the_road_to_user_mode.md

The Road to User Mode

Now that the kernel boots, prints to screen and reads from keyboard - what do we do? Usually, a kernel is not supposed to do the application logic itself, but leave that for applications. The kernel creates the proper abstractions (for memory, files, devices) to make application development easier, performs tasks on behalf of applications (system calls) and schedules processes.

User mode, in contrast with kernel mode, is the environment in which the user's programs execute. This environment is less privileged than the kernel, and will prevent (badly written) user programs from messing with other programs or the kernel. Badly written kernels are free to mess up what they want.

There's quite a way to go until the OS created in this book can execute programs in user mode, but this chapter will show how to easily execute a small program in kernel mode.

Loading an External Program

Where do we get the external program from? Somehow we need to load the code we want to execute into memory. More feature-complete operating systems usually have drivers and file systems that enable them to load the software from a CD-ROM drive, a hard disk or other persistent media.

Instead of creating all these drivers and file systems we will use a feature in GRUB called modules to load the program.

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/program

Now create the folder iso/modules:

    mkdir -p iso/modules

The 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.

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 program

This 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 [@multiboot]. 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 [@multiboot].

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!

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.

It's quite a lot of work and technical details to go through, but in a few chapters you'll have working user mode programs.