scos

This is a simple operating system, which I wrote from scratch in my high school in order to learn how an OS runs on a machine. It is partly a mimic of Linux 0.11. It contains a bootloader, a kernel, a shell and some tools that can run on it for demonstration.

It runs in x86 protected mode(32-bit), the kernal runs in ring 0, and other programs runs in ring 3.
It partially supports memory paging, with page mapping from vitual address space to physical address space and allocating physical pages on demand.
It (only) supports floppy disk controller and FAT-12 system. (the whole OS is installed in a 1.44 MB floppy disk)
It supports DMA to read/write the disk.
It supports preemptive multi-tasking, and each process(no thread) has 64MB memory (all process sharing the same system page table) and equal CPU time slice (a simple implementation).
It cantains a system API on which the shell and other tools base. It is called through software interruption.

build

The Makefile was originally wrote for djgpp on Windows, and I adapt it to OS X. There are still some problems now:

ld from djgpp(GNU) and from xcode are quite different. Our binaries need the text section being put at the begining of the file and specified address origins, but Apple's ld does not support link script and raw binary format, it always puts a head before anything else in the file. For now, we use -preload -segaddr __TEXT xxxxx flags to output a simplest file structure, and just use dd to cut off the head.
After El Capitan, Disk Utility seems not to support loading empty image file any more (file created by dd bs=1024 count=1440 if=/dev/zero of=disk.img) and images created by hdiutil always contains an extra sector at the begining of the disk (the boot sector is moved to the second sector). So I cannot find a way to create a standard FAT-12 disk image right now. For now, you could use any 1.44MB FAT-12 floppy disk image you have or found on the Internet, backup and format them.

To build is quite simple:

make sure that you have nasm installed

# use GNU-toolset
make
# use OSX toolset
make OSX_TOOLSET=1

There are 5 binary files built, boot.bin, kernel.bin, shell.bin, edit.bin, demo.bin.

Mount the disk image on your system
Use the boot.bin to replace the boot sector of the disk.

# replace '/dev/fda1' with the filename of your mounted image
dd bs=512 count=1 if=boot.bin of=/dev/fda1

Put other binaries files into the disk directly through File Explorer or Finder.

Caution: if you use the OSX toolset, please remember to cut off the head part from the files:

# our code and other sections start after 8192 bytes from the begining of the file
dd bs=8192 skip=1 if=old.bin of=new.bin

shenchi/scos

scos

build