®
INK
T H E C O M P U T E R A P P L I C A T I O N S J O U R N A L
Real-Time PC
Ingo Cyliax
Embedded RT-Linux
Part 1: General Introduction
In past columns dealing with real-time Linux the kernel, which is the true Linux Usually, it s not a problem. GPL only
operating systems, I ve mentioned Linux component, and the applications that have requires you to make available the sources
and explained how I use it as a develop- been developed and ported to Linux. The for components that are already under
ment system and the primary OS on my applications range from clones of simple GPL. Other components, applications, and
laptop. Starting this month, I want to show Unix command-line utilities like and modules that you develop can be excluded.
you how to use Linux as an embedded to C and Fortran compilers and even A real-time extender, which extends
OS even as a real-time embedded OS. commercial applications. the kernel using a dynamically loadable
I ve been a Linux user and developer Together, the Linux kernel and the module, is also freely available. It works
for about two years. I m also a fan of other extensive applications that come bundled similar to real-time extenders for OSs like
freely distributed Unix-like OSs, like Net- with it are called distributions. Windows NT, by dividing applications
BSD and FreeBSD. Both are direct descen- Also, the kernel and a number of into processes and threads that are real-
dents of the BSD (Berkeley Software Dis- applications are available under the GNU time aware and standard user processes
tribution) operating systems. I still use copyright, or GPL. Check out Pat Villani s that go unaffected. There s more, but I ll
them for other projects. For example, my series of excellent articles on FreeDOS get into Linux s real-time extension in a
web and mail server runs FreeBSD. (INK 95 96), where he describes the GPL later column.
But, Linux has such a huge following, I and the issues concerning the embedded-
wanted to make sure you had the latest systems integrator. WHY LINUX?
information. I also discuss some differences I should note that one of the differences Naturally, this question is tugging at
between Linux and FreeBSD/NetBSD. between the Linux kernel and the NetBSD/ your brain. There are several reasons to
FreeBSD kernel is that the Linux kernel is consider Linux as an embedded operat-
INTRODUCING LINUX not GPL protected (whereas the others are) ing system for many applications.
Linux is an alternative OS for PC (and and it is subject to a different licensing For one thing, it s freely available and
non-PC) platforms. There are really two agreement. Whether GPL is right for your includes a TCP/IP stack. Also, it has
aspects to what is generally considered project depends on your requirements. multiarchitecture support for Intel, 68k,
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PowerPC, Alpha, Sparc, SMP, and others.
SCSI Host Adapters
Ethernet Controllers
So, you can obtain and begin using Linux
SCSI driver for Symbios/NCR 53c700 series and
3Com 3c501 (throw it away!)
with very little trouble.
53c800 series host adapters
3Com EtherLink II
BusLogic MultiMaster (NOT Flashpoint) SCSI
One of the great benefits of using Linux 3Com Etherlink Plus
host adapter driver
3Com EtherLink16
is the large support network that exists for
NCR53c406a-based SCSI host adapter driver
3Com EtherLink III
it, and let s not forget the fact that Linux AdvanSys SCSI host adapter driver
3Com 3c590/3c595 Vortex
Adaptec AHA-152x host adapter driver
Ansel Communications Model 3200 EISA
supports many devices and bus architec-
Adaptec AHA-154x and 631x-based host
Ethernet adapter
tures right out of the box. Of course, Linux
adapter driver
Apricot 82596
Adaptec AHA-174x host adapter driver
ARCnet for IP driver
is somewhat modular and there are many
Adaptec AHA-2740, 28xx, 29xx, 39xx, aic7xxx-
Allied Telesis AT1700
tools and programming languages avail-
based host adapter driver
DE425, DE434, DE435, DE450, and DE500
DTC 3180/3280 host adapter driver
able. With features like this, Linux starts to DEC EtherWORKS cards
All DMA-capable DPT SCSI host adapters
D-Link DE-600 Ethernet pocket adapter
make more sense, huh?
