content: <PAGE>
   
                        SECURITIES AND EXCHANGE COMMISSION
                              Washington, D.C.  20549
  
                                     FORM 8-K
  
                                  CURRENT REPORT
  
      Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934
  
  
  Date of Report (Date of earliest event reported):     December 8, 1998
                                                   -------------------------------
   
                          QUICKTURN DESIGN SYSTEMS, INC.
  --------------------------------------------------------------------------------
              (Exact name of registrant as specified in its charter)
     Delaware                       0-22738                       77-0159619
  --------------------------------------------------------------------------------
  (State or other                 (Commission                   (IRS Employer
  jurisdiction of                 File Number)              Identification Number)
   incorporation)
   
                         55 W. TRIMBLE ROAD SAN JOSE, CA                  95131
  --------------------------------------------------------------------------------
                     (Address of principal executive offices)           (Zip Code)
   
  Registrant's telephone number, including area code: (408) 914-6000
                                                     ----------------------
                                        N/A
  --------------------------------------------------------------------------------
           (Former name or former address, if changed since last report)
  <PAGE>
   
  ITEM 5.   OTHER EVENTS.
            ------------- 
  
       On December 8, 1998, Quickturn Design Systems, Inc. ("Quickturn") and
  Cadence Design Systems, Inc. ("Cadence") entered into an Agreement and Plan of
  Merger (the "Merger Agreement") under which Cadence will acquire Quickturn in a
  tax-free, stock-for-stock transaction with an aggregate purchase price of
  approximately $253 million. Upon closing of the merger, each stockholder of
  Quickturn will receive shares of common stock of Cadence with a value of $14 per
  share. In addition, Quickturn has issued to Cadence an option to purchase 19.9%
  of the outstanding common stock of Quickturn for $14 per share, which will
  become exercisable under certain conditions set forth in the Stock Option
  Agreement attached as Exhibit 2.1 hereto. The Merger Agreement was approved
  unanimously by the Boards of Directors of both parties thereto.
  
       The merger will be accounted for as a pooling of interests. As a result of
  the merger, Quickturn will become a wholly-owned subsidiary of Cadence. The
  merger is subject to certain conditions described in the Merger Agreement
  attached hereto as Exhibit 2.2, including compliance with applicable regulatory
  requirements and approval by Quickturn's stockholders. It is expected to close
  in the first quarter of 1999.
  
  ITEM 7.   FINANCIAL STATEMENTS AND EXHIBITS.
            --------------------------------- 
  
       (c)  Exhibits
            --------
  
            2.1  Stock Option Agreement, dated as of December 8, 1998, between
                 Cadence Design Systems, Inc. and Quickturn Design Systems, Inc.
  
            2.2  Agreement and Plan of Merger, dated as of December 8, 1998, by
                 and among Cadence Design Systems, Inc., Quickturn Design Systems,
                 Inc., and CDSI Acquisition, Inc..
  
                                        -2-
  <PAGE>
   
                                    SIGNATURES
  
  
       Pursuant to the requirements of the Securities Exchange Act of 1934, the
  Registrant has duly caused this Report to be signed on its behalf by the
  undersigned hereunto duly authorized.
  
  
  Dated: December 16, 1998                   QUICKTURN DESIGN SYSTEMS, INC.
  
  
                                    /s/ RAYMOND K. OSTBY
                                   ---------------------------------------------
                                   Raymond K. Ostby
                                   Vice President, Finance and Administration,
                                    Chief Financial Officer and Secretary
  
                                        -3-
  <PAGE>
   
                                 INDEX TO EXHIBITS
                                 -----------------
  
  
    Exhibit
    Number            Description of Document
    ------            -----------------------
  
  
     2.1     Stock Option Agreement, dated as of December 8, 1998, between Cadence
             Design Systems, Inc. and Quickturn Design Systems, Inc.
  
