1. Introduction
2. Introduction about the x86 architecture and about our OS
3. Setup the development environment
4. First boot with GRUB
5. Backbone of the OS and C++ runtime
6. Base classes for managing x86 architecture
7. GDT
8. IDT and interrupts
9. Memory management: physical and virtual
10. Process management and multitasking
11. External program execution: ELF files
12. Userland and syscalls
13. Modular drivers
14. Some basics modules: console, keyboard
15. IDE Hard disks
16. DOS Partitions
17. EXT2 read-only filesystems
18. Standard C library (libC)
19. UNIX basic tools: sh, cat
20. Lua interpreter
Table of Contents
Online book about how to write a computer operating system in C/C++ from scratch.
Caution: This repository is a remake of my old course. It was written several years ago as one of my first projects when I
was in High School, I'm still refactoring some parts. The original course was in French and I'm not an English native. I'm
going to continue and improve this course in my free-time.
Book: An online version is available at http://samypesse.gitbooks.io/how-to-create-an-operating-system/ (PDF, Mobi and
ePub). It was been generated using GitBook.
Source Code: All the system source code will be stored in the src directory. Each step will contain links to the different
related files.
Contributions: This course is open to contributions, feel free to signal errors with issues or directly correct the errors with
pull-requests.
Questions: Feel free to ask any questions by adding issues. Please don't email me.
You can follow me on Twitter @SamyPesse or support me on Flattr or Gittip.
The goal is to build a very simple UNIX-based operating system in C++, not just a "proof-of-concept". The OS should be
able to boot, start a userland shell, and be extensible.
How to Make a Computer Operating System
What kind of OS are we building?
The term x86 denotes a family of backward compatible instruction set architectures based on the Intel 8086 CPU.
The x86 architecture is the most common instruction set architecture since its introduction in 1981 for the IBM PC. A large
amount of software, including operating systems (OS's) such as DOS, Windows, Linux, BSD, Solaris and Mac OS X,
function with x86-based hardware.
In this course we are not going to design an operating system for the x86-64 architecture but for x86-32, thanks to
backward compatibility, our OS will be compatible with our newer PCs (but take caution if you want to test it on your real
machine).
The goal is to build a very simple UNIX-based operating system in C++, but the goal is not to just build a "proof-of-concept".
The OS should be able to boot, start a userland shell and be extensible.
The OS will be built for the x86 architecture, running on 32 bits, and compatible with IBM PCs.
Specifications:
Code in C++
x86, 32 bit architecture
Boot with Grub
Kind of modular system for drivers
Kind of UNIX style
Multitasking
ELF executable in userland
Modules (accessible in userland using /dev/...) :
IDE disks
DOS partitions
Clock
EXT2 (read only)
Boch VBE
Userland :
API Posix
LibC
"Can" run a shell or some executables like Lua, ...
Chapter 1: Introduction to the x86 architecture and about our
OS
What is the x86 architecture?
Our Operating System
The first step is to setup a good and viable development environment. Using Vagrant and Virtualbox, you'll be able to
compile and test your OS from all the OSs (Linux, Windows or Mac).
Vagrant is free and open-source software for creating and configuring virtual development environments. It can be
considered a wrapper around VirtualBox.
Vagrant will help us create a clean virtual development environment on whatever system you are using. The first step is to
download and install Vagrant for your system at http://www.vagrantup.com/.
Oracle VM VirtualBox is a virtualization software package for x86 and AMD64/Intel64-based computers.
Vagrant needs Virtualbox to work, Download and install for your system at https://www.virtualbox.org/wiki/Downloads.
Once Vagrant and Virtualbox are installed, you need to download the ubuntu lucid32 image for Vagrant:
vagrant box add lucid32 http://files.vagrantup.com/lucid32.box
Once the lucid32 image is ready, we need to define our development environment using a Vagrantfile, create a file named
Vagrantfile. This file defines what prerequisites our environment needs: nasm, make, build-essential, grub and qemu.
