SC TO-T/A

                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549

                               ------------------

                                  SCHEDULE TO/A
                                 (RULE 14d-100)

        TENDER OFFER STATEMENT UNDER SECTION 14(d)(1) OR SECTION 13(e)(1)
                     OF THE SECURITIES EXCHANGE ACT OF 1934

                               (AMENDMENT NO. 1)

                              -------------------

                               IKOS SYSTEMS, INC.
                       (NAME OF SUBJECT COMPANY (ISSUER))

                           MENTOR GRAPHICS CORPORATION
                               FRESNO CORPORATION
                      (NAMES OF FILING PERSONS (OFFERORS))

                    COMMON STOCK, PAR VALUE $0.01 PER SHARE,
                         (TITLE OF CLASS OF SECURITIES)

                                    451716203
                      (CUSIP NUMBER OF CLASS OF SECURITIES)

                                WALDEN C. RHINES
                      PRESIDENT AND CHIEF EXECUTIVE OFFICER
                           MENTOR GRAPHICS CORPORATION
                             8005 S.W. BOECKMAN ROAD
                         WILSONVILLE, OREGON 97070-7777
                                 (503) 685-7000
       (NAME, ADDRESS AND TELEPHONE NUMBER OF PERSON AUTHORIZED TO RECEIVE
           NOTICES AND COMMUNICATIONS ON BEHALF OF THE FILING PERSON)

                                   COPIES TO:

       JOHN J. HUBER, ESQ.                          CHRISTOPHER L. KAUFMAN, ESQ.
        LATHAM & WATKINS                                  LATHAM & WATKINS
555 11TH STREET, N.W., SUITE 1000                      135 COMMONWEALTH DRIVE
     WASHINGTON, D.C. 20004                         MENLO PARK, CALIFORNIA 94025
         (202) 637-2200                                     (650) 328-4600


Check the appropriate boxes below to designate any transactions to which the
statement relates:

     [X]       third-party tender offer subject to Rule 14d-1

     [ ]       issuer tender offer subject to Rule 13e-4

     [ ]       going private transaction subject to Rule 13e-3

     [ ]       amendment to Schedule 13D under Rule 13d-2

Check the following box if the filing is a final amendment reporting the results
of the tender offer:  [ ]

         This Amendment No. 1 to Tender Offer Statement on Schedule TO amends
and supplements the Tender Offer Statement on Schedule TO filed with the
Securities and Exchange Commission on December 7, 2001 (the "Schedule TO"),
relating to a tender offer by Fresno Corporation, a Delaware corporation
("Purchaser") and a wholly-owned subsidiary of Mentor Graphics Corporation, an
Oregon corporation ("Mentor"), to purchase all outstanding shares of common
stock, par value $0.01 per share, and the related preferred stock purchase
rights, of IKOS Systems, Inc., a Delaware corporation (the "Company"), for a
purchase price of $11.00 per share, net to the seller in cash, without interest
thereon, upon the terms and subject to the conditions set forth in the Offer to
Purchase dated December 7, 2001 (the "Offer to Purchase") and in the related
Letter of Transmittal (the "Letter of Transmittal" which, together with the
Offer to Purchase, as each may be amended and supplemented from time to time,
constitute the "Offer"). Capitalized terms used herein and not defined herein
have the respective meanings assigned such terms in the Offer to Purchase.

ITEM 5. PAST CONTACTS, TRANSACTIONS, NEGOTIATIONS AND AGREEMENTS

         Item 5 is hereby amended to add the following:

                  On December 12, 2001, Christopher Kaufman of Latham & Watkins,
         counsel to Mentor and Purchaser, received a letter and form of
         confidentiality and standstill agreement from Diane Holt Frankle of
         Gray Cary Ware & Freidenrich, LLP, counsel to the Company, in the form
         attached to this Schedule TO as Exhibit (d)(2), which is incorporated
         by reference herein.

                  On December 17, 2001, Mr. Kaufman had a telephone conversation
         with Ms. Frankle, the contents of which were memorialized in a letter
         dated December 18, 2001, sent by Mr. Kaufman to Ms. Frankle in the form
         attached to this Schedule TO as Exhibit (d)(3), which is incorporated
         by reference herein.

