of the multiboot information data structure
push %eax #pass eax, so kernel can check for magic number
reloadSegments:
#Reload CS register containing code selector:
jmp $0x08,$reload_CS # 0x08 points at the new code selector
reload_CS:
mov $0x10, %ax
mov %ax, %ds
mov %ax, %es
mov %ax, %fs
mov %ax, %gs
mov %ax, %ss
call kernel_main
# In case the function returns, we'll want to put the computer into an
# infinite loop. To do that, we use the clear interrupt ('cli') instruction
# to disable interrupts, the halt instruction ('hlt') to stop the CPU until
# the next interrupt arrives, and jumping to the halt instruction if it ever
# continues execution, just to be safe. We will create a local label rather
# than real symbol and jump to there endlessly.
cli
hlt
.Lhang:
jmp .Lhang
# Set the size of the _start symbol to the current location '.' minus its start.
# This is useful when debugging or when you implement call tracing.
.size _start, . - _start
|
