Time Application Interface
A real time system can be defined as a "system capable of guaranteeing timing
requirements of the processes under its control".
The RTAI plug-in should help Linux to fulfill some real time constraints (few
milliseconds deadline, no event loss). It is based on a RTHAL: Real Time Hardware
Abstraction Layer. This concept is also known in Windows NT.
The HAL exports some Linux data & functions close related to HW. RTAI modifies
them to get control over the HW platform. That allows RTAI real time tasks to
run concurrently with Linux processes. The HAL defines a clear I/F between RTAI
OS requirements summary
Generally speaking, we would like Software components to rely on a platform offering
both real time support and "standard" general purpose API.
The Platform needs a real-time executive (small footprint, deadline fulfillment,
... ) and a comprehensive OS for Applications ( NT, UNIX, Linux, ... ).
market place already offers products for:
" NT OS: real time extensions
like Radisys' Intime or VenturCom's RTX
" UNIX : micro kernel based technologies
natively supporting real time and rich UNIX like API's.
The Linux OS exports the same application service level but suffers from a lack
of real time support. Fortunately some options are foreseen:
real time support in Linux : does not seem the Main stream of Linux.
Linux Modifications for real time constraints (KURT): no outstanding background.
Linux and real time sharing resources ( L4Linux) : experimental project.
Linux as a task of a real time executive: RTLinux, RTAI, eCos, Nucleus, ...
as a task of a real time executive" is maturing today. RTLinux is a better
known solution. RTAI , now requires a closer look and justifies this presentation.
1.2 A little
bit of ... Linux kernel core
Linux, like other OS, offers to the Applications at least the following services:
" HW management layer dealing with event polling or Processor/peripheral
" Scheduler classes dealing with process activation, priorities,
time slice, soft real-time
" Communications means among Applications (at
" Hardware Interrupts are served by handlers. On Intel machine
the handler addresses are store in the Interrupt Descriptor Table (IDT)...but
it is still possible to pool interrupt peripherals.
The Linux current releases
offer some soft-real time capabilities, but neither guaranties hard deadline nor
prevents event loss.
A little bit of ... RTAI
RTAI is such a small proprietary (see note 1) executive, still under development
at DIAMP (Dipartimento di Ingegneria Aerospaziale del Politecnico di Milano) by
Paolo Mantegazza's team . It offers some services related to:
" HW management
layer dealing with peripherals
" Scheduler classes dealing with tasks,
priorities, hard real-time
" Communications means among tasks & processes
(at least FIFO).
RTAI real time entities are simple mono-thread tasks whereas Linux applications
are full features (mono or multi threads) processes. RTAI is basically an Interrupt
dispatcher. The Intel processor interrupts (0..31) are still managed by Linux.
RTAI mainly traps the peripherals ISA IT's and if necessary re-routes them to
Linux (e.g.: disk IT). It is also able to manage other kind of ITs.
It supports, like Linux, both SMP (Symmetric Multi Processor) and UP (Uni-Processor)
Architecture. From a real time point of view, it is quite similar to RTLinux.
is an intrusive modification of the kernel, RTAI uses the concept of HAL to get
information from Linux and to trap some fundamental functions.
This HAL provides
few dependencies to Linux Kernel. This leads to:
" Simple adaptation
in Linux Kernel
" Easy RTAI port from version to version of Linux
Easier use of other Oses instead of RTAI
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