Start your box using:
vagrant up
You can now access your box by using ssh to connect to the virtual box using:
vagrant ssh
Put any file into the same directory of Vargrantfile, it will be available in the /vagrant directory of VM (in this case, Ubuntu
Lucid32):
cd /vagrant
The file Makefile defines some basics rules for building the kernel, the user libc and some userland programs.
Build:
make all
Chapter 2: Setup the development environment
Install Vagrant
Install Virtualbox
Start and test your development environment
Build and test our operating system
Test our operating system with qemu:
make run
The documentation for qemu is available at QEMU Emulator Documentation.
You can exit the emulator using: Ctrl-a.
When an x86-based computer is turned on, it begins a complex path to get to the stage where control is transferred to our
kernel's "main" routine ( kmain() ). For this course, we are only going to consider the BIOS boot method and not it's
successor (UEFI).
The BIOS boot sequence is: RAM detection -> Hardware detection/Initialization -> Boot sequence.
The most important step for us is the "Boot sequence", where the BIOS is done with its initialization and tries to transfer
control to the next stage of the bootloader process.
During the "Boot sequence", the BIOS will try to determine a "boot device" (e.g. floppy disk, hard-disk, CD, USB flash
memory device or network). Our Operating System will initially boot from the hard-disk (but it will be possible to boot it from
a CD or a USB flash memory device in future). A device is considered bootable if the bootsector contains the valid
signature bytes 0x55 and 0xAA at offsets 511 and 512 respectively (called the magic bytes of Master Boot Record (MBR),
This signature is represented (in binary) as 0b1010101001010101. The alternating bit pattern was thought to be a
protection against certain failures (drive or controller). If this pattern is garbled or 0x00, the device is not considered
bootable)
BIOS physically searches for a boot device by loading the first 512 bytes from the bootsector of each device into physical
memory, starting at the address 0x7C00 (1 KiB below the 32 KiB mark). When the valid signature bytes are detected, BIOS
transfers control to the 0x7C00 memory address (via a jump instruction) in order to execute the bootsector code.
Throughout this process the CPU has been running in 16-bit Real Mode (the default state for x86 CPUs in order to maintain
backwards compatibility). To execute the 32-bit instructions within our kernel, a bootloader is required to switch the CPU
into Protected Mode.
GNU GRUB (short for GNU GRand Unified Bootloader) is a boot loader package from the GNU Project. GRUB is the
reference implementation of the Free Software Foundation's Multiboot Specification, which provides a user the
choice to boot one of multiple operating systems installed on a computer or select a specific kernel configuration
available on a particular operating system's partitions.
To make it simple, GRUB is the first thing booted by the machine (a boot-loader) and will simplify the loading of our kernel
stored on the hard-disk.
GRUB is very simple to use
Make it very simple to load 32bits kernels without needs of 16bits code
Multiboot with Linux, Windows and others
Make it easy to load external modules in memory
GRUB uses the Multiboot specification, the executable binary should be 32bits and must contain a special header
(multiboot header) in its 8192 first bytes. Our kernel will be a ELF executable file ("Executable and Linkable Format", a
common standard file format for executables in most UNIX system).
The first boot sequence of our kernel is written in Assembly: start.asm and we use a linker file to define our executable
structure: linker.ld.
Chapter 3: First boot with GRUB
How the boot works?
What is GRUB?
Why are we using GRUB?
How to use GRUB?
This boot process also initializes some of our C++ runtime, it will be described in the next chapter.