All PIO-capable DPT SCSI host adapters
D-Link DE-620 Ethernet pocket adapter
Let s talk about these points in more Future Domain TMC-16xx SCSI host adapters
DEC DEPCA and EtherWORKS DE100,
IN2000 SCSI host adapters
DE101, DE200, DE201, DE202, DE210,
detail. I already mentioned that Linux is
Symbios/NCR 53C810, 53C815, 53C820,
DE422
freely available. Obviously, that s nice
53C825 SCSI host adapters
Digi RightSwitch SE-4, SE-6
Pro Audio Spectrum/Studio 16
Cabletron E2100
because you won t pay royalties for em-
IOMEGA PPA3/Parallel ZIP
EtherExpress Pro/10
bedding Linux in your product. But, you
Qlogic FAS408 SCSI host adapter
EtherExpress
QLogic ISP1020 SCSI host adapter
need to make sure all of the modules and ICL EtherTeam 16i/32 EISA
Seagate ST-01/02, Future Domain TMC-8xx
EtherWORKS 3: DE203, DE204, DE205
components you plan to use for your
SCSI host adapter
Fujitsu FMV-181/182/183/184
project are freely available. Most are. Trantor T128/T128F/T228 SCSI host adapter
HP PCLAN/plus
UltraStor 14F/34F (not 24F) SCSI host adapter
HP LAN
You can download most Linux distribu-
UltraStor 14F/24F/34F SCSI host adapter
HP10/100VG ANY LAN: J2577, J2573, 27248B,
tions from the Internet. But, this can be
WD7000-FASST2/WD7000-ASC/WD7000-AX/
J2577, J2573, J2585
WD7000-EX SCSI host adapter
Shared-memory IBM Token Ring 16/4
tedious over a slow link, so most of us opt
AMD PCnet32, PCnetPCI
to buy a CD-ROM. Yet, being able to
CD-ROM Drivers
NE1000, NE2000, and compatible
Aztech CD268 CD-ROM driver
download a distribution over the  Net is NI5210 Ethernet
Sony CDU-31A CD-ROM driver
NI6510 Ethernet
extremely useful if you re in the field and
Philips/LMS cm20 CD-ROM driver
Parallel Link Internet Protocol
need to upgrade you embedded system GoldStar R420 CD-ROM driver
Sangoma S502/S508 series multi-protocol PC
ISP16/MAD16/Mozart soundcard-based CD-
interface card
or get a new version of a device driver.
ROM driver
SMC Ultra, SMC EtherEZ ISA
Linux has a TCP/IP stack built in. The
Mitsumi CD-ROM driver
SMC 9000 series Ethernet
Mitsumi XA/Multisession CD-ROM driver
Starmode Radio IP
TCP/IP implementation is very robust,
SoundBlaster Pro/Matsushita/Panasonic/
DEC 21040, most 21*40 Ethernet
since many Internet-service providers use
Longshine/CreativeLabs/TEAC/ECS-AT CD-
AT&T GIS (nee NCR) WaveLAN Ethernet-like
ROM
Linux for their high-throughput web serv- radio transceiver
Sanyo CD-ROM device driver
WD8003- and WD8013-compatible ether cards
ers. The programming interface for the
Sony CDU-535 CD-ROM driver
TCP/IP stack is the standard Socket API.
Linux runs on a multitude of architectures.
