Thread: My Own OS

i dont really remember the exact but it could be 2 million.

ok

I had to do that at school last year.

First a little knowledge about the boot sequence on a PC:

On power up the BIOS looks for the first executable bootable device (floppy bootsector for example) and loads its contents into memory at the address 0:7C00 (512 bytes will be put out there) and then simply 'jumps' the processor to 0:7C00 and executes that code.

then, your code (at 0:7c00) is supposed to look for the rest of the code because 512 bytes is not enough for a whole OS :P

this part is in general called stage1.

that's where it starts getting interesting. the purpose of stage1 is to find stage2 on the floppy disk, load it in memory and then jump to it!

stage2 is a little bigger that stage1 and it purpose is to load the kernel somewhere in memory (before the 1MB mark obviously), disable interrupts, enable the A20 gate (= access to the rest of the memory), setup the GDT and IDT registry which is used by the protected mode and then jump into protected mode.

Then stage2 must move the kernel at the 1MB memory mark (just above the I/O area) an jump to it.

The kernel can be written in flat C++, but you'll have to write printf yourself and access the video card yourself. Also, you might need to program the keyboard and make a fgets() function.

because there will be no libraries available for you.

still intersted in writing an OS ?

Just for the record, I am not a newbie in assembly, neither in C, and neither in compiling and linking, and it still took me a whole month (fulltime) to debug that and make it work. The only reason why I did that is because we needed it at school to run some tests. Otherwise there is no way I would have done that.

if you're still interested, drop me a line at [email protected] and I'll email you some of the code and interesting links

Originally posted by xmdvp
ok

I had to do that at school last year.

First a little knowledge about the boot sequence on a PC:

On power up the BIOS looks for the first executable bootable device (floppy bootsector for example) and loads its contents into memory at the address 0:7C00 (512 bytes will be put out there) and then simply 'jumps' the processor to 0:7C00 and executes that code.

then, your code (at 0:7c00) is supposed to look for the rest of the code because 512 bytes is not enough for a whole OS :P

this part is in general called stage1.

that's where it starts getting interesting. the purpose of stage1 is to find stage2 on the floppy disk, load it in memory and then jump to it!

stage2 is a little bigger that stage1 and it purpose is to load the kernel somewhere in memory (before the 1MB mark obviously), disable interrupts, enable the A20 gate (= access to the rest of the memory), setup the GDT and IDT registry which is used by the protected mode and then jump into protected mode.

Then stage2 must move the kernel at the 1MB memory mark (just above the I/O area) an jump to it.

The kernel can be written in flat C++, but you'll have to write printf yourself and access the video card yourself. Also, you might need to program the keyboard and make a fgets() function.

because there will be no libraries available for you.

still intersted in writing an OS ?

i cannot imagine anything more exciting than what you described above (not that I dont understanf /have not heard it before)

Originally posted by xmdvp

still intersted in writing an OS ?

I love it

Originally posted by Travis Dane
I love it

it always makes me smile when I read this. People love doing things when the dirty job's been done before, or when they don't know what they are talking about.

someone said: there's 10 kinds of people: those who understand binary and those who don't

xmdvp

Originally posted by xmdvp
or when they don't know what they are talking about.

I've read through your post and what i made up of it is that
writing your own OS takers time and has to be done on a low
level meaning you have to do everything yourself (like printf),
And that's what i love.

I agree that I had fun writing my own printf() and getc() function.