ITEM 12.  EXHIBITS

         Item 12 of the Schedule TO is hereby amended and supplemented as
follows:

(a)(1)(A)    Offer to Purchase dated December 7, 2001.*

(a)(1)(B)    Letter of Transmittal.*

(a)(1)(C)    Notice of Guaranteed Delivery.*

(a)(1)(D)    Letter to Brokers, Dealers, Commercial Banks, Trust Companies and
             Other Nominees.*

(a)(1)(E)    Letter to Clients for use by Brokers, Dealers, Commercial Banks,
             Trust Companies and Other Nominees.*

(a)(1)(F)    Guidelines for Certification of Taxpayer Identification Number of
             Substitute Form W-9.*

(a)(1)(G)    Press release issued by Mentor Graphics Corporation on December 7,
             2001.*

(a)(1)(H)    Summary Advertisement, published December 7, 2001.*

(a)(1)(I)    Complaint filed in the United States District Court for the
             District of Delaware on December 7, 2001.*

(a)(1)(J)    Complaint filed in the Chancery Court, New Castle County, Delaware
             on December 7, 2001.*

(b)          None.

(c)          None.

(d)(1)       Confidentiality Agreement dated June 16, 2000, between Mentor
             Graphics Corporation and IKOS Systems, Inc.*

(d)(2)       Letter from Gray Cary Ware & Freidenrich, LLP to Latham & Watkins
             dated December 12, 2001, and form of confidentiality and
             standstill agreement.

(d)(3)       Letter from Latham & Watkins to Gray Cary Ware & Freidenrich, LLP
             dated December 18, 2001.

(e)          None.

(f)          None.

(g)          None.

(h)          None.

- ------------------
*Previously filed.


                                       2

                                    SIGNATURE

         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: December 18, 2001                MENTOR GRAPHICS CORPORATION


                                        By:  /s/  Gregory K. Hinckley
                                            ------------------------------------
                                        Name:  Gregory K. Hinckley
                                        Title:  President


                                        FRESNO CORPORATION


                                        By:  /s/  Gregory K. Hinckley
                                            ------------------------------------
                                        Name:  Gregory K. Hinckley
                                        Title:  Chief Financial Officer


                                       3

                                  EXHIBIT INDEX

(a)(1)(A)    Offer to Purchase dated December 7, 2001.*

(a)(1)(B)    Letter of Transmittal.*

(a)(1)(C)    Notice of Guaranteed Delivery.*

(a)(1)(D)    Letter to Brokers, Dealers, Commercial Banks, Trust Companies and
             Other Nominees.*

(a)(1)(E)    Letter to Clients for use by Brokers, Dealers, Commercial Banks,
             Trust Companies and Other Nominees.*

(a)(1)(F)    Guidelines for Certification of Taxpayer Identification Number of
             Substitute Form W-9.*

(a)(1)(G)    Press release issued by Mentor Graphics Corporation on December 7,
             2001.*

(a)(1)(H)    Summary Advertisement, published December 7, 2001.*

(a)(1)(I)    Complaint filed in the United States District Court for the
             District of Delaware on December 7, 2001.*

(a)(1)(J)    Complaint filed in the Chancery Court, New Castle County, Delaware
             on December 7, 2001.*

(b)          None.

(c)          None.

(d)(1)       Confidentiality Agreement dated June 16, 2000, between Mentor
             Graphics Corporation and IKOS Systems, Inc.*

(d)(2)       Letter from Gray Cary Ware & Freidenrich, LLP to Latham & Watkins
             dated December 12, 2001, and form of confidentiality and
             standstill agreement.

(d)(3)       Letter from Latham & Watkins to Gray Cary Ware & Freidenrich, LLP
             dated December 18, 2001.

(e)          None.

(f)          None.

(g)          None.

(h)          None.

- ------------------
*Previously filed.

EX-99.D.2

                                                                  Exhibit (d)(2)

               [GRAY CARY TECHNOLOGY'S LEGAL EDGE(R) LETTERHEAD]


December 12, 2001
VIA COURIER                                               OUR FILE NO. 1090303-1

Mr. Christopher Kaufman
Partner
Latham & Watkins
135 Commonwealth Drive
Menlo Park, CA 94025

RE:  MENTOR GRAPHICS CORPORATION OFFER TO PURCHASE ALL OUTSTANDING SHARES OF
     IKOS SYSTEMS, INC. COMMON STOCK

Dear Mr. Kaufman:

We represent IKOS Systems, Inc. ("IKOS") and we are writing at the request of
the IKOS Board of Directors with reference to the recently commenced cash tender
offer by your client, Mentor Graphics Corporation ("Mentor") to acquire all
outstanding shares of Common Stock of IKOS at $11.00 per share (the "Offer").

