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#  $Id: README,v 1.2 2001-09-27 11:59:04 chris Exp $

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Building RTEMS

==============

See the file README.configure.

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.