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<chapter id="architecture">
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<title>Architecture overview</title>
<section>
<title>Scheme</title>
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</section>
<section>
<title>Kernel primitives</title>
<para><termdef><glossterm>Thread</glossterm> is the basic execution
primitive.</termdef></para>
<para><termdef><glossterm>Thread context</glossterm> represents state of
the <emphasis>thread</emphasis>. Thread context is built of the context
registers contents, FPU state and the stack.</termdef></para>
<para><termdef> <glossterm>Task</glossterm> is a multi-purpose entity,
serving to <itemizedlist>
<listitem>incorporate set if its threads</listitem>
<listitem>provide common address space to its threads</listitem>
<listitem>be an end-point in IPC</listitem>
</itemizedlist> </termdef></para>
<para><termdef> <glossterm>Address space area</glossterm> is a mutually
disjunctive range of memory with the code, stack and data.
</termdef></para>
<para><termdef> <glossterm>Address space</glossterm> is a aggregating
entity for address space areas, connecting them to the task.
</termdef></para>
</section>
<section>
<title>Monolithic microkernel</title>
<para>Though HelenOS was initially planned as a microkernel, we were
trying to avoid several issues, connected with microkernels, such as much
higher overhead during memory management and hardware operations. For this
reason some of the subsystems, that are to be implemented as servers in
classic microkernel design, were implemented as a part of kernel, thus
minimizing this overhead.</para>
<formalpara>
<title>Memory management</title>
<para>Unlike the classic microkernel, HelenOS has all its memory
management functionality in the kernel, available to the memory
management server via the set of syscalls.</para>
</formalpara>
<formalpara>
<title>Kernel device drivers</title>
<para>HelenOS kernel has some of the very basic device drivers
<itemizedlist>
<listitem>
ACPI
</listitem>
<listitem>
APIC
</listitem>
<listitem>
SMP configuration
</listitem>
<listitem>
System clock
</listitem>
<listitem>
Interrupt controllers
</listitem>
<listitem>
Console
</listitem>
<listitem>
VESA & frame buffer
</listitem>
</itemizedlist></para>
</formalpara>
</section>
<section>
<title>IPC</title>
<para>HelenOS IPC is designed in analogy with telephone communication.
Each task has an <emphasis>answerbox</emphasis> and a set of
<emphasis>phones</emphasis> to call another tasks' answerboxes.</para>
<para>Communication is possible after the connection is established, and
can be either <emphasis>asynchronious</emphasis> or
<emphasis>synchronious</emphasis>.</para>
</section>
<section>
<title>Functionality model</title>
<para>As you know, microkernel design is very simple, just enough to
provide communication facility for tasks. Most of the OS functionality is
performed by server tasks, that are running in userspace. Thus most of the
system calls in monolithic kernels, are the IPC calls on server tasks in
microkernels.</para>
<para>Moreover, problems experience the device drivers. Running in the
user space, device driver still needs to recieve interrupts and access
hardware directly.</para>
<para>This raises two major problems in microkernels: <orderedlist
numeration="loweralpha">
<listitem>
What is the recipient address of the server (e.g. "memory manager" or a specific device driver) ?
</listitem>
<listitem>
How this server task is going to access hardware or kernel while running in the user mode?
</listitem>
</orderedlist></para>
<formalpara id="intro_ns">
<title>Name server</title>
<para>As every microkernel, HelenOS has a "Name server" task with "well
known" IPC address, that connects user task to any server just by the
string service indentification.</para>
</formalpara>
<formalpara id="intro_ddi">
<title>Device driver interface</title>
<para>Device drivers use special syscalls to map physical memory areas
into their address space, to map port regions (mostly ia32). Interrupts
are delivered to the device driver task by the standard IPC
means.</para>
</formalpara>
</section>
</chapter>