Multiboot header structure:
struct multiboot_info {
u32 flags;
u32 low_mem;
u32 high_mem;
u32 boot_device;
u32 cmdline;
u32 mods_count;
u32 mods_addr;
struct {
u32 num;
u32 size;
u32 addr;
u32 shndx;
} elf_sec;
unsigned long mmap_length;
unsigned long mmap_addr;
unsigned long drives_length;
unsigned long drives_addr;
unsigned long config_table;
unsigned long boot_loader_name;
unsigned long apm_table;
unsigned long vbe_control_info;
unsigned long vbe_mode_info;
unsigned long vbe_mode;
unsigned long vbe_interface_seg;
unsigned long vbe_interface_off;
unsigned long vbe_interface_len;
};
You can use the command mbchk kernel.elf to validate your kernel.elf file against the multiboot standard. You can also
use the command nm -n kernel.elf to validate the offset of the different objects in the ELF binary.
The script diskimage.sh will generate a hard disk image that can be used by QEMU.
The first step is to create a hard-disk image (c.img) using qemu-img:
qemu-img create c.img 2M
We need now to partition the disk using fdisk:
fdisk ./c.img
# Switch to Expert commands
> x
# Change number of cylinders (1-1048576)
> c
> 4
# Change number of heads (1-256, default 16):
> h
> 16
# Change number of sectors/track (1-63, default 63)
> s
> 63
# Return to main menu
> r
# Add a new partition
> n
# Choose primary partition
Create a disk image for our kernel and grub
> p
# Choose partition number
> 1
# Choose first cylinder (1-4, default 1)
> 1
# Choose last cylinder, +cylinders or +size{K,M,G} (1-4, default 4)
> 4
# Toggle bootable flag
> a
# Choose first partition for bootable flag
> 1
# Write table to disk and exit
> w
We need now to attach the created partition to the loop-device (which allows a file to be access like a block device) using
losetup. The offset of the partition is passed as an argument and calculated using: offset= start_sector *
bytes_by_sector.
Using fdisk -l -u c.img , you get: 63 * 512 = 32256.
losetup -o 32256 /dev/loop1 ./c.img
We create a EXT2 filesystem on this new device using:
mke2fs /dev/loop1
We copy our files on a mounted disk:
mount /dev/loop1 /mnt/
cp -R bootdisk/* /mnt/
umount /mnt/
Install GRUB on the disk:
grub --device-map=/dev/null << EOF
device (hd0) ./c.img
geometry (hd0) 4 16 63
root (hd0,0)
setup (hd0)
quit
EOF
And finally we detach the loop device:
losetup -d /dev/loop1
GNU GRUB on Wikipedia
Multiboot specification
See Also
A kernel can be programmed in C++, it is very similar to making a kernel in C, except that there are a few pitfalls you must
take into account (runtime support, constructors, ...)
The compiler will assume that all the necessary C++ runtime support is available by default, but as we are not linking in
libsupc++ into your C++ kernel, we need to add some basic functions that can be found in the cxx.cc file.
Caution: The operators new and delete cannot be used before virtual memory and pagination have been initialized.
The kernel code can't use functions from the standard libraries so we need to add some basic functions for managing
memory and strings:
void itoa(char *buf, unsigned long int n, int base);
void * memset(char *dst,char src, int n);
void * memcpy(char *dst, char *src, int n);
int strlen(char *s);
int strcmp(const char *dst, char *src);
int strcpy(char *dst,const char *src);
void strcat(void *dest,const void *src);
char * strncpy(char *destString, const char *sourceString,int maxLength);
int strncmp( const char* s1, const char* s2, int c );
These functions are defined in string.cc, memory.cc, itoa.cc
During the next step, we are going to use different types in our code, most of the types we are going to use unsigned types
(all the bits are used to stored the integer, in signed types one bit is used to signal the sign):
typedef unsigned char u8;
typedef unsigned short u16;
typedef unsigned int u32;
typedef unsigned long long u64;
typedef signed char s8;
typedef signed short s16;
typedef signed int s32;
typedef signed long long s64;
Compiling a kernel is not the same thing as compiling a linux executable, we can't use a standard library and should have
no dependencies to the system.
Our Makefile will define the process to compile and link our kernel.
