The Implementation of the ULIX Literate Operating System








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A diary documenting the implementation of ULIX-i386

Welcome to the ULIX blog.

Ulix (Literate Unix) is a Unix-like operating system developed at University of Erlangen-Nürnberg. I use D. E. Knuth's concept of Literate Programming for the implementation and documentation. The goal was a fully working system which can be used in operating system courses to show students how OS concepts (such as paging and scheduling) can be implemented. Literate programs are very accessible because they can be read like a book; the order of presentation is not enforced by program logic or compiler restrictions, but instead is guided by the implementer's creative process.

Ulix is written in C and assembler for the Intel architecture. The literate programming part is handled by noweb.

On this page I document my progress with the implementation.

Navigation: 2015 | 2014 | 2013 | 2012 | 2011

User Mode Shell (25.11.2012)

Ulix now boots into a user mode shell -- a real shell process which can fork new processes. Currently the new shell has only a few (internal) commands; it can show the process list (ps) and the files on the floppy (ls) and look into them with the head command. Ah, it also can exit...

The rewrite of the task-switching code seems to work a lot better than the original code which was based on a tutorial that I found on the web. For now, there are no more problems with the stacks. I've also changed the code for the system call handlers which now works with the same set of saved registers as interrupt handlers do; that makes life easier, because there are always the same data on the stack.

Also, the document which describes Ulix has crossed the 400 pages barrier today. It will likely grow to something like 600-700 pages before Ulix is finished.

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Redoing the scheduler and init / exec code; floppy access (19.11.2012)

Today I started rewriting the code for scheduling and process initialization (via fork() or load_from_disk()). The new code uses the fact that all handler functions (which includes system call handlers) can read the register contents of the process in user mode (before the interrupt occurred or the system call was initiated with int 0x80).

Also new in Ulix: I can now access a floppy drive. Reading and writing sectors works fine in both system mode (during the OS initialization) and in user mode (through sys calls). For testing stuff, there's also a rudimentary FAT-like filesystem (including a makefs command that creates new floppy images and fills them with files, a bit like genisoimage).

Since so many things depend on one another, there's also a usermode library (ulixlib) which allows Ulix programs to be (cross-) compiled with gcc. So it is now possible to write C programs that open() a file on the floppy disk and then read() from and write() to it.

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Executing processes (05.10.2012)

Ulix can run processes now. It does not handle several of them nicely yet, but the news is that programs can be compiled (externally) using a basic usermode library. So far it can printf() and--sort of--open files and read/write them.

Compiling requires a linker script which creates a flat binary: So far, Ulix does not support ELF binaries (but there'll be ELF support early next year, since a BSc student has just started working on that component). Such programs can call functions in the usermode library which in turn make system calls in order to do privileged things (such as writing to the screen and forking).

After launching a program (which has no way to properly exit() so far), Shift-Esc jumps back to the shell. The new ps command in that shell shows what's going on.

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Ulix has Hard Disk Support (sort of...) (02.10.2012)

I've been looking at a few implementations of hard disk drivers, since (obviously) Ulix needs this as well. Since none of the available solutions is simple enough for inclusion in the Ulix book, I've decided to cheat a little.

So I present: the serial hard disk. It relies on an external process that serves as some kind of simple storage server. Via the serial port, Ulix can request sectors (or send their content), and the storage process translates this to reading and writing a disk image.

It already works for non-blocking I/O (i.e. when the kernel wants to access the disk), when Ulix runs on qemu. (Bochs support follows soon.) In theory this would also work when running Ulix on a real machine and connecting the serial port to another machine running the storage process. Next up: blocking I/O for processes.

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Bachelor Theses (14.08.2012)

Two students from Nuernberg University of Applied Sciences ("Ohm-Hochschule Nürnberg") are now working on Bachelor theses, implementing parts of the ULIX operating system. I'll report on their progress and, later, on the integration of the new code into ULIX. The two theses are about RAM disks and an ELF program loader.

