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+****************************
+GHDL implementation of VITAL
+****************************
+
+.. index:: VITAL
+
+.. index:: IEEE 1076.4
+
+.. index:: 1076.4
+
+This chapter describes how VITAL is implemented in GHDL.  Support of VITAL is
+really in a preliminary stage.  Do not expect too much of it as now.
+
+.. _vital_packages:
+
+VITAL packages
+==============
+
+The VITAL standard or IEEE 1076.4 was first published in 1995, and revised in
+2000.
+
+The version of the VITAL packages depends on the VHDL standard.  VITAL
+1995 packages are used with the VHDL 1987 standard, while VITAL 2000
+packages are used with other standards.  This choice is based on the
+requirements of VITAL: VITAL 1995 requires the models follow the VHDL
+1987 standard, while VITAL 2000 requires the models follow VHDL 1993.
+
+The VITAL 2000 packages were slightly modified so that they conform to
+the VHDL 1993 standard (a few functions are made pure and a few one
+impure).
+
+.. _vhdl_restrictions_for_vital:
+
+VHDL restrictions for VITAL
+===========================
+
+The VITAL standard (partially) implemented is the IEEE 1076.4 standard
+published in 1995.
+
+This standard defines restriction of the VHDL language usage on VITAL
+model.  A :dfn:`VITAL model` is a design unit (entity or architecture)
+decorated by the `VITAL_Level0` or `VITAL_Level1` attribute.
+These attributes are defined in the `ieee.VITAL_Timing` package.
+
+Currently, only VITAL level 0 checks are implemented.  VITAL level 1 models
+can be analyzed, but GHDL doesn't check they comply with the VITAL standard.
+
+Moreover, GHDL doesn't check (yet) that timing generics are not read inside
+a VITAL level 0 model prior the VITAL annotation.
+
+The analysis of a non-conformant VITAL model fails.  You can disable the
+checks of VITAL restrictions with the *--no-vital-checks*.  Even when
+restrictions are not checked, SDF annotation can be performed.
+
+.. _backannotation:
+
+Backannotation
+==============
+
+.. index:: SDF
+
+:dfn:`Backannotation` is the process of setting VITAL generics with timing
+information provided by an external files.
+
+The external files must be SDF (Standard Delay Format) files.  GHDL
+supports a tiny subset of SDF version 2.1, other version number can be
+used, provided no features added by the next version are used.
+
+Hierarchical instance names are not supported. However you can use a list of
+instances.  If there is no instance, the top entity will be annotated and
+the celltype must be the name of the top entity.  If there is at least one
+instance, the last instance name must be a component instantiation label, and
+the celltype must be the name of the component declaration instantiated.
+
+Instances being annotated are not required to be VITAL compliant.  However
+generics being annotated must follow rules of VITAL (e.g., type must be a
+suitable vital delay type).
+
+Currently, only timing constraints applying on a timing generic of type
+`VitalDelayType01` has been implemented.  This SDF annotator is
+just a proof of concept.  Features will be added with the following GHDL
+release.
+
+Negative constraint calculation
+===============================
+
+Negative constraint delay adjustment are necessary to handle negative
+constraint such as a negative setup time.  This step is defined in the VITAL
+standard and should occur after backannotation.
+
+GHDL does not do negative constraint calculation.  It fails to handle models
+with negative constraint.  I hope to be able to add this phase soon.