For x86 architecture, the followings arguments will be used for gcc/g++/ld:
# Linker
LD=ld
LDFLAG= -melf_i386 -static -L ./ -T ./arch/$(ARCH)/linker.ld
Chapter 4: Backbone of the OS and C++ runtime
C++ kernel run-time
Basic C/C++ functions
C types
Compile our kernel
# C++ compiler
SC=g++
FLAG= $(INCDIR) -g -O2 -w -trigraphs -fno-builtin -fno-exceptions -fno-stack-protector -O0 -m32 -fno-rtti -nostdlib -nodefaultlibs
# Assembly compiler
ASM=nasm
ASMFLAG=-f elf -o
Now that we know how to compile our C++ kernel and boot the binary using GRUB, we can start to do some cool things in
C/C++.
We are going to use VGA default mode (03h) to display some text to the user. The screen can be directly accessed using
the video memory at 0xB8000. The screen resolution is 80x25 and each character on the screen is defined by 2 bytes: one
for the character code, and one for the style flag. This means that the total size of the video memory is 4000B (80B25B2B).
In the IO class (io.cc),:
x,y: define the cursor position on the screen
real_screen: define the video memory pointer
putc(char c): print a unique character on the screen and manage cursor position
printf(char* s, ...): print a string
We add a method putc to the IO Class to put a character on the screen and update the (x,y) position.
/* put a byte on screen */
void Io::putc(char c){
kattr = 0x07;
unsigned char *video;
video = (unsigned char *) (real_screen+ 2 * x + 160 * y);
// newline
if (c == '\n') {
x = 0;
y++;
// back space
} else if (c == '\b') {
if (x) {
*(video + 1) = 0x0;
x--;
}
// horizontal tab
} else if (c == '\t') {
x = x + 8 - (x % 8);
// carriage return
} else if (c == '\r') {
x = 0;
} else {
*video = c;
*(video + 1) = kattr;
x++;
if (x > 79) {
x = 0;
y++;
}
}
if (y > 24)
scrollup(y - 24);
}
We also add a useful and very known method: printf
/* put a string in screen */
void Io::print(const char *s, ...){
va_list ap;
char buf[16];
int i, j, size, buflen, neg;
unsigned char c;
int ival;
Chapter 5: Base classes for managing x86 architecture
Printing to the screen console
unsigned int uival;
va_start(ap, s);
while ((c = *s++)) {
size = 0;
neg = 0;
if (c == 0)
break;
else if (c == '%') {
c = *s++;
if (c >= '0' && c <= '9') {
size = c - '0';
c = *s++;
}
if (c == 'd') {
ival = va_arg(ap, int);
if (ival < 0) {
uival = 0 - ival;
neg++;
} else
uival = ival;
itoa(buf, uival, 10);
buflen = strlen(buf);
if (buflen < size)
for (i = size, j = buflen; i >= 0;
i--, j--)
buf[i] =
(j >=
0) ? buf[j] : '0';
if (neg)
print("-%s", buf);
else
print(buf);
}
else if (c == 'u') {
uival = va_arg(ap, int);
itoa(buf, uival, 10);
buflen = strlen(buf);
if (buflen < size)
for (i = size, j = buflen; i >= 0;
i--, j--)
buf[i] =
(j >=
0) ? buf[j] : '0';
print(buf);
} else if (c == 'x' || c == 'X') {
uival = va_arg(ap, int);
itoa(buf, uival, 16);
buflen = strlen(buf);
if (buflen < size)
for (i = size, j = buflen; i >= 0;
i--, j--)
buf[i] =
(j >=
0) ? buf[j] : '0';
print("0x%s", buf);
} else if (c == 'p') {
uival = va_arg(ap, int);
itoa(buf, uival, 16);
size = 8;
buflen = strlen(buf);
if (buflen < size)
for (i = size, j = buflen; i >= 0;
i--, j--)
buf[i] =
(j >=
0) ? buf[j] : '0';
print("0x%s", buf);
} else if (c == 's') {
print((char *) va_arg(ap, int));
}
} else
putc(c);
}
return;
}
A large number of instructions are available in Assembly but there is not equivalent in C (like cli, sti, in and out), so we need
an interface to these instructions.