As you know, IKOS is a party to an Agreement and Plan of Merger and
Reorganization by and among Synopsys, Inc., Oak Merger Corporation and IKOS
dated July 2, 2001, as amended (the "Merger Agreement"). Pursuant to Section
5.2 of the Merger Agreement, we enclose a form of nondisclosure agreement for
execution by Mentor.

We look forward to receipt of an executed copy of the enclosed nondisclosure
agreement, which has identical terms to the Confidentiality Agreement in effect
between Synopsys and IKOS.

Very truly yours,

GRAY CARY WARE & FREIDENRICH LLP


By: /s/ Diane Holt Frankle
   _____________________________

Diane Holt Frankle
dfrankle@graycary.com


Enclosure

Cc:  Ramon A. Nunez
     Joseph W. Rockom

                                                               December 12, 2001

Mentor Graphics Corporation
8005 S.W. Boeckman Road
Wilsonville, OR 97070-7777

Dear Sirs:

     In connection with your consideration of a possible transaction with IKOS
Systems, Inc. and/or its subsidiaries or affiliates (collectively, with such
subsidiaries or affiliates, the "Company"), the Company is prepared to make
available to you certain information concerning the business, financial
condition, operations, assets and liabilities of the Company. As a condition to
such information being furnished to you and your directors, officers, employees,
agents or advisors (including, without limitation, attorneys, accountants,
consultants, bankers and financial advisors)(collectively, "Representatives"),
you agree to treat any information concerning the Company (whether prepared by
the Company, its advisors or otherwise and irrespective of the form of
communication) which has been or will be furnished to you or to your
Representatives by or on behalf of the Company (herein collectively referred to
as the "Evaluation Material") in accordance with the provisions of this letter
agreement, and to take or abstain from taking certain other actions hereinafter
set forth.

     The term "Evaluation Material" shall be deemed to include all notes,
analyses, compilations, studies, interpretations or other documents prepared by
you or your Representatives which contain, reflect or are based upon, in whole
or in part, the information furnished to you or your Representatives pursuant
hereto. The term "Evaluation Material" does not include information which (i) is
or becomes generally available to the public other than as a result of a
disclosure by you or your Representatives, (ii) you can demonstrate was within
your possession prior to its being furnished to you by or on behalf of the
Company pursuant hereto, provided that the source of such information was not
known by you to be bound by a confidentiality agreement with or other
contractual, legal or fiduciary obligation of confidentiality to the Company or
any other party with respect to such information; (iii) becomes available to you
on a non-confidential basis from a source other than the Company or any of its
Representatives, provided that such source is not known by you to be bound by a
confidentiality agreement with or other contractual, legal or fiduciary
obligation of confidentiality to the Company or any other party with respect to
such information; or (iv) is independently developed by you without the use of
any Evaluation Material.

     You hereby agree that you and your Representatives shall use the Evaluation
Material solely for the purpose of evaluating a possible transaction between the
Company and you, that the Evaluation Material will be kept confidential and that
you and your Representatives will not disclose any of the Evaluation Material in
any manner whatsoever; provided, however, that (i) you may make any disclosure
of such information to which the Company gives its prior written consent and
(ii) any of such information may be disclosed to your Representatives who


                                       1

December __, 2001
Page 2


need to know such information for the sole purpose of evaluating a possible
transaction with the Company, who agree to keep such information confidential
and who are provided with a copy of this letter agreement and agree to be bound
by the terms hereof to the same extent as if they were parties hereto. In any
event, you shall be responsible for any breach of this letter agreement by any
of your Representatives and you agree, at your sole expense, to take all
reasonable measures (including but not limited to court proceedings) to restrain
your Representatives from prohibited or unauthorized disclosure or use of the
Evaluation Material.

     Contrary provisions notwithstanding, Mentor Graphics Corporation shall be
free to use any information in intangible form, including ideas, concepts,
techniques and know-how contained therein, which may be retained in the minds of
its employees who have had rightful access to Evaluation Material hereunder and
who have not intentionally memorized such information for the specific purpose
of retaining and subsequently disclosing it in violation of the restriction on
disclosure herein, subject only to the valid patents and copyrights of the
Company.

