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Embedded software often runs on processors with limited computation power, thus optimizing the code becomes a necessity. In this article we will explore the following optimization techniques for C and C++ code developed for Real-time and Embedded Systems.

Adjust structure sizes to power of two
Place case labels in narrow range
Place frequent case labels first
Break big switch statements into nested switches
Minimize local variables
Declare local variables in the inner most scope
Reduce the number of parameters
Use references for parameter passing and return value for types bigger than 4 bytes
Don't define a return value if not used
Consider locality of reference for code and data
Prefer int over char and short
Define lightweight constructors
Prefer initialization over assignment
Use constructor initialization lists
Do not declare "just in case" virtual functions
In-line 1 to 3 line functions

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This article describes some of the code optimization techniques used by the GNU C Compiler, in order to give the reader a feel of what code optimization is and how it can increase the efficiency of the generated object code.

Introduction
Assembly Language Code for a C Program
Constant Folding
Common Subexpression Elimination
Dead Code Elimination
Strength Reduction using Induction Variable
Conclusion
Acknowledgments
References

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C optimisation tutorial

This document has evolved over time and contains a number of the best ways to hand-optimise your C-code. Compilers are good, but they can't do everything, and here I hope to help you squeeze the best performance out of your code. This is not intended for beginners, but for more experienced programmers.

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Origins of UNIX

In 1969, Ken Thompson from AT&Tâ€™s Bell Telephone Labs wrote the first version of the UNIX operating system, on a DEC PDP-7. Disillusioned with the inefficiency of the Multics (Multiplexed Information and Computing Service) project, Thompson decided to create a programmer-friendly operating system that limited the functions contained within the kernel and allowed greater flexibility in the design and implementation of applications. The PDP-7 was a modest system on which to build a new operating system. it had only an assembler and a loader,and it would allow only a single user login at any one time. It didn't even have a hard disk, the developers were forced to partition physical memory into an operating system segment and a RAM disk segment.

Thus, the first UNIX file system was emulated entirely in RAM!

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Security Considerations with Diameter

 This memo does not describe a stand-alone protocol, but a particular
   application for the Diameter protocol [RFC3588].  Consequently, all
   the security considerations applicable to Diameter automatically
   apply to this memo.  In particular, Section 13 of RFC 3588 applies to
   this memo.

   This Diameter SIP application allows a Diameter client to use the
   properties of HTTP Digest authentication [RFC2617] by evaluating or
   sending to the Diameter server the credentials supplied by a user.
   The discussion of HTTP Digest authentication in Section 4 of RFC 2617
   [RFC2617] is also applicable to this memo.

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Migration from RADIUS to DIAMETER

 	RADIUS offers support for HTTP Digest authentication in the RADIUS
   Extension for Digest Authentication [RFC4590].  A number of AVPs (the
   Digest-* AVPs) of this Diameter SIP application are imported from the
   RADIUS attributes namespace, thus making the migration from RADIUS to
   Diameter smooth.

   Note that the RADIUS Extension for Digest Authentication [RFC4590]
   provides a more limited scope than this Diameter SIP application.
   Specifically, the RADIUS extension for Digest Authentication merely
   provides support for HTTP Digest authentication, whereas the Diameter
   SIP application provides support for user location, profile
   downloading and update, etc.

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Diameter SIP Application AVPs

 This section defines new AVPs used in this Diameter SIP application.
   Applications compliant with this specification MUST implement these
   AVPs.

   Table 2 lists the new AVPs defined in this Diameter SIP application.
   The following abbreviations are used in the Data-Type column:

   o  DURI: DiameterURI
   o  E: Enumerated
   o  G: Grouped
   o  OS: OctetString
   o  UTF8S: UTF8String
   o  U32: Unsigned32

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Diameter SIP Application Command Codes

All the Diameter implementations conforming to this specification
   MUST implement and support the list of Diameter commands listed in
   Table 1.

   +-------------------------------------+-------+------+--------------+
   | Command Name                        | Abbr. | Code | Reference    |
   +-------------------------------------+-------+------+--------------+
   | User-Authorization-Request          |  UAR  |  283 | Section 8.1  |
   | User-Authorization-Answer           |  UAA  |  283 | Section 8.2  |
   | Server-Assignment-Request           |  SAR  |  284 | Section 8.3  |
   | Server-Assignment-Answer            |  SAA  |  284 | Section 8.4  |
   | Location-Info-Request               |  LIR  |  285 | Section 8.5  |
   | Location-Info-Answer                |  LIA  |  285 | Section 8.6  |
   | Multimedia-Auth-Request             |  MAR  |  286 | Section 8.7  |
   | Multimedia-Auth-Answer              |  MAA  |  286 | Section 8.8  |
   | Registration-Termination-Request    |  RTR  |  287 | Section 8.9  |
   | Registration-Termination-Answer     |  RTA  |  287 | Section 8.10 |
   | Push-Profile-Request                |  PPR  |  288 | Section 8.11 |
   | Push-Profile-Answer                 |  PPA  |  288 | Section 8.12 |
   +-------------------------------------+-------+------+--------------+

                      Table 1: Defined command codes