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)
 
Payment of Filing Fee (Check the appropriate box):
 
/X/  No fee required.
/ /  Fee computed on table below per Exchange Act Rules 14a-6(i)(1)
     and 0-11.
     (1) Title of each class of securities to which transaction applies:
         -----------------------------------------------------------------------
     (2) Aggregate number of securities to which transaction applies:
         -----------------------------------------------------------------------
     (3) Per unit price or other underlying value of transaction computed
         pursuant to Exchange Act Rule 0-11 (set forth the amount on which the
         filing fee is calculated and state how it was determined):
         -----------------------------------------------------------------------
     (4) Proposed maximum aggregate value of transaction:
         -----------------------------------------------------------------------
     (5) Total fee paid:
         -----------------------------------------------------------------------
/ /  Fee paid previously with preliminary materials.
/ /  Check box if any part of the fee is offset as provided by Exchange Act Rule
     0-11(a)(2) and identify the filing for which the offsetting fee was paid
     previously. Identify the previous filing by registration statement number,
     or the Form or Schedule and the date of its filing.
     (1) Amount Previously Paid:
         -----------------------------------------------------------------------
     (2) Form, Schedule or Registration Statement No.:
         -----------------------------------------------------------------------
     (3) Filing Party:
         -----------------------------------------------------------------------
     (4) Date Filed:
         -----------------------------------------------------------------------

             MENTOR GRAPHICS SAYS DELAWARE COURT OF CHANCERY DENIES
                     QUICKTURN MOTION FOR SUMMARY JUDGMENT
 
             --DELAWARE TRIAL ON VALIDITY OF QUICKTURN POISON PILL AND
                      BY-LAW AMENDMENTS TO BEGIN OCT. 19--
 
        --Separately, Special Master in Oregon Patent Litigation Denies
                 All of Quickturn's Summary Judgment Motions--
 
WILSONVILLE, OR, OCTOBER 12, 1998--Mentor Graphics Corporation (Nasdaq: MENT)
today announced that the Court of Chancery of the State of Delaware on Friday,
October 9 denied a motion for summary judgment by Quickturn Design Systems, Inc.
(Nasdaq: QKTN) as to the validity of amendments to Quickturn's by-laws and
poison pill, which Quickturn had adopted after Mentor Graphics commenced its
$12.125 per share, all-cash offer on August 12, 1998.
 
    The Delaware trial with respect to the validity of Quickturn's amended
by-laws and poison pill--which together would delay the ability of Quickturn
stockholders to accept Mentor Graphics' premium offer until a minimum of 11
months from the launch of Mentor Graphics' offer--has been scheduled to commence
on October 19, 1998.
 
    Separately, Mentor Graphics announced that the Special Master in the patent
litigation with Quickturn in federal court in Oregon recommended to the court on
Friday, October 9 that summary judgment be granted in favor of Mentor Graphics
respecting one of Quickturn's patents and that all other motions for summary
judgment by the parties be denied. Trial in the case is presently scheduled for
December 1998.
 
    Dr. Walden C. Rhines, President and Chief Executive Officer of Mentor
Graphics, said: "We are very pleased with the Delaware Chancery Court's denial
of Quickturn's motion for summary judgment, and now look forward to trying the
case on October 19. This opinion represents an important step in ultimately
ensuring Quickturn stockholders are able to decide whether to accept Mentor
Graphics' fully financed, all-cash, premium offer.
 
    "We also are gratified by the recommendation of the Special Master in our
Oregon patent dispute with Quickturn that summary judgment of non-infringement
as to one Quickturn patent be granted in Mentor Graphics' favor. In addition, we
are pleased that the Special Master recommended against granting any of
Quickturn's motions for summary judgment on the other patents involved. We will
be prepared to proceed to trial in December," concluded Dr. Rhines.
 
    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.
 
Contacts:    Anne M. Wagner               Roy Winnick/Todd Fogarty
             Vice President, Marketing    Kekst and Company
             503/685-1462                 212/521-4800
 
                                     # # #