VHDL-2008: Just the New Stuff

Peter J. Ashenden and Jim Lewis

Published by Morgan Kaufmann Publishers (an imprint of Elsevier), 2008, ISBN 978-0-12-374249-0

Information about this book

Front matter, including Contents

Sample - Chapter 5


VHDL Updates

Since VHDL-2008: Just the New Stuff was published, there have been some late changes to the IEEE VHDL-2008 standard.

Unconstrained record types

The definition of an unconstrained record type on page 106 of the book is not quite right. The correct definition, now included in the standard, is that the record type has at least one composite element and all composite elements are unconstrained.

Implicit application of the ?? operator

On page 132, the book lists the places where the condition operator (??) is implicitly applied. The list omits guard expressions of block statements. The VHDL standard has been updated to include that place.

Reading out-mode signal parameters

On page 164, the book says that we can an read out-mode signal parameter. The VHDL standard is now updated to prohibit this, since allowing it introduced some subtle subtype compatibility problems. The description in the book about reading out-mode signal parameters should be disregarded. However, reading out-mode variable parameters remains legal.

'Path_name and 'instance_name attributes

Section 9.17 on pages 221-222 describes updates to the 'path_name and 'instance_name attributes. One aspect that was overlooked was provision for unlabeled loops. The VHDL standard has been updated to allow for an empty element in a pathname corresponding to an unlabeled loop. This is similar to the way in which a pathname can include an empty element corresponding to an unlabeled process statement.

You might also be interested in the following books:

The Designer's Guide to VHDL, 3rd Edition

The Student's Guide to VHDL, 2nd Edition

Other sources of VHDL-related information:

The EDA Industry Working Groups Web Server (www.eda.org)

The comp.lang.vhdl Usenet news group Frequently Asked Questions (FAQ) file


I would be pleased to hear your comments about the book. You can email comments and errata to me at vhdl-book@ashenden.com.au.

Thanks, and happy reading!