Compilers, assemblers, linkers, loaders, and programming issues For embedded Systems

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Compilers, assemblers, linkers, loaders, and
programming issuesFor embedded Systems

• Chris Gregg
• January 29, 2009

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is programming for an embedded system different?

• Speed
• Size
• Correctness
• Portability
• Real-Time or not
• Other Hardware Considerations

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How does one actually program an embedded system?

• Has this changed over the years? Has there been
  significant progress?
• How does Ford program their Fuel Injection System
  computer?
• How does GE program their Microwave Ovens (and
  was this different 30 years ago?)
• How does Nikon program their digital SLR cameras?
• How does Apple/RIM/Microsoft program their
  iPhone/Blackberry/SmartPhone?
• How does a hobbyist program a Pic microprocessor?
• How does NASA program (and reprogram) the Mars
  Rover?
• How did James Gosling at Sun want his set-top
  boxes programmed?
• (How was the original 128K Macintosh programmed?)
  

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Programming Languages
Machine Code
Fixed Rom, Ram, Firmware
Assembly Code
Higher Level Languages
Compiled C, nesC, C, Ada, Forth, etc.
Interpreted (?) Perl, Python, Javascript
Markup HTML, XML
Java
All Eventually End up as Machine Code
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So youve chosen C. Lets Go!
(Not so fast.)

• What does a programmer need to know about
  programming for an embedded system?
• Shed better know about the hardware.
• Purpose
• How data flows (to include getting the program
  onto the system, I/O), how to interface with
  sensors, actuators, etc.
• Whether there is an operating system, and how it
  runs programs
• Limitations memory, speed, upgradability
  (firmware?)
• How are hardware errors handled? (think Mars
  Rover)
• Plan on debugging hardware issues

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So youve chosen C. Lets Go!
(Not so fast.)

• What does a programmer need to know about
  programming for an embedded system?
• Shed better know about the software tools
  related to programming for the specific hardware.
• Is there a compiler/linker/assembler?
  (hopefully!)
• How is the memory addressed, what bit-level
  knowledge of the hardware is necessary, how does
  one access pins, etc.?
• How will debugging be managed?
• How will testing be done?
• What licensing needs to be organized (this can be
  tricky!)
• Does the software need to be portable? (using C
  is probably going to help yousee the second
  point above).

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The Embedded Software Development Process
Barr, M. Massa, A. Oram, A. (ed.) Programming
Embedded Systems in C and C, 2nd Edition.
O'Reilly Associates, Inc., 2006 , p.55
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The Tools

• Compiler Translates human readable code into
  assembly language or opcodes for a particular
  processor (or possibly into machine-independent
  opcodes a la Java). Produces an object file.
• Assembler Translates assembly language into
  opcodes (it is really a compiler, too). Also
  produces an object file.
• Linker Organizes the object files, necessary
  libraries, and other data and produces a
  relocatable file.
• Locator Takes the relocatable file and
  information about the memory of the system and
  produces an executable.
• (By the way gcc takes care of all of these
  functions at once)

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The Tools Embedded System Specifics

• All of the tools run on the host computer, not
  the embedded computer.
• Compiler Has to know about the specific hardware
  (except in very trivial cases). Should be able
  to optimize for size.
• Assembler Produces startup code not inserted
  automatically as in general purpose computers
  (i.e., the programmer needs to compile it
  independently).
• Linker Needs the correct libraries (open source
  c libraries, such as newlib, are available).
• Locator Needs programmer input for information
  about memory.

Bottom Line There can be a lot of extra work
for the programmer, although certain systems
(e.g. Pic programming) tools can automate most of
it.
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Moving the program onto the embedded system

• Remember, the program is written (and possibly
  run in an emulator) on a host computer, but it
  still needs to get onto the embedded system.
• Methods
• Build/burn the program into the hardware
  (firmware or other flash memory)
• Bootloader a bootloader resides on the embedded
  system and facilitates loading programs onto the
  system.
• Debug Monitor The debug monitor is a more robust
  program on an embedded system that helps with
  debugging and other chores, and can include a
  bootloader as well.

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Debugging

• Debugging embedded systems can be facilitated
  with a Debug Monitor, or through a remote
  debugger on the host computer. A serial link is
  normally set up, and the debugger acts more or
  less like a general purpose debugger.
• Emulators can be used to test the system without
  utilizing the actual hardware (but this has many
  caveats, and nothing beats testing on the real
  system).
• Software Simulators allow the programmer to debug
  completely on the host system, which can be
  quicker and can allow faster code turnaround.
• When it comes down to it, an oscilloscope and a
  multimeter can be your best friend for debugging.

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Final Thoughts

• Programming for embedded systems has come a long
  way since the days of toggle switches, but it
  still involves greater programmer involvement in
  the hardware aspect of the system.
• There are many software tools built for embedded
  systems, and many embedded system hardware
  designers include debugging tools with the
  hardware.
• You had probably learn C (and a fair amount of
  assembly) to program embedded systems.

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Final Thoughts

• Is this a necessary topic for an Embedded Systems
  class?
• Yes
• Youve got to get your program onto the system
  some how, and flipping toggle switches doesnt
  put food on the table any more.
• Hardware only does what you tell it to, via
  software.
• Discussion of the tradeoffs necessary to program
  an embedded system versus a GP system is a
  necessity.

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6 Questions

• Discuss the steps needed to get a program from
  source code to executable in an embedded system,
  and the differences between similar steps on a
  general purpose system.
• What trade-offs are necessary when thinking about
  how you are going to program for an embedded
  system?
• Why is C generally considered a better language
  to program in for embedded systems than, say,
  Java or Perl?

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6 Questions

• Name three different ways you might debug the
  code for your embedded system.
• Why is it imperative that an embedded system
  programmer know the details of the hardware that
  she is programming for? Compare this to someone
  writing a Java applet for a web site.
• Talk about the role firmware plays in developing
  software for an embedded system.

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Bibliography

• Barr, M. Massa, A. Oram, A. (ed.)Programming
  Embedded Systems in C and C, 2nd Edition.
  O'Reilly Associates, Inc., 2006
• Wolfe, M. How compilers and tools differ for
  embedded systems. 2005. http//www.pgroup.com/lit
  /pgi_article_cases.pdf
• Gay, D. Levis, P. von Behren, R. Welsh, M.
  Brewer, E. Culler, D. The nesC language A
  holistic approach to networked embedded
  systemsSIGPLAN Not., ACM, 2003, 38, 1-11
• Williams, B. Ingham, M. Chung, S. Elliott,
  P.Model-based programming of intelligent
  embedded systems and robotic space
  explorersProceedings of the IEEE, 2003, 91,
  212-237