How to create your own OS #2

5 min readJul 23, 2021

Hello friends welcome back this is the second part of the series if you miss part #1 go throw this link : How to create your own OS?. Developing an operating system (OS) is… | by Rasalingam Ragul | Jul, 2021 | Medium

Implement with C

This part I will show you how to use C instead of assembly code as the programming language for the OS. Assembly is very good for interacting with the CPU and enables maximum control over every aspect of the code. I would like to use C as much as possible and use assembly code only where it make sense.

2.1 Setting Up a Stack

One prerequisite for using C is a stack, since all non-trivial C programs use a stack. Setting up a stack is not harder than to make the esp register point to the end of an area of free memory (remember that the stack grows towards lower addresses on the x86) that is correctly aligned (alignment on 4 bytes is recommended from a performance perspective).

We could point esp to a random area in memory since, so far, the only thing in the memory is GRUB, BIOS, the OS kernel and some memory-mapped I/O. This is not a good idea - we don’t know how much memory is available or if the area esp would point to is used by something else. A better idea is to reserve a piece of uninitialized memory in the bss section in the ELF file of the kernel. It is better to use the bss section instead of the data section to reduce the size of the OS executable. Since GRUB understands ELF, GRUB will allocate any memory reserved in the bss section when loading the OS.

The NASM pseudo-instruction resb can be used to declare uninitialized data:

There is no need to worry about the use of uninitialized memory for the stack, since it is not possible to read a stack location that has not been written (without manual pointer fiddling). A (correct) program can not pop an element from the stack without having pushed an element onto the stack first. Therefore, the memory locations of the stack will always be written to before they are being read.

The stack pointer is then set up by pointing esp to the end of the kernel_stack memory:

2.2 Calling C Code From Assembly

The next step is to call a C function from assembly code. There are many different conventions for how to call C code from assembly code . This series uses the cdecl calling convention, since that is the one used by GCC. The cdecl calling convention states that arguments to a function should be passed via the stack (on x86). The arguments of the function should be pushed on the stack in a right-to-left order, that is, you push the rightmost argument first. The return value of the function is placed in the eax register. The following code shows an example:

and put below part in loader.s,

end of these parts your loader.s file look like this:

2.2.1 Packing Structs

In the rest of this series, you will often come across “configuration bytes” that are a collection of bits in a very specific order. Below follows an example with 32 bits:

Bit:     | 31     24 | 23          8 | 7     0 |
Content: | index     | address       | config  |

Instead of using an unsigned integer, unsigned int, for handling such configurations, it is much more convenient to use “packed structures”:

struct example {
        unsigned char config;   /* bit 0 - 7   */
        unsigned short address; /* bit 8 - 23  */
        unsigned char index;    /* bit 24 - 31 */
    };

When using the struct in the previous example there is no guarantee that the size of the struct will be exactly 32 bits - the compiler can add some padding between elements for various reasons, for example to speed up element access or due to requirements set by the hardware and/or compiler. When using a struct to represent configuration bytes, it is very important that the compiler does not add any padding, because the struct will eventually be treated as a 32 bit unsigned integer by the hardware. The attribute packed can be used to force GCC to not add any padding:

struct example {
        unsigned char config;   /* bit 0 - 7   */
        unsigned short address; /* bit 8 - 23  */
        unsigned char index;    /* bit 24 - 31 */
    } __attribute__((packed));

Note that __attribute__((packed)) is not part of the C standard - it might not work with all C compilers.

2.3 Compiling C Code

When compiling the C code for the OS, a lot of flags to GCC need to be used. This is because the C code should not assume the presence of a standard library, since there is no standard library available for our OS. For more information about the flags, see the GCC manual.

The flags used for compiling the C code are:

-m32 -nostdlib -nostdinc -fno-builtin -fno-stack-protector -nostartfiles
    -nodefaultlibs

As always when writing C programs we recommend turning on all warnings and treat warnings as errors:

-Wall -Wextra -Werror

You can now create a function kmain in a file called kmain.c that you call from loader.s. At this point, kmain probably won’t need any arguments (but in later series it will).

2.4 Build Tools

Now is also probably a good time to set up some build tools to make it easier to compile and test-run the OS. We recommend using make, but there are plenty of other build systems available. A simple Makefile for the OS could look like the following example:

you put your OS name in place of windOS.

The contents of your working directory should now look like the following figure:

.
    |-- bochsrc.txt
    |-- iso
    |   |-- boot
    |     |-- grub
    |       |-- menu.lst
    |       |-- stage2_eltorito
    |-- kmain.c
    |-- loader.s
    |-- Makefile

in final step, You should now be able to start the OS with the simple command make run, which will compile the kernel and boot it up in Bochs.

Now you can a window like this,

then Goto your terminal and type c like this,

now you can see your OS is successfully boot,

After quitting Bochs, display the log produced by Boch:

cat bochslog.txt

You should now see the contents of the registers of the CPU simulated by Bochs somewhere in the output. If you find EAX=00000006 in the output then your OS has successfully booted!