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authorDrashna Jaelre <drashna@live.com>2019-05-13 20:58:46 -0700
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***************************
GHDL implementation of VHDL
***************************

This chapter describes several implementation defined aspect of VHDL in GHDL.

.. _VHDL_standards:

VHDL standards
==============

.. index:: VHDL standards

.. index:: IEEE 1076

.. index:: IEEE 1076a

.. index:: 1076

.. index:: 1076a

.. index:: v87

.. index:: v93

.. index:: v93c

.. index:: v00

.. index:: v02

This is very unfortunate, but there are many versions of the VHDL
language, and they aren't backward compatible.

The VHDL language was first standardized in 1987 by IEEE as IEEE 1076-1987, and
is commonly referred as VHDL-87. This is certainly the most important version,
since most of the VHDL tools are still based on this standard.

Various problems of this first standard have been analyzed by experts groups
to give reasonable ways of interpreting the unclear portions of the standard.

VHDL was revised in 1993 by IEEE as IEEE 1076-1993.  This revision is still
well-known.

Unfortunately, VHDL-93 is not fully compatible with VHDL-87, i.e. some perfectly
valid VHDL-87 programs are invalid VHDL-93 programs.  Here are some of the
reasons:

* the syntax of file declaration has changed (this is the most visible source
  of incompatibility),
* new keywords were introduced (group, impure, inertial, literal,
  postponed, pure, reject, rol, ror, shared, sla, sll, sra, srl,
  unaffected, xnor),
* some dynamic behaviours have changed (the concatenation is one of them),
* rules have been added.

Shared variables were replaced by protected types in the 2000 revision of
the VHDL standard.  This modification is also known as 1076a.  Note that this
standard is not fully backward compatible with VHDL-93, since the type of a
shared variable must now be a protected type (there was no such restriction
before).

Minors corrections were added by the 2002 revision of the VHDL standard.  This
revision is not fully backward compatible with VHDL-00 since, for example,
the value of the `'instance_name` attribute has slightly changed.