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<title>Architecture overview</title>
<para>The HelenOS operating system is designed as a relatively small
microkernel assisted with a set of userspace drivers and server tasks.
HelenOS is not very radical in what subsystems should or should not be
implemented in the kernel - in some cases, both kernel and userspace drivers
exist. The reason for creating the system as a microkernel is prosaic. Even
though it is initially more difficult to get the same level of functionality
from a microkernel than it is in the case of a simple monolithic kernel, a
microkernel is much easier to maintain once the pieces have been put to work
together. Therefore, the kernel of HelenOS, as well as the essential
userspace libraries thereof can be maintained by only a few developers who
understand them completely. In addition, a microkernel based operating
system reaches completion sooner than monolithic kernels as the system can
be used even without some traditional subsystems (e.g. block devices,
filesystems and networking).</para>
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<caption>HelenOS architecture overview</caption>
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<para>HelenOS is comprised of the kernel and userspace server tasks. The
kernel provides scheduling, memory management and IPC. It also contains
essential device drivers that control the system clock and other devices
necessary to guarantee a safe environment. Userspace communicates with the
kernel through a small set of syscalls. The userspace layer consists of
tasks with different roles, capabilities and privileges. Some of the tasks
serve as device drivers, naming servers, managers of various kinds and some
are just ordinary user programs. All of them communicate with other threads
via kernel-provided IPC.</para>
<section>
<title>Scheduling</title>
<para>Kernel's unit of execution flow is a thread. A thread is an entity
that executes code and has a stack that takes up some space in memory. The
relation between kernel and userspace threads is 1:1:n, meaning that there
can be several pseudo threads running within one userspace thread that
maps to one kernel thread. Threads are grouped into tasks by functionality
they provide (i.e. several threads implement functionality of one task).
Tasks serve as containers of threads, they provide linkage to address
space and are communication endpoints for IPC.</para>
<para>The scheduler deploys several run queues on each processor. A thread
ready for execution is put into one of the run queues, depending on its
priority and its current processor, from where it is eventually picked up
by the scheduler. Special purpose kernel threads strive to keep processors
balanced by thread migration. Threads are scheduled by the round robing
scheduling policy with respect to multiple priority run queues.</para>
</section>
<section>
<title>Memory management</title>
<para>Memory management is another large subsystem in HelenOS. It serves
the kernel to satisfy its own memory allocation requests, provides
translation between virtual and physical memory addresses and manages
virtual address spaces of userspace tasks.</para>
<para>Kernel allocates memory from the slab allocator, which itself
allocates memory from a buddy system based allocator of physical memory
frames.</para>
<para>The virtual address translation layer currently supports two
mechanisms for mapping virtual memory pages to physical memory frames
(i.e. 4-level hierarchical page tables and global page hash table), and is
further extensible to other mechanisms.</para>
<para>Userspace tasks depend on support of address spaces provided by the
kernel. Each address space is a set of mutually dijunctive address space
areas that group pages of common attributes. An address space area is
usually connected to, and backed by, anonymous memory, executable image of
some program or continuous region of physical memory. However, swapping
pages in and out to external memory is not supported. Address space areas
can be easily shared among address spaces.</para>
</section>
<section>
<title>IPC</title>
<para>Due to the fact that HelenOS is a microkernel, strong emphasis is
put on its IPC (Inter-Process Communication<footnote>
<para>The term Inter-Process Communication is slightly confusing
because in HelenOS terminology there are tasks instead of processes.
However, its abbreviation, IPC, is being publicly used as a standard
name for similar facilities. This book will therefore use the term IPC
to refer to communication among tasks.</para>
</footnote>). Tasks communicate by passing very short messages to one
another or by sending (i.e. sharing) address space areas when larger data
is to be transfered.</para>
<para>The abstraction uses terms like phones, calls and answerboxes, but
is pretty similar to well-known abstraction of message queues. A task can
have multiple simultaneous simplex connections to several other tasks. A
connection leads from one of the source task's phones to the destination
task's answerbox. The phones are used as handles for making calls to other
tasks. Calls can be synchronous or asynchronous and can be forwarded from
one task to another.</para>
</section>
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