|author||Joel Sherrill <joel.sherrill@OARcorp.com>||1995-05-11 17:39:37 +0000|
|committer||Joel Sherrill <joel.sherrill@OARcorp.com>||1995-05-11 17:39:37 +0000|
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+This is the top level of the RTEMS directory structure. The following
+is a description of the files and directories in this directory:
+ Rudimentary installation instructions. For more detailed
+ information please see the Release Notes. The Postscript
+ version of this manual can be found in the file
+ Required legalese.
+ This file.
+ 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.
+ 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
+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
+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
+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"
+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.