Added information on RTEMS Time/Date Representatin.

doc/FAQ/FAQ.texi

@@ -71,6 +71,7 @@ END-INFO-DIR-ENTRY
 @include embeddedinfo.texi
 @include hwdebugaids.texi
 @include projects.texi
+@include endoftime.texi
 
 @ifinfo
 @node Top, Basic Information, (dir), (dir)
@@ -93,6 +94,7 @@ Common questions have been divided into the following categories.
 * Resources for Embedded Information::
 * Hardware to Ease Debugging::
 * RTEMS Projects::
+* Date/Time Issues in Systems Using RTEMS::
 @end menu
 
 @c * Command and Variable Index::

doc/FAQ/Makefile

@@ -19,7 +19,7 @@ COMMON_FILES=../common/cpright.texi ../common/setup.texi
 
 GENERATED_FILES= \
     basic.texi tools.texi concepts.texi bsp.texi debug.texi freesw.texi \
-    embeddedinfo.texi hwdebugaids.texi projects.texi
+    embeddedinfo.texi hwdebugaids.texi projects.texi endoftime.texi
 
 FILES=$(PROJECT).texi $(GENERATED_FILES)
 
@@ -104,9 +104,14 @@ hwdebugaids.texi: hwdebugaids.t Makefile
 	    -u "Top" \
 	    -n "" ${*}.t
 
-
 projects.texi: projects.t Makefile
 	$(BMENU) -c -p "" \
 	    -u "Top" \
 	    -n "" ${*}.t
 
+
+endoftime.texi: endoftime.t Makefile
+	$(BMENU) -c -p "" \
+	    -u "Top" \
+	    -n "" ${*}.t
+

doc/FAQ/endoftime.t

@@ -0,0 +1,111 @@
+@c
+@c  COPYRIGHT (c) 1988-1999.
+@c  On-Line Applications Research Corporation (OAR).
+@c  All rights reserved.
+@c
+@c  $Id$
+@c
+
+@chapter Date/Time Issues in Systems Using RTEMS
+
+This section provides technical information regarding
+date/time representation issues and RTEMS.  The Y2K problem has
+lead numerous people to ask these questions.  The answer to
+these questions are actually more complicated than most 
+people asking the question expect.  RTEMS supports multiple
+standards and each of these standards has its own epoch and
+time representation.  These standards include both programming
+API and programming language standards.
+
+In addition to the issues inside RTEMS
+itself, there is the complicating factor that the Board
+Support Package or application itself may interface with hardware
+or software that has its own set of date/time representation
+issues.  
+
+In conclusion, viewing date/time representation as "the Y2K problem"
+is very short-sighted.  Date/time representation should be viewed as
+a systems level issue for the system you are building.  Each software
+and hardware component in the system as well as the systems being
+connected to is a factor in the equation.
+
+@section Hardware Issues
+
+Numerous Real-Time Clock (RTC) controllers provide only a two-digit
+Binary Coded Decimal (BCD) representation for the current year.  Without
+software correction, these chips are a classic example of the Y2K problem.
+When the RTC rolls the year register over from 99 to 00, the device
+has no idea whether the year is 1900 or 2000.  It is the responsibility
+of the device driver to recognize this condition and correct for it.
+The most common technique used is to assume that all years prior
+to either the existence of the board or RTEMS are past 2000.  The
+starting year (epoch) for RTEMS is 1988.  Thus,
+
+@itemize @bullet
+@item Chip year values 88-99 are interpreted as years 1988-1999.
+@item Chip year values 00-87 are interpreted as years 2000-2087.
+@end itemize
+
+Using this technique, a RTC using a
+two-digit BCD representation of the current year will overflow on
+January 1, 2088.
+
+@section RTEMS Specific Issues
+
+Internally, RTEMS uses an unsigned thirty-two bit integer to represent the
+number of seconds since midnight January 1, 1988.  This counter will
+overflow on February 5, 2124. 
+
+The time/date services in the Classic API will overflow when the 
+RTEMS internal date/time representation overflows.
+
+The POSIX API uses the type @i{time_t} to represent the number of
+seconds since January 1, 1970.  Many traditional UNIX systems as
+well as RTEMS define @i{time_t} as a signed thirty-two bit integer.
+This representation overflows on January 18, 2038.  The solution
+usually proposed is to define @i{time_t} as a sixty-four bit
+integer.  This solution is appropriate for for UNIX workstations
+as many of them already support sixty-four bit integers natively.
+At this time, this imposes a burden on embedded systems which are
+still primarily using processors with native integers of thirty-two
+bits or less.
+
+@section Language Specific Issues
+
+The Ada95 Language Reference Manual requires that the @i{Ada.Calendar}
+package support years through the year 2099.  However, just as the
+hardware is layered on top of hardware and may inherit its limits,
+the Ada tasking and run-time support is layered on top of an operating
+system.  Thus, if the operating system or underlying hardware fail
+to correctly report dates after 2099, then it is possible for the
+@i{Ada.Calendar} package to fail prior to 2099.
+
+@section Date/Time Conclusion
+
+Each embedded system could be impacted by a variety of date/time
+representation issues.  Even whether a particular date/time
+repsentation issue impacts a system is questionable.  A system
+using only the RTEMS Classic API is not impacted by the
+date/time representation issues in POSIX.  A system not using
+date/time at all is not impacted by any of these issues.  Also
+the planned end of life for a system may make these issues
+moot.
+
+The following is a timeline of the date/time representation
+issues presented in this section:
+
+@itemize @bullet
+
+@item 2000 - Two BCD Digit Real-Time Clock Rollover
+
+@item 2038 - POSIX @i{time_t} Rollover
+
+@item 2088 - Correction for Two BCD Digit Real-Time Clock Rollover
+
+@item 2099 - Ada95 @i{Ada.Calendar} Rollover
+
+@item 2124 - RTEMS Internal Seconds Counter Rollover
+
+@end itemize
+
+