In C, we can include Assembly using the directive "asm()", gcc use gas to compile the assembly.
Caution: gas uses the AT&T syntax.
/* output byte */
void Io::outb(u32 ad, u8 v){
asmv("outb %%al, %%dx" :: "d" (ad), "a" (v));;
}
/* output word */
void Io::outw(u32 ad, u16 v){
asmv("outw %%ax, %%dx" :: "d" (ad), "a" (v));
}
/* output word */
void Io::outl(u32 ad, u32 v){
asmv("outl %%eax, %%dx" : : "d" (ad), "a" (v));
}
/* input byte */
u8 Io::inb(u32 ad){
u8 _v; \
asmv("inb %%dx, %%al" : "=a" (_v) : "d" (ad)); \
return _v;
}
/* input word */
u16 Io::inw(u32 ad){
u16 _v; \
asmv("inw %%dx, %%ax" : "=a" (_v) : "d" (ad)); \
return _v;
}
/* input word */
u32 Io::inl(u32 ad){
u32 _v; \
asmv("inl %%dx, %%eax" : "=a" (_v) : "d" (ad)); \
return _v;
}
Assembly interface
Thanks to GRUB, your kernel is no longer in real-mode, but already in protected mode, this mode allows us to use all the
possibilities of the microprocessor such as virtual memory management, paging and safe multi-tasking.
The GDT ("Global Descriptor Table") is a data structure used to define the different memory areas: the base address, the
size and access privileges like execute and write. These memory areas are called "segments".
We are going to use the GDT to define different memory segments:
"code": kernel code, used to stored the executable binary code
"data": kernel data
"stack": kernel stack, used to stored the call stack during kernel execution
"ucode": user code, used to stored the executable binary code for user program
"udata": user program data
"ustack": user stack, used to stored the call stack during execution in userland
GRUB initializes a GDT but this GDT is does not correspond to our kernel. The GDT is loaded using the LGDT assembly
instruction. It expects the location of a GDT description structure:
And the C structure:
struct gdtr {
u16 limite;
u32 base;
} __attribute__ ((packed));
Caution: the directive __attribute__ ((packed)) signal to gcc that the structure should use as little memory as possible.
Without this directive, gcc include some bytes to optimize the memory alignment and the access during execution.
Now we need to define our GDT table and then load it using LGDT. The GDT table can be stored wherever we want in
memory, its address should just be signaled to the process using the GDTR registry.
The GDT table is composed of segments with the following structure:
Chapter 6: GDT
What is the GDT?
How to load our GDT?
And the C structure:
struct gdtdesc {
u16 lim0_15;
u16 base0_15;
u8 base16_23;
u8 acces;
u8 lim16_19:4;
u8 other:4;
u8 base24_31;
} __attribute__ ((packed));
We need now to define our GDT in memory and finally load it using the GDTR registry.
We are going to store our GDT at the address:
#define GDTBASE 0x00000800
The function init_gdt_desc in x86.cc initialize a gdt segment descriptor.
void init_gdt_desc(u32 base, u32 limite, u8 acces, u8 other, struct gdtdesc *desc)
{
desc->lim0_15 = (limite & 0xffff);
desc->base0_15 = (base & 0xffff);
desc->base16_23 = (base & 0xff0000) >> 16;
desc->acces = acces;
desc->lim16_19 = (limite & 0xf0000) >> 16;
desc->other = (other & 0xf);
desc->base24_31 = (base & 0xff000000) >> 24;
return;
}
And the function init_gdt initialize the GDT, some parts of the below function will be explained later and are used for
multitasking.