Table 1 Here is a list of some of the devices that are typically supported with a standard Linux
kernel. In addition to these devices, many peripheral-card vendors also offer Linux-driver
I m only going to talk about the Intel port,
modules for their cards.
but it s good to know that you re not stuck
with Intel. Some of the details in each port into the Linux kernel development effort. quality of device drivers for Linux varies,
are different, but for the most part, Linux is There is yet another side effect to this many popular cards and chip sets are
Linux, at least at the application level. large support group. Many of the fame- well supported. Also, there s hope of
Linux is ported to 68k, which includes and-glory developers are students who fixing or adapting the device driver for
VME-bus modules, Alpha, and PowerPCs. have learned OS internals by observing your purposes, since sources are avail-
And, there s a port in progress for MIPS the Linux kernel, and they ll eventually able for most device drivers.
processors. enter the job market. Drivers are available for just about all
The US Robotics Palm Pilot may be the Consequently, the number of software of the Ethernet chip sets, all standard AT
smallest system that Linux is ported for. It s engineers familiar with Linux is growing, peripherals (COM, IDE, floppy, etc.), and
a PDA based on Motorola s 68328 and they re even proficient at the kernel many VGA chip sets. Also, many CD-ROMs
Dragonball processor. There s also work level. That s good news, and perhaps one and SCSI devices are supported. Table 1
in progress to port Linux to smaller Intel compelling reason why much of Linux will shows a list of devices that are supported
processors, like the 80188. stay freely available with the GPL. in an older distribution of RedHat.
Since Linux has such a great following, There are a great number of device However, a flash file-system driver isn t
there s a large support network. Although drivers available for Linux. Because Linux supported. That s unfortunate. It would be
many people provide Linux support by has been ported to so many different nice for working with embedded systems.
writing or porting applications and drivers architectures, it supports various bus ar- Hopefully, someone is working on it.
in an effort to attain fame and glory, there chitectures, like ISA bus, PCMCIA, PCI, But, all is not lost it s still possible to
are also several companies that are pro- VME bus, S-bus, and others. boot Linux from a flash file system. Flash
viding commercial support for Linux. Of- Many vendors of PC-compatible cards disks, like SanDisk s CompactFlash de-
ten, these companies reintegrate the fixes have Linux drivers available. While the vices, work under Linux because it acts just
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CIRCUIT CELLAR INK NOVEMBER 1998
rtf_create(unsigned int fifo, int size) create a FIFO
RUNNING SUP-
rtf_destroy(unsigned int fifo) get rid of the FIFO
PORT
rtf_resize(unsigned int minor, int size) change the size of the FIFO
rt_fifo_put(unsigned int fifo, char * buf, int count) write to FIFO
Although the recommended
rt_fifo_get(unsigned int fifo, char * buf, int count) read from FIFO
minimum size for running Linux with
rtf_create_handler(unsigned int fifo, create a handler that is called when a
X-Windows is usually 8 16 MB, it s
int (*handler)(unsigned int fifo)) user process reads or writes to a
overkill for running Linux in an embedded
FIFO device
rt_task_init(RT_TASK *task, void (*fn) create a task
system. What is the minimum configura-
(int data), int data, int stack_size, int priority);
tion we need?
rt_task_make_periodic(RT_TASK arrange so that the task is
To run Linux on a PC architecture, we
*task, RTIME start_time, RTIME period); called at a periodic interrupt
need at least an i 386-class machine be-
rt_task_delete(RT_TASK *task); get rid of task
rt_task_wait(void); give up time slice, wait until next time slice
cause Linux has to run in 32-bit protected
rt_task_suspend(RT_TASK *task); suspend any task
mode. Several embedded  386 boards
rt_get_time(void); read the current time
out there should run Linux just fine.
rt_request_timer(void (*fn)(void)); allocate a timer, which will call a handler
In fact, I was even able to boot and run
when it expires
rt_free_timer(void); return timer
Linux on my 16-MHz  386SX laptop with
rt_set_timer(RTIME time); set the timer to go off at a specific time
5 MB of memory. It runs fine, although you
rt_no_timer(void); clear timer
probably don t want to compile a pro-
Table 2 The real-time extension to Linux (RT-Linux) provides this API for programs that are
gram or run an Xserver on it. But then,
written to run in this mode. Most of the services are pretty standard interfaces you d see in a
that s not the point of a minimum system.
bigger RTOS. But, the FIFO interface can be used to communicate with non-real-time threads.
