panthema / 2006 / SDIOS06 / README.source (Download File)
                                    SDI OS 06
                            In lack of a better name...

1. Introduction
2. Installation
3. Features
4. Operating System
5. Userspace Libraries
6. Contact


1. Introduction

This is a toy operating system developed during the System Design and
Implementation course 2006 at the University of Karlsruhe. It was designed and
written by Timo Bingmann, Matthias Braun, Torsten Geiger and Andreas Maehler.

1.1 Code/Libraries used

This project wouldn't have been possible without code from the following
projects that was available under open source licenses:

* L4
* sdios base
* dietlibc
* zlib
* libpng
* libjpeg
* SDL_image
* supertux
* sdljump

1.2 Development Tools

Such a project would also not be possible without good development tools:

* Linux
* VMWare
* All the good GNU development tools
* subversion
* A mediawiki wiki
* Eclipse CDT, xemacs, vim


2. Installation

2.1 Prerequesites

* The usual suspects: gcc, make, autoconf, binutils, objdump, ...
  We used gcc 4.1.x for development. You should make sure that libsupc++ is in
  your library path!
* IDL4, version 1.0.2 is required
* L4 pistachio 0.4 kernel (you should consider applying the timer patch in the
  contrib directory to improve timing in vmware machines)

2.2 Building

Use the following commands to build and install sdios in INSTALLDIR

./configure --prefix=INSTALLDIR
make install

2.3 Booting the OS

You need a multiboot compliant bootloader to boot SDIos. The bootloader has 
to load the operating system servers and the ramdisk images as boot modules.

Development happened with the grub boot loader on a floppy disk that tried
tried to load the additional files from a tftp server in the network.

You have to make sure that the first modules loaded are: kickstart, ia32-kernel,
sigma0, root, locator. After that you can load any modules in any order. The
configuration used during the presentation was the following:

default 0
timeout 0

title SdiOS



3. Features

* 32 bit multi-tasking operating system
* Protected address spaces, paging support
* Text mode output to graphics card, supports a subset of ANSI escape sequences
* Virtual Consoles with scrollback support
* Input from keyboard
* Ramdisk support (loaded as grub modules)
* Minix filesystem
* Uniform global namespace (used for tasks, services, filesystem)
* Elf loading
* PCI support
* VMWare graphics card framebuffer driver
* Supports a subset of the POSIX API
* Ports of several gaming related libraries: SDL, zlib, jpeg6, png, SDL_image
* Command line tools: shell, cat, ls
* Ports of two games: supertux, sdljump

3.1 Not working yet

* Write access crashes the minixfs server
* The console sometimes stops working properly and only scrolls the last line
* ThreadIDs are not managed (we just assign new IDs and wrap around if too many
  have been assigned)

3.2 Could be improved

* libc write functions are not buffered yet, so things like fputc and esp.
  fprintf which uses fputc internally produce a write IPC per character. This
  is inefficient!
* Heap management algorithm is only O(n)
* More POSIX functions could be implemented
* Console only understands a subset of ANSI escape sequences

3.3 Would be nice to have

* IDE block device driver
* A sound card driver
* More applications :)

3.4 Not a design goal

* No security
* No multiuser system (but still multitasking)
* No networking


4. Operating System

4.0 The boot process / roottask

First all the code of the roottask and all loaded boot modules is pinned in the
roottask, so that the sigma0 server of L4 won't release the memory to other
parties. The next step is to start the following code modules, which are all
linked into the roottask binary.
    * Logger 
        Thread in the address pace of the roottask
    * Sigma1 Pager:
        Started as a thread in its own address space but with roottask as
        backing pager. This way data/code segments are shared with the
        roottask, but pages mapped from sigma0 are protected from the threads
        in the roottask.
    * Syscall Server
        Thread in the address space of the roottask
    * Ramdisk
        Thread in the address space of the roottask
    * Elfexec
        Init thread in the address space of the roottask.

Finally the roottask enters into a loop and serves pagefault requests so it can
be used as pager for the sigma1 server.

The rest of the bootup process is executed by the elfexec thread. It is
implemented as a thread because we need a running sigma1 pager to start further
tasks, but for the sigma1 pager to work the roottask must server pagefault
IPCs.  The elfexec thread inspects the boot modules loaded by grub. If a module
is an elf-file then a new task is started (see elf loading), if it is a minix
filesystem image, then the module gets registered as ramdisk.

4.1 Memory Management

The pager sigma1 starts by fetching all available anonymous memory from sigma0
via the RPC protocol L4_Sigma0_GetAny. This transfers ownership of all
remaining conventional memory pages to sigma1, it does not include special
address ranges marked as bootloader, architecture-specific or reserved in the
KIP. Afterwards the roottask cannot allocate more anonymous memory from sigma0,
thus the roottask has no dynamic heap (no working malloc). Sigma1's interface
contains a function GetPageGrant, which may be used by the root task (or any
other task) if more memory is required.

Since sigma1 itself has the same memory view as the root task, it too has no
working dynamic heap. Therefore sigma1 backs it's dynamic data structures on
different slab allocators. The slab allocators are a list of slab pages from
which fixed-sized memory blocks can be allocated. Initially a few slab pages
from memory allocated within the data segment (which is reserved by the
roottask's memory pinning) are added to each slab allocator's pool. If the slab
allocator requires further memory, it reserves a page from the free anonymous
memory pool.