void init_gdt(void)
{
default_tss.debug_flag = 0x00;
default_tss.io_map = 0x00;
default_tss.esp0 = 0x1FFF0;
default_tss.ss0 = 0x18;
/* initialize gdt segments */
init_gdt_desc(0x0, 0x0, 0x0, 0x0, &kgdt[0]);
init_gdt_desc(0x0, 0xFFFFF, 0x9B, 0x0D, &kgdt[1]); /* code */
init_gdt_desc(0x0, 0xFFFFF, 0x93, 0x0D, &kgdt[2]); /* data */
init_gdt_desc(0x0, 0x0, 0x97, 0x0D, &kgdt[3]); /* stack */
How to define our GDT table?
init_gdt_desc(0x0, 0xFFFFF, 0xFF, 0x0D, &kgdt[4]); /* ucode */
init_gdt_desc(0x0, 0xFFFFF, 0xF3, 0x0D, &kgdt[5]); /* udata */
init_gdt_desc(0x0, 0x0, 0xF7, 0x0D, &kgdt[6]); /* ustack */
init_gdt_desc((u32) & default_tss, 0x67, 0xE9, 0x00, &kgdt[7]); /* descripteur de tss */
/* initialize the gdtr structure */
kgdtr.limite = GDTSIZE * 8;
kgdtr.base = GDTBASE;
/* copy the gdtr to its memory area */
memcpy((char *) kgdtr.base, (char *) kgdt, kgdtr.limite);
/* load the gdtr registry */
asm("lgdtl (kgdtr)");
/* initiliaz the segments */
asm(" movw $0x10, %ax \n \
movw %ax, %ds \n \
movw %ax, %es \n \
movw %ax, %fs \n \
movw %ax, %gs \n \
ljmp $0x08, $next \n \
next: \n");
}
An interrupt is a signal to the processor emitted by hardware or software indicating an event that needs immediate
attention.
There are 3 types of interrupts:
Hardware interrupts: are sent to the processor from an external device (keyboard, mouse, hard disk, ...). Hardware
interrupts were introduced as a way to reduce wasting the processor's valuable time in polling loops, waiting for
external events.
Software interrupts: are initiated voluntarily by the software. It's used to manage system calls.
Exceptions: are used for errors or events occurring during program execution that are exceptional enough that they
cannot be handled within the program itself (division by zero, page fault, ...)
When the user pressed a key on the keyboard, the keyboard controller will signal an interrupt to the Interrupt Controller. If
the interrupt is not masked, the controller will signal the interrupt to the processor, the processor will execute a routine to
manage the interrupt (key pressed or key released), this routine could, for example, get the pressed key from the keyboard
controller and print the key to the screen. Once the character processing routine is completed, the interrupted job can be
resumed.
The PIC (Programmable interrupt controller)is a device that is used to combine several sources of interrupt onto one or
more CPU lines, while allowing priority levels to be assigned to its interrupt outputs. When the device has multiple interrupt
outputs to assert, it asserts them in the order of their relative priority.
The best known PIC is the 8259A, each 8259A can handle 8 devices but most computers have two controllers: one master
and one slave, this allows the computer to manage interrupts from 14 devices.
In this chapter, we will need to program this controller to initialize and mask interrupts.
The Interrupt Descriptor Table (IDT) is a data structure used by the x86 architecture to implement an interrupt vector
table. The IDT is used by the processor to determine the correct response to interrupts and exceptions.
Our kernel is going to use the IDT to define the different functions to be executed when an interrupt occurred.
Like the GDT, the IDT is loaded using the LIDTL assembly instruction. It expects the location of a IDT description structure:
struct idtr {
u16 limite;
u32 base;
} __attribute__ ((packed));
The IDT table is composed of IDT segments with the following structure:
struct idtdesc {
u16 offset0_15;
u16 select;
u16 type;
u16 offset16_31;
} __attribute__ ((packed));
Chapter 7: IDT and interrupts
The keyboard example:
What is the PIC?
What is the IDT?
Caution: the directive __attribute__ ((packed)) signal to gcc that the structure should use as little memory as possible.
Without this directive, gcc includes some bytes to optimize the memory alignment and the access during execution.
