SC 14D1/A

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                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549
 
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                                AMENDMENT NO. 12
                               TO SCHEDULE 14D-1
 
                             TENDER OFFER STATEMENT
     (PURSUANT TO SECTION 14(D)(1) OF THE SECURITIES EXCHANGE ACT OF 1934)
 
                         QUICKTURN DESIGN SYSTEMS, INC.
 
                           (Name of Subject Company)
 
                          MENTOR GRAPHICS CORPORATION
                                   MGZ CORP.
 
                                   (Bidders)
 
                    COMMON STOCK, PAR VALUE $.001 PER SHARE
 
                       (including the Associated Rights)
 
                         (Title of Class of Securities)
 
                                   74838E102
 
                     (CUSIP Number of Class of Securities)
 
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                                WALDEN C. RHINES
                     PRESIDENT AND CHIEF EXECUTIVE OFFICER
                          MENTOR GRAPHICS CORPORATION
                            8005 S.W. BOECKMAN ROAD
                         WILSONVILLE, OREGON 97070-7777
                                 (503) 685-1200
 
           (Name, Address and Telephone Number of Persons Authorized
          to Receive Notices and Communications on Behalf of Bidders)
 
                                    COPY TO:
 
         JOHN J. HUBER, ESQ.                   CHRISTOPHER L. KAUFMAN, ESQ.
           LATHAM & WATKINS                          LATHAM & WATKINS
    1001 PENNSYLVANIA AVENUE, N.W.                    75 WILLOW ROAD
         WASHINGTON, DC 20004                  MENLO PARK, CALIFORNIA 94025
            (202) 637-2200                            (650) 328-4600
 
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    MGZ Corp., a Delaware corporation ("Purchaser"), and Mentor Graphics
Corporation, an Oregon corporation ("Parent"), hereby amend and supplement their
Tender Offer Statement on Schedule 14D-1 filed on August 12, 1998 (the
"Statement"), as amended, with respect to the offer by Purchaser to purchase all
outstanding shares of Common Stock, par value $.001 per share, of Quickturn
Design Systems, Inc., a Delaware corporation, for a purchase price of $12.125
per share, net to the seller in cash, without interest thereon, as set forth in
this Amendment No. 12. Capitalized terms used herein and not defined have the
meanings ascribed to them in the Statement.
 
ITEM 10. ADDITIONAL INFORMATION.
 
    Item 10(f) of the Statement is hereby amended and supplemented by the
following:
 
    1. On October 2, 1998, Parent issued a press release, a copy of which is
attached hereto as Exhibit (a)(23) and is incorporated herein by reference.
 
ITEM 11. MATERIAL TO BE FILED AS EXHIBITS.
 
    (a)(23) Press Release dated October 2, 1998.
 
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                                   SIGNATURES
 
    After due inquiry and to the best of my knowledge and belief, I certify that
the information set forth in this statement is true, complete and correct.
 
                               
Dated: October 2, 1998          MENTOR GRAPHICS CORPORATION
 
                                By:  /s/ Gregory K. Hinckley
 
                                Name: Gregory K. Hinckley
 
                                Title: Executive Vice President, Chief Operating
                                     Officer and Chief Financial Officer
 
                                MGZ CORP.
 
                                By:  /s/ Gregory K. Hinckley
 
                                Name: Gregory K. Hinckley
 
                                Title: Secretary and Chief Financial Officer


 
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EX-99.(A)(23)

            MENTOR GRAPHICS SAYS QUICKTURN BOARD'S CONTINUED DELAYS
                    ARE COSTING QUICKTURN STOCKHOLDERS MONEY
 
WILSONVILLE, OR, OCTOBER 2, 1998 -- Mentor Graphics Corporation (Nasdaq: MENT)
today said that the attempt by the Board of Directors of Quickturn Design
Systems, Inc. (Nasdaq: QKTN) to set the date of the Quickturn Special Meeting
for January 8, 1999, at the earliest, unreasonably delays the ability of
Quickturn's stockholders to receive $12.125 per share in cash for their shares.
 
Mentor stated its belief that Quickturn's delaying tactics--which include an
amended bylaw under which Quickturn is attempting to set the Special Meeting for
at least three months from now, as well as an amended poison pill that would
delay the Mentor offer for at least six more months beyond that--are invalid,
and are costing Quickturn stockholders money every day the Quickturn Board
prevents Mentor's all-cash offer from being considered and accepted. Mentor is
challenging Quickturn's bylaw amendment and poison pill amendment in Delaware
state court.
 
Mentor Graphics has previously called a Special Meeting of Quickturn
stockholders for October 29, 1998, with a record date of September 10, 1998,
after delivering, on September 11, 1998, Agent Designations representing 17.26%
of Quickturn's outstanding shares. Citing a bylaw that Quickturn purported to
adopt after Mentor's offer was launched, Quickturn conditionally set January 8,
1999 as the date for the Special Meeting.
 
Dr. Walden C. Rhines, President and Chief Executive Officer of Mentor Graphics,
said: "We believe that Quickturn's attempt to delay the Special Meeting should
be held invalid, and we are challenging it in Delaware state court. If the bylaw
is upheld, it would force Quickturn stockholders to wait until at least January
next year just to take the actions that would allow them to replace Quickturn's
Board with directors whom we expect, subject to their fiduciary duties, would
support Mentor's all-cash, premium offer. Ultimately, the delays caused by
Quickturn's stonewalling tactics would, if valid, prevent Quickturn stockholders
from receiving the benefits of our offer until July 1999 at the earliest--nearly
one year after we commenced our offer in August 1998.
 
"Notwithstanding the current volatility of the financial markets, we are still
prepared to consummate our fully financed offer as soon as possible. Given this
volatility and the time value of the money that is being lost by Quickturn
stockholders every day our offer is delayed, we urge stockholders to send the
Quickturn Board a message by tendering their shares into our offer by October 6,
1998," Dr. Rhines said.
 
Mentor Graphics' Offer to Purchase and ancillary documents are available on a
Mentor Graphics World Wide Web site at http://www.mentorg.com/file.
 
The Dealer Manager for the Offer is Salomon Smith Barney. The Information Agent
for the Offer is MacKenzie Partners, Inc., which can be reached toll-free at
800-322-2885 or by collect call at 212-929-5500.
 
                                
CONTACT:     Anne M. Wagner           Roy Winnick/Todd Fogarty
             Vice President,          Kekst and Company
             Marketing                212/521-4800
             503/685-1462