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Moodle... (01.06.2012)

When Ulix is finished, there will also be a Moodle course which can be used as a free introduction to operating systems. Today I've setup a temporary Moodle site.
[Update 2012/10/20: I've registered a new domain ulixos.org which will later host the course.]

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Processes... (30.05.2012)

The simple scheduling of several processes works. Each of them uses their own memory (address spaces), and a simple round robin scheduler cycles between them. For some reason they cannot access the keyboard input buffer yet. Next: Fix this input problem; understand why fork only works when manually moving the child's stack pointer 8 bytes up; create a user mode library so that processes can be compiled C programs (instead of inline assembler). Here's the first running process:

    call dofork;
    call dofork;
    mov ebx, esp;      // print ESP
    mov eax, 0x1003;
    int 0x80;
    mov eax, 20;       // call getpid()
    int 0x80;
    mov eax, 0x1000;   // store PID in 0x1000
    mov [eax], ebx;
    call doshowpid;
    jmp loop;

    mov eax,2;         // syscall 2: fork
    int 0x80;

    mov eax,0x1000;
    mov ebx,[eax];
    add ebx,'a'-1;
    mov eax,0x1001;    // syscall 0x1001: putchar
    int 0x80;
It forks twice, so there are 4 processes running. The screenshot shows the beautiful output of process 4 (printing "d"s).

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Filesystems: read and write via mmap? (22.05.2012)

Just a note to myself: While reading a blog posting about improved file I/O performance via memory mapped files, I wondered whether it might make sense to write an mmap() implementation first and then create open, lseek, read and write as functions which use an mmap()ped file. Every read() or write() would just turn into a memcpy(), an lseek() would just set an offset variable associated with an mmap()ped file. Plus, the code for transfering disk blocks to memory is likely to be similar to the code needed for paging in a page that was written to disk.

There would be a problem with large files, obviously, because a mapping cannot be larger than the address space.

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Processes, little updates, serial console (20.05.2012)

I'm working on the process code. So far I can create the initial process, run it (in usermode) and make it fork (via a syscall). There's also an initial implementation of a simple scheduler that is supposed to switch between tasks 1 and 2 (which does not work yet; after changing to the second process the system crashes, because the stack is corrupt. I can fix the stack, but then I get the same problem when switching back to task 1. So there is some weird error in there).

Other new stuff in Ulix: I have a serial console (thanks to the code from the xv6 operating system, but I'm only using the functionality of duplicating all terminal output in a terminal window on the host (good for long memory dumps which scroll out of the window). I've also added a "hlt" instruction in the kgetch() function, so my notebook does not get hot any longer :)

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Bug fix :) (19.05.2012)

I found a bug in memory management that came into being after including code for creating new address spaces: When there is more than one address space, we have (identical) copies of the page directory's mapping for c0000000..ffffffff -- so when an entry in that area is modifed (i.e. a new page table is created and linked from that page directory) it must also be modified in all existing copies... Took me quite a while to find it, but now the code is OK :) Memory setup works for processes (as long as no more than 4 MB are requested; can be fixed easily), so the system is ready for process creation.

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Ulix has address spaces (05.04.2012)

Another step on the way to proper process management: Ulix now has address spaces which can be switched on. So there is "process-private" memory (however: no process yet).
To do: more work on address spaces (there are some bugs in there); integrate address spaces + usermode switch + process table (not defined yet). For starters, only cooperative scheduling.

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User mode stuff: side project (02.04.2012)

Since I'm giving the lecture "System Level Programming" at Nuernberg Univ. of Applied Sciences, I use the practical coursework to develop some user mode tools for Ulix. Currently I'm working on a simple shell. So far it can evaluate parameters and launch processes (assuming there are working fork() and exec() functions).
fork() should not be a problem at all (once I have a process table), exec() can't be done properly yet since there is no filesystem code at all. However, exec() could load a binary from RAM. I had already experimented with precompiled programs stored in the Ulix code as byte arrays.

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Update: Development goes on, Moved to Erlangen (12.01.2012)

I've moved to Erlangen (where research and Ulix development are done), so now I'm closer to the project, and things will continue faster this year. Keep watching for new features :)

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