Now we need to define our IDT table and then load it using LIDTL. The IDT table can be stored wherever we want in
memory, its address should just be signaled to the process using the IDTR registry.
Here is a table of common interrupts (Maskable hardware interrupt are called IRQ):
IRQ Description
0 Programmable Interrupt Timer Interrupt
1 Keyboard Interrupt
2 Cascade (used internally by the two PICs. never raised)
3 COM2 (if enabled)
4 COM1 (if enabled)
5 LPT2 (if enabled)
6 Floppy Disk
7 LPT1
8 CMOS real-time clock (if enabled)
9 Free for peripherals / legacy SCSI / NIC
10 Free for peripherals / SCSI / NIC
11 Free for peripherals / SCSI / NIC
12 PS2 Mouse
13 FPU / Coprocessor / Inter-processor
14 Primary ATA Hard Disk
15 Secondary ATA Hard Disk
This is a simple method to define an IDT segment
void init_idt_desc(u16 select, u32 offset, u16 type, struct idtdesc *desc)
{
desc->offset0_15 = (offset & 0xffff);
desc->select = select;
desc->type = type;
desc->offset16_31 = (offset & 0xffff0000) >> 16;
return;
}
And we can now initialize the interupts:
#define IDTBASE 0x00000000
#define IDTSIZE 0xFF
idtr kidtr;
void init_idt(void)
{
/* Init irq */
int i;
for (i = 0; i < IDTSIZE; i++)
init_idt_desc(0x08, (u32)_asm_schedule, INTGATE, &kidt[i]); //
How to initialize the interrupts?
/* Vectors 0 -> 31 are for exceptions */
init_idt_desc(0x08, (u32) _asm_exc_GP, INTGATE, &kidt[13]); /* #GP */
init_idt_desc(0x08, (u32) _asm_exc_PF, INTGATE, &kidt[14]); /* #PF */
init_idt_desc(0x08, (u32) _asm_schedule, INTGATE, &kidt[32]);
init_idt_desc(0x08, (u32) _asm_int_1, INTGATE, &kidt[33]);
init_idt_desc(0x08, (u32) _asm_syscalls, TRAPGATE, &kidt[48]);
init_idt_desc(0x08, (u32) _asm_syscalls, TRAPGATE, &kidt[128]); //48
kidtr.limite = IDTSIZE * 8;
kidtr.base = IDTBASE;
/* Copy the IDT to the memory */
memcpy((char *) kidtr.base, (char *) kidt, kidtr.limite);
/* Load the IDTR registry */
asm("lidtl (kidtr)");
}
After intialization of our IDT, we need to activate interrupts by configuring the PIC. The following function will configure the
two PICs by writting in their internal registries using the output ports of the processor io.outb . We configure the PICs
using the ports:
Master PIC: 0x20 and 0x21
Slave PIC: 0xA0 and 0xA1
For a PIC, there are 2 types of registries:
ICW (Initialization Command Word): reinit the controller
OCW (Operation Control Word): configure the controller once initialized (used to mask/unmask the interrupts)
void init_pic(void)
{
/* Initialization of ICW1 */
io.outb(0x20, 0x11);
io.outb(0xA0, 0x11);
/* Initialization of ICW2 */
io.outb(0x21, 0x20); /* start vector = 32 */
io.outb(0xA1, 0x70); /* start vector = 96 */
/* Initialization of ICW3 */
io.outb(0x21, 0x04);
io.outb(0xA1, 0x02);
/* Initialization of ICW4 */
io.outb(0x21, 0x01);
io.outb(0xA1, 0x01);
/* mask interrupts */
io.outb(0x21, 0x0);
io.outb(0xA1, 0x0);
}
The registries have to be configured in order.
