Our basic kernel consists of two source files, one assembler file that will contain our executable header and call our main function, and a c file that contains our bare bones kernel.

The following is the start.asm. I basically stole it from a tutorial online: http://www.cs.vu.nl/~herbertb/misc/writingkernels.txt. All it really does is create our stack area, define the Multiboot header then call the cmain function. We probably will never have to modify this file once we compile it.

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; ;; start.asm ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; ; This is the kernel's entry point. We could either call main here, ; or we can use this to setup the stack or other nice stuff, like ; perhaps setting up the GDT and segments. Please note that interrupts ; are disabled at this point. [BITS 32] global start  start:  mov esp, _sys_stack ; This points the stack to our new stack area jmp stublet  ; This part MUST be 4byte aligned, so we solve that issue using 'ALIGN 4' ALIGN 4  mboot:  ; Multiboot macros to make a few lines later more readable MULTIBOOT_PAGE_ALIGN equ 1<<0 MULTIBOOT_MEMORY_INFO equ 1<<1 MULTIBOOT_HEADER_MAGIC equ 0x1BADB002 MULTIBOOT_HEADER_FLAGS equ MULTIBOOT_PAGE_ALIGN | MULTIBOOT_MEMORY_INFO MULTIBOOT_CHECKSUM equ -(MULTIBOOT_HEADER_MAGIC + MULTIBOOT_HEADER_FLAGS)  ; This is the GRUB Multiboot header. A boot signature dd MULTIBOOT_HEADER_MAGIC dd MULTIBOOT_HEADER_FLAGS dd MULTIBOOT_CHECKSUM  ; A call to main (the C kernel) followed by an infinite loop (jmp $) stublet:  EXTERN cmain ; start of our kernel call cmain jmp $  ; Here is the definition of our BSS section. Right now, we'll use ; it just to store the stack. Remember that a stack actually grows ; downwards, so we declare the size of the data before declaring ; the identifier '_sys_stack' SECTION .bss resb 8192 ; This reserves 8KBytes of memory here _sys_stack: ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

Next is mos.c the meat and potatoes of our kernel, this is where we start our kernel. All we are currently doing is pointing two pointers to the start of video memory and then using them to clear the screen, then write "Hello world" in the top left corner.

// The main procedure for our kernel. This is called from the assember startup  // file. I am using cmain only because that was what was used in the tutorial I // found. int cmain() {     char* string = "Hello World";     int i;           // we are using volatile, which essentially means no caching, if my memory     // serves.     volatile short *v = (volatile short * )0xB8000;           // clear the screen, set all characters to a space 0x20     // note that the 0x0720 seems backwards compared to our printing     // of the string below. This is because the Intel IA32 is Little Endian.     for (i =0; i < 25 * 80; i++)     {         *v = 0x0720;         v++;     }           volatile char * video=(volatile char*)0xB8000;     // loop through the string Hello World, 0x07 simply sets the colour as      // light grey on black      while(*string!=0)     {         *video=*string;         string++;         video++;         *video=0x07;         video++;     }     return 0; }

Next all we need is a couple of scripts to facilitate the compiling and linking. They are as follows

link.ld (again taken from http://www.cs.vu.nl/~herbertb/misc/writingkernels.txt):

ENTRY(start) phys = 0x00100000; SECTIONS { .text phys : AT(phys) { code = .; *(.text) *(.rodata) . = ALIGN(4096); } .data : AT(phys + (data - code)) { data = .; *(.data) . = ALIGN(4096); } .bss : AT(phys + (bss - code)) { bss = .; *(.bss) . = ALIGN(4096); } end = .; }

Essentially all it does is tell the linker where to put the different data sections, as well as where to start the program. I tried to just let linker do it on its own, grub would not even load the file.

makefile:

CFLAGS := -fno-builtin -nostdinc -O -g -Wall -I. LDFLAGS := -nostdlib -Wl,-N -Wl,-Ttext -Wl,100000  all: mos.bin  mos.bin: start.o mos.o     /usr/cross/i586-elf/bin/ld -T link.ld -o mos.bin start.o mos.o     @echo Done!  start.o: start.asm     nasm -f elf -o start.o start.asm  mos.o: mos.c      /usr/cross/bin/i586-elf-gcc-4.2.3 $(CFLAGS) -c -o mos.o mos.c  clean:      rm -f *.o *.bin  install:     cp mos.bin a:/boot/mos.bin

You may have to change some of your locations. This allows us to use three different commands with make. all, clean and install which compiles and links, removes all binaries, and copies the kernel to the floppy drive respectively.

At this point you should be able to from your Cygwin console change directory to wherever you placed the previous four files and compile:

cd /cygdrive/r/mos make all make install

If you did not experience any errors you should now be able to boot off of your floppy and see the words "Hello World" at the top of the screen.

Now where do you go? Well from here I would recommend writing an output module using what you already know. It would make life much more easier if you could write debut information to the screen. So writing the various output routines that would allow you to display information on the screen would be very helpful. 

After that, It is up to you. It all depends on where you plan on going with your OS. But as a general rule it is always easier to start at the bottom by building the basics and working on more complicated modules using those you have already created. It is much better to learn to crawl before you attempt to run. Don’t let your ego get ahead of what you are capable of accomplishing and things will go much easier for you.

At some point I may post my output routines, If I get them finished. 

Tools:

Cygwin: http://www.cygwin.com/
RawWrite For Windows: http://www.chrysocome.net/rawwrite
DSL Linux: http://www.damnsmalllinux.org/ 
JEdit Editor http://www.jedit.org 
Bochs IA-32 Emulator http://bochs.sourceforge.net/

Source:

MOS http://downloads.terdos.com/mos/mos1.zip
Grub Boot Disk http://downloads.terdos.com/mos/grubboot.zip (bif format?)
GNU Utilities Source: http://ftp.gnu.org/gnu/

References:

Cygwin FAQ: http://www.cs.nyu.edu/~yap/prog/cygwin/FAQs.html
Making a Grub Boot Floppy: http://linux-sxs.org/administration/grubflop.html
Writing Kernel Tutorial: http://www.cs.vu.nl/~herbertb/misc/writingkernels.txt
Cross Compiler Tutorial: http://wiki.osdev.org/GCC_Cross-Compiler

Further Reading: