L4Re – L4 Runtime Environment
Moe, the Root-Task

Moe is the default root-task implementation for L4Re-based systems.

Moe is the first task which is usually started in L4Re-based systems. The micro kernel starts Moe as the Root-Task.

Moe objects

Moe provides a default implementation for the basic L4Re abstractions, such as data spaces (L4Re::Dataspace), region maps (L4Re::Rm), memory allocators (L4::Factory, L4Re::Mem_alloc), name spaces (L4Re::Namespace) and so on (see L4Re Interface). These are described in the following subsections.


The factory in Moe is responsible for all kinds of dynamic object allocation.

Moe's factory allows allocation of the following objects:

L4::Scheduler objects can be only created through the user factory provided by Moe to the initial application. Other factory instances cannot create this object.

Passing parameters to the create stream

L4::Factory.create() returns a create stream that allows arguments to be forwarded to the the object creation in Moe.

Objects that support additional parameters on their creation are presented next with a non-empty list of parameters. The parameters are listed in the order they should be provided to a create stream. Optional parameters are identified by their default values. Multiple entries in the next list denote different ways of initializing an object.

  • L4Re::Namespace ()
  • L4Re::Dataspace (l4_mword_t size, l4_umword_t flags = 0, l4_umword_t align = 0)
    • Argument size: size in bytes (mandatory)
    • Argument flags: special dataspace properties, see L4Re::Mem_alloc::Mem_alloc_flags
    • Argument align: Log2 alignment of dataspace if supported by allocator
    • See detailed description of the parameters in L4Re::Mem_alloc::alloc()
    • For details on the types of dataspaces provided by Moe, see Dataspace
  • L4Re::Dma_space ()
  • L4Re::Rm ()
  • L4::Vcon (char const *label, l4_mword_t color = 7)
    • Argument label: label used as prefix for the console output
    • Argument color: color code 0..15
    • For more details see Log Subsystem
  • L4::Vcon (char const *label, char const *color = "w")
    • Argument label: label used as prefix for the console output
    • Argument color: color code
      • The color is identified by a single character
      • Supported colors: N, n, R, r, G, g, Y, y, B, b, M, m, C, c, W, w
    • For more details see Log Subsystem
  • L4::Scheduler (l4_mword_t limit, l4_mword_t offset, l4_umword_t bitmap = ~0UL)
    • Argument limit: maximum priority
    • Argument offset: priority offset
    • Argument bitmap: bitmap of CPUs
    • Argument limit must be greater than offset
    • For more details see Scheduler subsystem
  • L4::Factory (l4_mword_t quota)
    • Argument quota: limit in bytes (not zero)
    • The limit is deducted from the limit of the factory that creates the new factory


Moe provides a name space conforming to the L4Re::Namespace interface (see Name-space API). Per default Moe creates a single name space for the Boot FS. That is available as rom in the initial objects of the init process.

Boot FS

The Boot FS subsystem provides access to the files loaded during the platform boot (or available in ROM). These files are either linked into the boot image or loaded via a flexible boot loader, such as GRUB.

The subsystem provides an L4Re::Namespace object as directory and an L4Re::Dataspace object for each file.

By default all files are read only and visible in the namespace rom. As an option, files can be supplied with the argument :rw to mark them as writable modules. Moe will allow read and write access to these dataspaces and make them visible in a different namespace called rwfs.

An example entry in 'modules.list' would look like this:

module somemodule :rw
In order for a client to receive write permissions to the dataspace, the corresponding cap also needs write permissions.


Dataspaces can be allocated with an arbitrary size. The granularity for memory allocation however is the machine page size (L4_PAGESIZE). A dataspace user must be aware that, as a result of this page-size granularity, there may be padding memory at the end of a dataspace which is accessible to each client. Moe currently allows most dataspace operations on this padding area. Nonetheless, the client must not make any assumptions about the size or content of the padding area, as this behaviour might change in the future.

The provided data spaces can have different characteristics:

  • Physically contiguous and pre-allocated
  • Non contiguous and on-demand allocated with possible copy on write (COW)

Log Subsystem

The logging facility of Moe provides per application tagged and synchronized log output.

DMA Space

Scheduler subsystem

The scheduler subsystem provides a simple scheduler proxy for scheduling policy enforcement.

The priority offset provided on the creation of a scheduler proxy defines the minimum priority assigned to threads which are scheduled by that instance of the scheduler proxy. The offset is implicitly added to priorities provided to L4::Scheduler.run_thread().

Region Map

Command Line Options

Moe's command-line syntax is:

moe [--debug=<flags>] [--init=<binary>] [--l4re-dbg=<flags>] [--ldr-flags=<flags>] [-- <init options>]
--debug=<debug flags>
This option enables debug messages from Moe itself, the <debug flags> values are a combination of info, warn, boot, server, loader, exceptions, and ns (or all for full verbosity).
--init=<init process>
This options allows to override the default init process binary, which is 'rom/ned'.
--l4re-dbg=<debug flags>
This option allows to set the debug options for the L4Re runtime environment of the init process. The flags are the same as for --debug=.
--ldr-flags=<loader flags>
This option allows setting some loader options for the L4Re runtime environment. The flags are pre_alloc, all_segs_cow,and pinned_segs.
-- <init options>
All command-line parameters after the special -- option are passed directly to the init process.