ICW1 (port 0x20 / port 0xA0)
|0|0|0|1|x|0|x|x|
| | +--- with ICW4 (1) or without (0)
| +----- one controller (1), or cascade (0)
+--------- triggering by level (level) (1) or by edge (edge) (0)
PIC ICW configurations details
ICW2 (port 0x21 / port 0xA1)
|x|x|x|x|x|0|0|0|
| | | | |
+----------------- base address for interrupts vectors
ICW2 (port 0x21 / port 0xA1)
For the master:
|x|x|x|x|x|x|x|x|
| | | | | | | |
+------------------ slave controller connected to the port yes (1), or no (0)
For the slave:
|0|0|0|0|0|x|x|x| pour l'esclave
| | |
+-------- Slave ID which is equal to the master port
ICW4 (port 0x21 / port 0xA1)
It is used to define in which mode the controller should work.
|0|0|0|x|x|x|x|1|
| | | +------ mode "automatic end of interrupt" AEOI (1)
| | +-------- mode buffered slave (0) or master (1)
| +---------- mode buffered (1)
+------------ mode "fully nested" (1)
You should have noticed that when I'm initializing our IDT segments, I'm using offsets to segment the code in Assembly.
The different functions are defined in x86int.asm and are of the following scheme:
%macro SAVE_REGS 0
pushad
push ds
push es
push fs
push gs
push ebx
mov bx,0x10
mov ds,bx
pop ebx
%endmacro
%macro RESTORE_REGS 0
pop gs
pop fs
pop es
pop ds
popad
%endmacro
%macro INTERRUPT 1
global _asm_int_%1
_asm_int_%1:
SAVE_REGS
push %1
call isr_default_int
pop eax ;;a enlever sinon
mov al,0x20
out 0x20,al
Why do idt segments offset our ASM functions?
RESTORE_REGS
iret
%endmacro
These macros will be used to define the interrupt segment that will prevent corruption of the different registries, it will be
very useful for multitasking.
In the chapter related to the GDT, we saw that using segmentation a physical memory address is calculated using a
segment selector and an offset.
In this chapter, we are going to implement paging, paging will translate a linear address from segmentation into a physical
address.
Paging will allow our kernel to:
use the hard-drive as a memory and not be limited by the machine ram memory limit
to have a unique memory space for each process
to allow and unallow memory space in a dynamic way
In a paged system, each process may execute in its own 4gb area of memory, without any chance of effecting any other
process's memory, or the kernel's. It simplifies multitasking.
The translation of a linear address to a physical address is done in multiple steps:
1. The processor use the registry CR3 to know the physical address of the pages directory.
2. The first 10 bits of the linear address represent an offset (between 0 and 1023), pointing to an entry in the pages
directory. This entry contains the physical address of a pages table.
3. the next 10 bits of the linear address represent an offset, pointing to an entry in the pages table. This entry is pointing
to a 4ko page.
4. The last 12 bits of the linear address represent an offset (between 0 and 4095), which indicates the position in the 4ko
page.
Chapter 8: Memory management: physical and virtual
Why do we need paging?
How does it work?
The two types of entries (table and directory) look like the same. Only the field in gray will be used in our OS.
P : indicate if the page or table is in physical memory
R/W : indicate if the page or table is accessible in writting (equals 1)
U/S : equals 1 to allow access to non-preferred tasks
A : indicate if the page or table was accessed
D : (only for pages table) indicate if the page was written
PS (only for pages directory) indicate the size of pages:
0 = 4ko
1 = 4mo
Note: Physical addresses in the pages diretcory or pages table are written using 20 bits because these addresses are
aligned on 4ko, so the last 12bits should be equal to 0.
A pages directory or pages table used 1024*4 = 4096 bytes = 4k
A pages table can address 1024 * 4k = 4 Mo
A pages directory can address 1024 (1024 4k) = 4 Go
To enable pagination, we just need to set bit 31 of the CR0 registry to 1:
asm(" mov %%cr0, %%eax; \
Format for pages table and directory
How to enable pagination?
or %1, %%eax; \
mov %%eax, %%cr0" \
:: "i"(0x80000000));
But before, we need to initialize our pages directory with at least one pages table.
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