The minimum memory configuration to
like a IDE drive. Next month, I ll look at
feature might not be important for build- load a standard Linux kernel and have it be
these issues in detail when I embed Linux.
ing embedded systems, but it s nice to able to do something is 4 MB. It should be
Linux supports dynamic module loading,
have access to all the applications on your possible to configure and build a minimum
which means libraries and devices driv- development system. size kernel that should work in about 2 MB.
ers can be loaded by the OS after boot
The normal compiler used for Linux is Now we have an i386 and 4 MB of
time to extend the base functionality of the
the GNU C compiler (a complete C, C++, memory, what else do we need? Well, to
kernel. Dynamic modules are nothing new,
and Objective C compiler). The companion boot Linux, we need a boot device.
but they do make configuration easy.
debugger (gdb) is flexible and can even Linux can t use flash memory as disks,
You build a simple kernel that can be
deal with remote debugging via Ethernet but it can boot from it. In fact, Linux can
configured to deal with different device
or serial port. This capability means that you load and initialize the kernel using any
configurations by changing a configuration
can debug your embedded system remotely kind of boot device the BIOS can handle.
file. This construction also improves memory
from a desktop or, in my case, a laptop. So, it s possible to boot Linux from a flash
use, since only the required device drivers
Of course, you can also run the debugger memory after all, but it can t access the
are loaded when needed. Of course, you
on the console of the embedded system. boot flash-memory disk once it s loaded.
can still configure a Linux kernel to be
In conjunction with the GNU C compiler You may ask, what good is it to boot
static by compiling in all the device drivers
and related tools, you ll find commercial from flash memory if you can t access the
it needs for a particular application.
compiler and development environments. I disk after booting? Well, there are at least
Many applications and programming
listed one compiler vendor in my sources, two techniques you can use to initialize an
languages are available for Linux. This
and you can find others in the Linux Journal. initial RAM disk while booting, and this
RAM-disk image can contain the essentials
of what s needed to run Linux.
Listing 1 You need this minimum set of files to boot Linux and start an application
What do we need to run Linux once the
program. Of course, you need to add more device-driver entries in the directory
if your application needs to access other devices besides the screen and the RAM disk.
kernel has booted? By incorporating our
system into one Linux program, we can
get away with a small set of files. Listing 1
has the details.
The most critical components are the
console device entry and
whatever device entries that are needed.
Once booted, the kernel opens and runs
whatever program or command script is
contained in . That s about all
you need for a minimal configuration.
Even though Linux wasn t designed as
an embedded OS, it ended up having
features like small initial size and capability
for RAM disks and modular device drivers.
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card-based Linux installation.
a small system. After that, you ll learn
more about Linux embedded software
INSTALLING LINUX
development and how to use RT-Linux.
Obtaining Linux is easy. As I said, you
You re also likely to see Linux in some
can get most Linux distributions from the
of my projects. It may not be the answer for
 Net for free. All you need is patience
every embedded-systems problem, but it s
while downloading it. You can also pur- another tool in the toolbox. And since it s still
chase a CD-ROM of various distributions.
being developed and is constantly evolving,
Most commonly available distributions
it will be interesting to see what problems
have enough of the basic components
people end up solving with Linux. RPC.EPC
necessary to allow you to embed Linux.
Once you have an installation, you
Ingo Cyliax has been writing for INK for
should build a desktop system to serve as
two years on topics such as embedded
your development platform. This process
systems, FPGA design, and robotics. He
is relatively painless and there are a lot of
is a research engineer at Derivation Sys-
resources on the  Net. If your target system
tems Inc., a San Diego based formal
Photo 1 The Motorola NLX55 motherboard
features a Pentium MMX CPU, graphics con- is capable enough to serve as a develop-
synthesis company, where he works on
troller, and 10-/100-Mbps Ethernet control-
ment environment, you can install it there.
formal-method design tools for high-assur-
ler. Unique to this board, it also includes a
ance systems and develops embedded-sys-
socket for a SanDisk flash-based ATA card.
