Implementation of an Operating System( 6-The road to user mode)
This is my sixth article of my article series about Implementation of an operating system. And in this article I will talk about The road to user mode in an operating system.
If you new to my series fist you must read in in beginning. Because if you don’t read it in beginning then you couldn’t understand what am I say now, In here you can go to first one.
What do we do now that the kernels boots, prints to screen and reads from keyboard? The application logic is usually left for applications. To make application development easier, the kernels creates proper abstractions for memory, files, devices, and performs tasks on behalf of applications.
The environment in which the user’s programs execute is called user mode. The environment is less privileged and will prevent user programs from messing with other programs. Badly written kernels can mess up what they want.
This chapter will show you how to execute a small program in kernels mode, even though the OS created in this book can only execute programs in user mode.
Loading an External Program
Where do we get the external program? We need to load the code we want to execute into memory. More feature-complete operating systems usually have drivers and file systems that allow them to load the software from a CD-ROM drive, a hard disk or other persistent media.
We will use a feature in GRUB called modules to load the program instead of creating all these drivers and file systems.
The next thing is to talk about GRUB Modules.
GRUB Modules
GRUB can load arbitrary files into memory from the ISO image, and these files are usually referred to as modules. To make GRUB load a module, edit the file iso/boot/grub/menu.lst and add the following line at the end of the file:
module /modules/programNow create the folder iso/modules:
mkdir -p iso/modulesThe application program will be created later in this chapter.
The code that calls kmain must be updated to pass information to kmain about where it can find the modules. We also want to tell GRUB that it should align all the modules on page boundaries when loading them (see the chapter “Paging” for details about page alignment).
To instruct GRUB how to load our modules, the “multiboot header” — the first bytes of the kernel — must be updated as follows:
; in file `loader.s`
MAGIC_NUMBER equ 0x1BADB002 ; define the magic number constant
ALIGN_MODULES equ 0x00000001 ; tell GRUB to align modules ; calculate the checksum (all options + checksum should equal 0)
CHECKSUM equ -(MAGIC_NUMBER + ALIGN_MODULES) section .text: ; start of the text (code) section
align 4 ; the code must be 4 byte aligned
dd MAGIC_NUMBER ; write the magic number
dd ALIGN_MODULES ; write the align modules instruction
dd CHECKSUM ; write the checksum
GRUB will also store a pointer to a struct in the register ebx that, among other things, describes at which addresses the modules are loaded. Therefore, you probably want to push ebx on the stack before calling kmain to make it an argument for kmain.
Now we will be look at how to Executing a Program.
Executing a Program
A Very Simple Program
A program written at this stage can only perform a few actions. Therefore, a very short program that writes a value to a register suffices as a test program. Halting Bochs after a while and then check that register contains the correct number by looking in the Bochs log will verify that the program has run. This is an example of such a short program:
; set eax to some distinguishable number, to read from the log afterwards
mov eax, 0xDEADBEEF ; enter infinite loop, nothing more to do
; $ means "beginning of line", ie. the same instruction
jmp $
Compiling
Since our kernel cannot parse advanced executable formats we need to compile the code into a flat binary. NASM can do this with the flag -f:
nasm -f bin program.s -o programThis is all we need. You must now move the file program to the folder iso/modules.
Finding the Program in Memory
Before jumping to the program we must find where it resides in memory. Assuming that the contents of ebx is passed as an argument to kmain, we can do this entirely from C.
The pointer in ebx points to a multiboot structure. Download the multiboot.h file from http://www.gnu.org/software/grub/manual/multiboot/html_node/multiboot.h.html, which describes the structure.
The pointer passed to kmain in the ebx register can be cast to a multiboot_info_t pointer. The address of the first module is in the field mods_addr. The following code shows an example:
int kmain(/* additional arguments */ unsigned int ebx)
{
multiboot_info_t *mbinfo = (multiboot_info_t *) ebx;
unsigned int address_of_module = mbinfo->mods_addr;
}However, before just blindly following the pointer, you should check that the module got loaded correctly by GRUB. This can be done by checking the flags field of the multiboot_info_t structure. You should also check the field mods_count to make sure it is exactly 1. For more details about the multiboot structure, see the multiboot documentation.
Jumping to the Code
The only thing left to do is to jump to the code loaded by GRUB. Since it is easier to parse the multiboot structure in C than assembly code, calling the code from C is more convenient (it can of course be done with jmp or call in assembly code as well). The C code could look like this:
typedef void (*call_module_t)(void);
/* ... */
call_module_t start_program = (call_module_t) address_of_module;
start_program();
/* we'll never get here, unless the module code returns */If we start the kernel, wait until it has run and entered the infinite loop in the program, and then halt Bochs, we should see 0xDEADBEEF in the register eax via the Bochs log. We have successfully started a program in our OS!
In this chapter we can lastly see about the beginning of user mode.
The Beginning of User Mode
The program we’ve written now runs at the same privilege level as the kernel — we’ve just entered it in a somewhat peculiar way. To enable applications to execute at a different privilege level we’ll need to, beside segmentation, do paging and page frame allocation.
Thank you!
