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# $Id: README,v 1.2 2001-09-27 11:59:04 chris Exp $
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#
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Building RTEMS
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==============
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See the file README.configure.
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Directory Overview
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==================
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This is the top level of the RTEMS directory structure. The following
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is a description of the files and directories in this directory:
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INSTALL
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Rudimentary installation instructions. For more detailed
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information please see the Release Notes. The Postscript
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version of this manual can be found in the file
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c_or_ada/doc/relnotes.tgz.
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LICENSE
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Required legalese.
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README
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This file.
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c
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This directory contains the source code for the C
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implementation of RTEMS as well as the test suites, sample
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applications, Board Support Packages, Device Drivers, and
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support libraries.
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doc
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This directory contains the PDL for the RTEMS executive.
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Ada versus C
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============
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There are two implementations of RTEMS in this source tree --
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in Ada and in C. These two implementations are functionally
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and structurally equivalent. The C implementation follows
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the packaging conventions and hiearchical nature of the Ada
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implementation. In addition, a style has been followed which
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allows one to easily find the corresponding Ada and C
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implementations.
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File names in C and code placement was carefully designed to insure
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a close mapping to the Ada implementation. The following file name
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extensions are used:
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.adb - Ada body
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.ads - Ada specification
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.adp - Ada body requiring preprocessing
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.inc - include file for .adp files
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.c - C body (non-inlined routines)
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.inl - C body (inlined routines)
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.h - C specification
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In the executive source, XYZ.c and XYZ.inl correspond directly to a
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single XYZ.adb or XYZ.adp file. A .h file corresponds directly to
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the .ads file. There are only a handful of .inc files in the
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Ada source and these are used to insure that the desired simple
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inline textual expansion is performed. This avoids scoping and
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calling convention side-effects in carefully constructed tests
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which usually test context switch behavior.
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In addition, in Ada code and data name references are always fully
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qualified as PACKAGE.NAME. In C, this convention is followed
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by having the package name as part of the name itself and using a
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capital letter to indicate the presence of a "." level. So we have
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PACKAGE.NAME in Ada and _Package_Name in C. The leading "_" in C
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is used to avoid naming conflicts between RTEMS and user variables.
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By using these conventions, one can easily compare the C and Ada
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implementations.
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The most noticeable difference between the C and Ada83 code is
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the inability to easily obtain a "typed pointer" in Ada83.
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Using the "&" operator in C yields a pointer with a specific type.
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The 'Address attribute is the closest feature in Ada83. This
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returns a System.Address and this must be coerced via Unchecked_Conversion
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into an access type of the desired type. It is easy to view
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System.Address as similar to a "void *" in C, but this is not the case.
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A "void *" can be assigned to any other pointer type without an
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explicit conversion.
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The solution adopted to this problem was to provide two routines for
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each access type in the Ada implementation -- one to convert from
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System.Address to the access type and another to go the opposite
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direction. This results in code which accomplishes the same thing
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as the corresponding C but it is easier to get lost in the clutter
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of the apparent subprogram invocations than the "less bulky"
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C equivalent.
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A related difference is the types which are only in Ada which are used
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for pointers to arrays. These types do not exist and are not needed
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in the C implementation.
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