Sigma1 organises free pages in a buddysystem. Free pages of any size greater
than 4096 can be allocated by breaking up larger pages if required. The
buddysystem will also coalesce smaller adjacent pages into larger blocks. The
status of the buddysystem can be view by reading /task/freelist.

Each task which managed by the sigma1 pager requires some specific variables,
which are held in the TaskEntry structure. It is allocated when a new task is
created and the task list enables the pager to function as task server. Each
TaskEntry holds the MappingList of the managed task. It contains a sorted
linked list describing current page mappings. A memory range of the managed
address space is associated with an anonymous memory page of equal size, which
was retrieved from sigma0. When a pagefault occurs on the managed task, the
pager first looks into the current mapping list and sends a corresponding
MapItem if the address is already mapped. Otherwise the pager checks in which
address range the touched address lies. The address space layout can be found
in src/pager/vmemlayout.txt. The fault address determines whether a new
zero-fill page of anonymous memory is allocated and returned, or if the pager
will panic and thus simulate a segmentation fault. Currently a segfault will
stop the whole system. It can easily be changed to kill the faulting task. The
pager's interface contains a brk() call to change the upper limit of the
heap. This only modifies the selection between segfault and free-mem

The pager sigma1 also functions as task server, because it already contains a
full task list of managed address spaces. Therefore the elf-loader needs only
to call CreateTask on the sigma1 pager to make a new address space and initial
thread. When a task wishes to terminate (by calling exit), the task must call
KillMe() on the sigma1 pager, which stops the threads execution and cleans up
the address space. Because sigma1 does not run in the roottask, it must use the
syscall server for the privileged calls to ThreadControl and SpaceControl. The
KillMe() function takes a return code as an argument. This return code is saved
in the TaskEntry and the task's status changes to zombie. A different task can
then call WaitFor() with corresponding parameters to get this return code and
clean up the zombie task. Linux coders will recognize the semantics of wait()
and waitpid().

Many internal structures of the sigma1 pager are visible on the name space
(file system) in the /task directory.

4.2 Elf Loading

The elf loader is used to create a new task from an elf image. The loading
function sdi_elfexec is implemented in the libsdi, so any thread can create a
new task. Contrary to exec()'s semantics the sdl_elfexec does not replace the
currently running task. An elf image can be started from memory or from the
file system.

First the sigma1 pager's function CreateTask is called to create a new empty
address space.

Then code and data from the elf image is loaded into the new addreess space by
creating "shared" mappings. The creating thread calls sigma1's GetSharedPage
function to get an area of memory which will be placed into the new task's
address space at a given position. The call itself returns a MapItem (mapped to
0x90000000) which is then filled with the code/data from the elf
image. FreeSharedPage releases the mapping from the creator's address
space. Using multiple calls and copies the whole elf program image is
transfered into the new address space. The elfexec function completely ignores
rwx protection flags and the pager does not implement them.

After the binary code is in place, the elfexec function constructs the stack of
the new task. On top of the stack it places the environment and command line
parameters in the same way as it is done on Linux. The environment variables
and command line parameters are copied from the sdi_elfexec's parameter
list. In the end the three calling parameters of the new thread's main()
function are "pushed" onto the created stack. The stask is transfered to the
new address space in the same fashion as the binary code using GetSharedPage.

Finally the new task is kick-started by the pager using the StartTask function.

4.3 Heap Management

4.4 Naming System

4.5 Blockdevices

Blockdevices are storage devices which are separated into several blocks. A
block is a fixed size array of bytes, each device can have a different but
constant blocksize. The only operations possible are reading and writing of a
block (identified by its block number).
Blockdevices are the base on which filesystems get implemented. Currently there
is only an implementation of a ramdisk in the roottask which allows
reading/writing to grub modules.
Blockdevices implement the IF_BLOCKDEV interface.

4.6 Filesystems

4.7 Console

4.8 PCI driver, vmware graphics card framebuffer driver


5. Userspace libraries

There are a set of libraries used to support user applications and servers:

5.1 libio

Came with SDI and allows reading/writing to the serial port (where you
typically connect a terminal or terminal emulator application like minicom)

5.2 libsdi

This library contains convenience functions for working with SDI specific
features. It contains functions that loading and starting elf-files, stopping
the kernel in exceptional situations (panic), using the logging server,
resolving namespace paths, as well as inline assembly functions for accessing

5.3 libc

Implements a POSIX subset to allow easy creation and porting of user
applications. It features:

* Heap management
* Assert
* Functions for working with environment variables
* Nearly complete IO support (fopen, fprintf & frineds)
* Some few math functions
* opendir/readdir and limited stat support

Some parts of the libc code (like printf, scanf, most math functions and the
random number generator were copied from dietlibc).

5.4 libstdc++

A very incomplete and not really conformant implementation of the standard C++
library. Featuring a string class, a vector class and an insert only map based
on a redblack tree.

Basically it implements all features used by supertux :)

5.5 png

Straight forward port of the libpng library

5.6 zlib

Straight forward port of the zlib library

5.7 jpeg

Straight forward port of the jpeg library

5.8 SDL

A port of the SDL library (TODO: write more)


6. Contact

... TODO ...
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