MAKING AN operating system IMPLEMENT with c (part 02)

7 min readJul 23, 2021

welcome back.first of all, you watch my first os(my own creating os) go throw this link click here.

GIThub:link

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.

We need to modify loader.s file with following coding…..

KERNEL_STACK_SIZE equ 4096                  ; size of stack in bytes

    section .bss
    align 4                                     ; align at 4 bytes
    kernel_stack:                               ; label points to beginning of memory
        resb KERNEL_STACK_SIZE                  ; reserve stack for the kernel

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:

mov esp, kernel_stack + KERNEL_STACK_SIZE   ; point esp to the start of the
                                                ; stack (end of memory area)

after you can see like this…..

global loader                   ; the entry symbol for ELF

    MAGIC_NUMBER equ 0x1BADB002     ; define the magic number constant
    FLAGS        equ 0x0            ; multiboot flags
    CHECKSUM     equ -MAGIC_NUMBER  ; calculate the checksum
                                    ; (magic number + checksum + flags should equal 0)KERNEL_STACK_SIZE equ 4096                  ; size of stack in bytes

    section .bss
    align 4                                     ; align at 4 bytes
    kernel_stack:                               ; label points to beginning of memory
        resb KERNEL_STACK_SIZE                  ; reserve stack for the kernel
    section .text:                  ; start of the text (code) section
    align 4                         ; the code must be 4 byte aligned
        dd MAGIC_NUMBER             ; write the magic number to the machine code,
        dd FLAGS                    ; the flags,
        dd CHECKSUM                 ; and the checksum

    loader:                         ; the loader label (defined as entry point in linker script)
        mov eax, 0xCAFEBABE         ; place the number 0xCAFEBABE in the register eax
    .loop:
        jmp .loop                   ; loop forever

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 [25]. This book 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:

/* The C function */
    int sum_of_three(int arg1, int arg2, int arg3)
    {
        return arg1 + arg2 + arg3;
    }

and put below part in loader.s,

; The assembly code
    external sum_of_three   ; the function sum_of_three is defined elsewhere

    push dword 3            ; arg3
    push dword 2            ; arg2
    push dword 1            ; arg1
    call sum_of_three       ; call the function, the result will be in eax

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

3.Packing Structs

In the rest of this book, 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.

4.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 chapters it will).

5.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 [13], but there are plenty of other build systems available. A simple Makefile for the OS could look like the following example:

OBJECTS = loader.o kmain.o
    CC = gcc
    CFLAGS = -m32 -nostdlib -nostdinc -fno-builtin -fno-stack-protector \
             -nostartfiles -nodefaultlibs -Wall -Wextra -Werror -c
    LDFLAGS = -T link.ld -melf_i386
    AS = nasm
    ASFLAGS = -f elf

    all: kernel.elf

    kernel.elf: $(OBJECTS)
        ld $(LDFLAGS) $(OBJECTS) -o kernel.elf

    os.iso: kernel.elf
        cp kernel.elf iso/boot/kernel.elf
        genisoimage -R                              \
                    -b boot/grub/stage2_eltorito    \
                    -no-emul-boot                   \
                    -boot-load-size 4               \
                    -A os                           \
                    -input-charset utf8             \
                    -quiet                          \
                    -boot-info-table                \
                    -o os.iso                       \
                    iso

    run: os.iso
        bochs -f bochsrc.txt -q

    %.o: %.c
        $(CC) $(CFLAGS)  $< -o $@

    %.o: %.s
        $(AS) $(ASFLAGS) $< -o $@

    clean:
        rm -rf *.o kernel.elf os.iso

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

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 (as defined in the Makefile above).