     2.2     Agreement and Plan of Merger, dated as of December 8, 1998, by and
             among Cadence Design Systems, Inc., Quickturn Design Systems, Inc.,
             and CDSI Acquisition, Inc.
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text: SECURITIES AND EXCHANGE COMMISSION
                              Washington, D.C.  20549
  
                                     FORM 8-K
  
                                  CURRENT REPORT
  
      Pursuant to Section 13 or 15(d) of the Securities Exchange Act of 1934
  
  Date of Report (Date of earliest event reported):     December 8, 1998
                                                   -------------------------------
   
                          QUICKTURN DESIGN SYSTEMS, INC.
  --------------------------------------------------------------------------------
              (Exact name of registrant as specified in its charter)
     Delaware                       0-22738                       77-0159619
  --------------------------------------------------------------------------------
  (State or other                 (Commission                   (IRS Employer
  jurisdiction of                 File Number)              Identification Number)
   incorporation)
   
                         55 W. TRIMBLE ROAD SAN JOSE, CA                  95131
  --------------------------------------------------------------------------------
                     (Address of principal executive offices)           (Zip Code)
   
  Registrant's telephone number, including area code: (408) 914-6000
                                                     ----------------------
                                        N/A
  --------------------------------------------------------------------------------
           (Former name or former address, if changed since last report)
  ITEM 5.   OTHER EVENTS.
            ------------- 
  
       On December 8, 1998, Quickturn Design Systems, Inc. ("Quickturn") and
  Cadence Design Systems, Inc. ("Cadence") entered into an Agreement and Plan of
  Merger (the "Merger Agreement") under which Cadence will acquire Quickturn in a
  tax-free, stock-for-stock transaction with an aggregate purchase price of
  approximately $253 million. Upon closing of the merger, each stockholder of
  Quickturn will receive shares of common stock of Cadence with a value of $14 per
  share. In addition, Quickturn has issued to Cadence an option to purchase 19.9%
  of the outstanding common stock of Quickturn for $14 per share, which will
  become exercisable under certain conditions set forth in the Stock Option
  Agreement attached as Exhibit 2.1 hereto. The Merger Agreement was approved
  unanimously by the Boards of Directors of both parties thereto.
  
       The merger will be accounted for as a pooling of interests. As a result of
  the merger, Quickturn will become a wholly-owned subsidiary of Cadence. The
  merger is subject to certain conditions described in the Merger Agreement
  attached hereto as Exhibit 2.2, including compliance with applicable regulatory
  requirements and approval by Quickturn's stockholders. It is expected to close
  in the first quarter of 1999.
  
  ITEM 7.   FINANCIAL STATEMENTS AND EXHIBITS.
            --------------------------------- 
  
       (c)  Exhibits
            --------
  
            2.1  Stock Option Agreement, dated as of December 8, 1998, between
                 Cadence Design Systems, Inc. and Quickturn Design Systems, Inc.
  
            2.2  Agreement and Plan of Merger, dated as of December 8, 1998, by
                 and among Cadence Design Systems, Inc., Quickturn Design Systems,
                 Inc., and CDSI Acquisition, Inc..
  
                                        -2-
  SIGNATURES
  
       Pursuant to the requirements of the Securities Exchange Act of 1934, the
  Registrant has duly caused this Report to be signed on its behalf by the
  undersigned hereunto duly authorized.
  
  Dated: December 16, 1998                   QUICKTURN DESIGN SYSTEMS, INC.
  
                                    /s/ RAYMOND K. OSTBY
                                   ---------------------------------------------
                                   Raymond K. Ostby
                                   Vice President, Finance and Administration,
                                    Chief Financial Officer and Secretary
  
                                        -3-
  INDEX TO EXHIBITS
                                 -----------------
  
    Exhibit
    Number            Description of Document
    ------            -----------------------
  
     2.1     Stock Option Agreement, dated as of December 8, 1998, between Cadence
             Design Systems, Inc. and Quickturn Design Systems, Inc.
  
     2.2     Agreement and Plan of Merger, dated as of December 8, 1998, by and
             among Cadence Design Systems, Inc., Quickturn Design Systems, Inc.,
             and CDSI Acquisition, Inc.