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README

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