RT EXTENSION OF LINUX
tem products. Before joining DSI, Ingo worked
These features are there because the devel- The standard Linux kernel doesn t pre-
for over 12 years as a system and re-
opers wanted Linux to be easy to install. tend to be real time in the traditional way.
search engineer for several universities
A single floppy can hold a complete Like many Unix kernels, the kernel is not
and as an independent consultant. You
mini-Linux system, complete with enough preemptive and thus can t be relied on for
may reach him at [e-mail].
utilities to format and initialize the file predictable interrupt response. But, some
REFERENCES
system on a hard disk and do a bootstrap clever people developed a real-time ex-
www.linux.org
install of Linux from another medium. We tender for Linux that turns it into RT-Linux.
RT-Linux project at NASA, aol11.wff.nasa.gov/rtlinux/
can use this to our advantage by including RT-Linux has a layer added between
SOURCES
our embedded applications, instead of the Linux kernel and the hardware timer
C/C++/F77 compilers for Linux
the installation utilities to build an embed- interface. After installing the RT extension, The Portland Group
[phone]
ded Linux system. the Linux kernel and its processes turn into
Fax: [phone]
Now let s look at a system I ve been a single real-time task that always runs at www.pgroup.com
using to play around with embedded Linux. the lowest priority.
NLX55 motherboard
It s based on Motorola s embedded Pentium Real-time applications are loaded into Motorola Computer Group
[phone], ext. PR
SBC, the NLX 55, shown in Photo 1. the kernel memory space using the Linux
[phone], ext. PR
The motherboard contains everything module facility. Once the application has www.mcg.mot.com
needed to build a system and run an loaded and started real-time tasks, the
OpenLinux
embedded application. It has a 233-MHz tasks have control of all the hardware and [phone]
Fax: [phone]
Pentium MMX CPU, slots for two DIMM memory in the system. RT-Linux provides
www.caldera.com
modules, 10-/100-Mbps Ethernet control- calls to start, suspend, and destroy tasks.
RT-Linux
ler, dual-IDE controller, floppy control, There s also a timer facility to set up timers
r52h146.res.gatech.edu/~bdixon/rtlinux
and SVGA controller. It also contains a with real-time responses.
Linux distributions
flash-card socket on the motherboard. A FIFO is used for real-time tasks to
RedHat
SanDisk s CompactFlash is a flash- communicate with regular user processes. www.redhat.com
based media that contains an IDE control- The FIFOs appear as standard Unix char-
S.u.S.E., Gmbh
ler on the card itself. To the system, it looks acter devices that a user process opens [phone]
Fax: [phone]
just like another IDE drive. Motorola also and then reads and writes to. Table 2 shows
www.suse.de
sells a starter system that includes a case the basic API available in the RT extender.
Linux Journal
and power supply as well as a standard Although, RT-Linux is rather primitive in
(888) 66-Linux
IDE drive for development. its implementation, it has the essentials you [phone]
Fax: [phone]
With this system, you can develop need to implement real-time applications.
www.linuxjournal.com
embedded applications while running off WHAT S NEXT
the hard disk and build a flash-card I ve given you an overview of Linux
module of our embedded system. Then, and RT-Linux as well as how you can use it
just reboot, change the boot sequence in as an embedded OS. I ll concentrate on
the BIOS setup, and have the system boot the details of Linux in upcoming articles.
©Circuit Cellar INK, the Computer Applications Journal.
from the flash card to test your applica- Next month, I ll show you how to
Reprinted by permission. For subscription information,
tion. Next month, I ll use it to build a flash embed a minimal Linux configuration into call [phone] or subscribe ©circellar.com
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CIRCUIT CELLAR INK NOVEMBER 1998