ISO/IEC TR 18037:2008 プログラミング言語— C —組み込みプロセッサをサポートするための拡張機能 | ページ 2

Foreword

ISO (the International Organization for Standardization) and IEC (the International Electrotechnical Commission) form the specialized system for worldwide standardization. National bodies that are members of ISO or IEC participate in the development of International Standards through technical committees established by the respective organization to deal with particular fields of technical activity. ISO and IEC technical committees collaborate in fields of mutual interest. Other international organizations, governmental and non-governmental, in liaison with ISO and IEC, also take part in the work. In the field of information technology, ISO and IEC have established a joint technical committee, ISO/IEC JTC 1.

International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, 2.

The main task of the joint technical committee is to prepare International Standards. Draft International Standards adopted by the joint technical committee are circulated to national bodies for voting. Publication as an International Standard requires approval by at least 75 % of the national bodies casting a vote.

In exceptional circumstances, the joint technical committee may propose the publication of a Technical Report of one of the following types:

• type 1, when the required support cannot be obtained for the publication of an International Standard, despite repeated efforts;
• type 2, when the subject is still under technical development or where for any other reason there is the future but not immediate possibility of an agreement on an International Standard;
• type 3, when the joint technical committee has collected data of a different kind from that which is normally published as an International Standard (“state of the art”, for example).

Technical Reports of types 1 and 2 are subject to review within three years of publication, to decide whether they can be transformed into International Standards. Technical Reports of type 3 do not necessarily have to be reviewed until the data they provide are considered to be no longer valid or useful.

Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. ISO and IEC shall not be held responsible for identifying any or all such patent rights.

ISO/IEC TR 18037, which is a Technical Report of type 2, was prepared by Joint Technical Committee ISO/IEC JTC 1, Information technology, Subcommittee SC 22, Programming languages, their environments, and system software interfaces.

This second edition cancels and replaces the first edition (ISO/IEC TR 18037:2004), of which it constitutes a minor revision. It includes a number of corrections and updates, based on implementation experiences.

Introduction

In the fast growing market of embedded systems there is an increasing need to write application programs in a high-level language such as C. Basically there are two reasons for this trend: programs for embedded systems become more complex (and hence are difficult to maintain in assembly language), and processor models for embedded systems have a decreasing lifespan (which implies more frequent re-adapting of applications to new instruction sets). The code re-usability achieved by C-level programming is considered to be a major step forward in addressing these issues.

Various technical areas have been identified where functionality offered by processors (such as DSPs) that are used in embedded systems cannot easily be exploited by applications written in C. Examples are fixed-point operations, usage of different memory spaces, low level I/O operations and others. The current proposal addresses only a few of these technical areas.

Embedded processors are often used to analyze analogue signals and process these signals by applying filtering algorithms to the data received. Typical applications can be found in all wireless devices. The common data type used in filtering algorithms is the fixed-point data type, and in order to achieve the necessary speed, embedded processors are often equipped with special hardware for fixed-point data. The C language (as defined in ISO/IEC 9899:1999) does not provide support for fixed-point arithmetic operations, currently leaving programmers with no option but to handcraft most of their algorithms in assembly language. This Technical Report specifies a fixed-point data type for C, definable in a range of precision and saturation options. Optimizing C compilers can generate highly efficient code for fixed-point data as easily as for integer and floating-point data.

Many embedded processors have multiple distinct banks of memory and require that data be grouped in different banks to achieve maximum performance. Ensuring the simultaneous flow of data and coefficient data to the multiplier/accumulator of processors designed for FIR filtering, for example, is critical to their operation. In order to allow the programmer to declare the memory space from which a specific data object must be fetched, this Technical Report specifies basic support for multiple address spaces. As a result, optimizing compilers can utilize the ability of processors that support multiple address spaces, for instance, to read data from two separate memories in a single cycle to maximize execution speed.

As the C language has matured over the years, various extensions for accessing basic I/O hardware (iohw) registers have been added to address deficiencies in the language. Today almost all C compilers for freestanding environments and embedded systems support some method of direct access to iohw registers from the C source level. However, these extensions have not been consistent across dialects.

This Technical Report provides an approach to codifying common practice and providing a single uniform syntax for basic iohw register addressing.

The functional differences between the first edition of ISO/IEC TR 18037 and this version are detailed in Annex G. Due to the different fonts used and the detailed lay-out, in a fully differences-marqued-up-document these differences are blurred. If however such a version is necessary for a proper review, please visit http://standards.iso.org/iso .

1 Scope

This Technical Report specifies a series of extensions of the programming language C, which is specified by ISO/IEC 9899:1999. These extensions support embedded processors.

Each clause in this Technical Report deals with a specific topic. The first subclauses of clauses 4, 5 and 6 contain a technical description of the features of the topic. These subclauses provide an overview but do not contain all the fine details. The last subclause of each clause contains the editorial changes to the standard necessary to fully specify the topic in the standard, and thereby provides a complete definition. Additional explanation and rationale are provided in the Annexes.

2 Normative references

The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.