SCHEDULE 14A INFORMATION
 
                  Proxy Statement Pursuant to Section 14(a) of
                      the Securities Exchange Act of 1934
 
    Filed by the Registrant / /
    Filed by a Party other than the Registrant /X/
 
    Check the appropriate box:
    / /  Preliminary Proxy Statement
    / /  Confidential, for Use of the Commission Only (as permitted by Rule
         14a-6(e)(2))
    / /  Definitive Proxy Statement
    /X/  Definitive Additional Materials
    / /  Soliciting Material Pursuant to Section240.14a-11(c) or
         Section240.14a-12
 
                                QUICKTURN DESIGN SYSTEMS, INC.
- --------------------------------------------------------------------------------
                (Name of Registrant as Specified In Its Charter)
 
                                 MENTOR GRAPHICS CORPORATION
- --------------------------------------------------------------------------------
    (Name of Person(s) Filing Proxy Statement, if other than the Registrant)
 
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                       [MENTOR GRAPHICS CORPORATION LOGO]
 
                                                              September 15, 1998
 
Dear Quickturn Stockholder:
 
    On September 11, 1998, we commenced mailing to Quickturn stockholders
important proxy materials regarding a Special Meeting of stockholders which we
had taken the steps to call for October 29, 1998.
 
    The enclosed supplements the information discussed on pages 4 and 5 of the
proxy materials.
 
    Please review the enclosed supplement together with our earlier proxy
materials.
 
    WE URGE YOU TO VOTE YOUR GOLD PROXY EARLY TO SUPPORT MENTOR'S $12.125 ALL
CASH OFFER. BY VOTING NOW YOU WILL BE SENDING THE QUICKTURN BOARD A STRONG
MESSAGE TO STOP WASTING TIME AND MONEY ON EXPENSIVE LITIGATION AND ENTRENCHMENT
TACTICS.
 
    We appreciate your prompt consideration of this extremely important vote. We
believe our offer is good for stockholders and good for customers and employees
of both companies.
 
                                          Sincerely,
 
                                          /s/ Walden C. Rhines
 
                                          Dr. Walden C. Rhines
                                          President and
                                            Chief Executive Officer
 
If you have any questions or need assistance in completing the GOLD proxy card,
                                please contact:
 
                        [MACKENZIE PARTNERS, INC. LOGO]
 
                                156 Fifth Avenue
                            New York, New York 10010
           CALL TOLL-FREE (800) 322-2885 OR (212) 929-5500 (COLLECT)

                               SUPPLEMENT TO THE
                                PROXY STATEMENT
                                       OF
                          MENTOR GRAPHICS CORPORATION
                                     FOR A
                       SPECIAL MEETING OF STOCKHOLDERS OF
                         QUICKTURN DESIGN SYSTEMS, INC.
 
                                                              September 15, 1998
 
To the Stockholders of Quickturn Design Systems, Inc.:
 
    This Supplement to the Proxy Statement of Mentor Graphics Corporation (the
"Supplement") amends and supplements the Proxy Statement dated September 11,
1998 of Mentor Graphics Corporation (the "Proxy Statement"). Except as otherwise
set forth in this Supplement, the terms and conditions set forth in the Proxy
Statement remain applicable in all respects, and this Supplement should be read
in conjunction with the Proxy Statement. Unless the context requires otherwise,
terms not defined herein have the meaning ascribed to them in the Proxy
Statement.
 
    The information set forth on pages 4 and 5 of the Proxy Statement under the
heading "The Special Meeting--The Proposals--Repeal of Bylaws or any Amendment
thereto Adopted Subsequent to March 30, 1998 and Prior to the Effectiveness of
the Proposals" is amended and supplemented with the following information:
 
    As you know, Mentor Graphics is involved in patent, disclosure, corporate
and fiduciary duty litigation with the Company and its directors. One aspect of
the corporate litigation is the validity of the Company Board's amendment to
Section 2.3 of the Company Bylaws purportedly adopted on August 21, 1998 (the
"Bylaw Amendment") which, if valid, would have the effect of delaying beyond
October 29, 1998 the date of the Special Meeting which Mentor Graphics has
called. According to the Amended and Supplemental Counterclaims that the Company
filed with the United States District Court for the District of Delaware on
September 14, 1998, the Special Meeting "could be held only at a time and date
set by the directors of Quickturn, which under the Bylaws would have to be
between December 10, 1998 and December 20, 1998 (i.e., 90 to 100 days after
holders purportedly requested such a meeting)."
 
    While Mentor Graphics has selected October 29, 1998 as the date of the
Special Meeting based on its understanding of the Company Bylaws prior to August
21, 1998, the date of the Special Meeting will be resolved based on the rulings
of the federal and state courts. Proceedings regarding and disclosure concerning
Mentor Graphics' claim as to the invalidity of the Bylaw Amendment, the hearings
for which are scheduled to be held before the Delaware state and federal courts
in October 1998, could result in a delay of the Special Meeting until after the
dates specified by Mentor Graphics and the Company.
 
    Mentor Graphics continues to stand ready to negotiate with the Company and
urges you to vote the GOLD Proxy Card in favor of the Proposals to send a
message to the Company Board to stop needlessly delaying the consummation of the
Offer.
 
    YOUR VOTE IS IMPORTANT, NO MATTER HOW FEW OR HOW MANY SHARES YOU OWN. MENTOR
GRAPHICS URGES YOU TO MARK, SIGN AND DATE AND RETURN THE ENCLOSED GOLD PROXY
CARD TO VOTE FOR THE PROPOSALS.
 
    A VOTE FOR THE PROPOSALS WILL ENABLE YOU--AS THE OWNERS OF THE COMPANY--TO
SEND A MESSAGE TO THE COMPANY BOARD THAT YOU ARE IN FAVOR OF A SALE OF THE
COMPANY TO MENTOR GRAPHICS.
 
    If you have any questions about executing or delivering your GOLD Proxy Card
or require assistance, please contact:
 
                        [MACKENZIE PARTNERS, INC. LOGO]
 
                                156 Fifth Avenue
                            New York, New York 10010
                         (212) 929-5500 (call collect)
                                       or
                         CALL TOLL-FREE: (800) 322-2885
                              FAX: (212) 929-0308