     In addition, you agree that, without the prior written consent of the
Company, you and your Representatives will not disclose to any other person the
fact that the Evaluation Material has been made available to you, that
discussions or negotiations are taking place concerning a possible transaction
involving the Company or any of the terms, conditions or other facts with
respect thereto (including the status thereof), unless in the written opinion of
your counsel such disclosure is required by law and then only with as much prior
written notice to the Company as is practical' under the circumstances. Without
limiting the generality of the foregoing, you further agree that, without the
prior written consent of the Company, you will not, directly or indirectly,
enter into any agreement, arrangement or understanding, or any discussions which
might lead to such agreement, arrangement or understanding, with any other
person regarding a possible transaction involving the Company. The term "person"
as used in this letter agreement shall be broadly interpreted to include the
media and any corporation, partnership, group, individual or other entity.

     In the event that you or any of your Representatives are requested or
required (by deposition, interrogatories, requests for information or documents
in legal proceedings, subpoena, civil investigative demand or other similar
process) to disclose any of the Evaluation Material, you shall provide the
Company with prompt written notice of any such request or requirement so that
the Company may seek a protective order or other appropriate remedy and/or waive
compliance with the provisions of this letter agreement. If, in the absence of a
protective order or other remedy or the receipt of a waiver by the Company, you
or any of your Representatives are nonetheless, in the written opinion of your
counsel, legally compelled to disclose Evaluation Material to any tribunal or
else stand liable for contempt or suffer other censure or penalty, you or your
Representative may, without liability hereunder, disclose to such tribunal only
that portion of the Evaluation Material which such counsel advises you is
legally required to be disclosed, provided that you exercise your best efforts
to preserve the confidentiality of the Evaluation Material, including, without
limitation, by cooperating with the Company to obtain an appropriate protective
order or other reliable assurance that confidential treatment will be accorded
the Evaluation Material by such tribunal.

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December __, 2001
Page 3

     If you decide that you do not wish to proceed with a transaction with the
Company, you will promptly inform the Company of that decision. In that case, or
at any time upon the request of the Company for any reason, you will promptly
deliver to the Company all documents (and all copies thereof) furnished to you
or your Representatives by or on behalf of the Company pursuant hereto and
provide written documentation that all such documents have been returned. In the
event of such a decision or request, all other Evaluation Material prepared by
you or your Representatives shall be destroyed and no copy thereof shall be
retained. Notwithstanding the return or destruction of the Evaluation Material,
you and your Representatives will continue to be bound by your obligations of
confidentiality and other obligations hereunder.

     You understand and acknowledge that neither the Company nor any of its
Representatives (including without limitation Needham & Company) make any
representation or warranty, express or implied, as to the accuracy or
completeness of the Evaluation Material. You agree that neither the Company nor
any of its Representatives (including without limitation Needham & Company)
shall have any liability to you or to any of your Representatives relating to or
resulting from the use of the Evaluation Material. Only those representations or
warranties which are made in a final definitive agreement regarding the
transactions contemplated hereby, when, as and if executed, and subject to such
limitations and restrictions as may be specified therein, will have any legal
effect.

     In consideration of the Evaluation Material being furnished to you, you
hereby agree that, for a period of 2 years from the date hereof, neither you nor
any of your affiliates will solicit to employ any of the officers or employees
of the Company so long as they are employed by the Company and the Company has
neither been acquired nor licensed and or sold substantially all of its assets
to a third party, without obtaining the prior written consent of the Company.

     In consideration of the Evaluation Material being furnished to you, you
hereby further agree that, without the prior written consent of the Board of
Directors of the Company, for a period of 2 years from the date hereof, neither
you nor any of your affiliates (as such term is defined in Rule 12b-2 of the
Securities Exchange Act of 1934, as amended), acting alone or as part of a
group, will acquire or offer or agree to acquire, directly or indirectly, by
purchase or otherwise, any voting securities (or direct or indirect rights or
options to acquire any voting securities) of the Company, or otherwise seek to
influence or control, in any manner whatsoever, the management or policies of
the Company.

     The Company reserves the right to assign all of its rights, powers and
privileges under this letter agreement (including, without limitation, the right
to enforce all of the terms of this letter agreement) to any person who enters
into the transactions contemplated by this letter agreement.

     It is understood and agreed that no failure or delay by the Company in
exercising any right, power or privilege hereunder shall operate as a waiver
thereof, nor shall any single or partial exercise thereof preclude any other or
further exercise thereof or the exercise of any other right, power or privilege
hereunder.

     It is further understood and agreed that money damages would not be a
sufficient remedy for any breach of this letter agreement by you or any of your
Representatives and that the

                                       3

December __, 2001
Page 4

Company shall be entitled to equitable relief, including injunction and specific
performance, as a remedy for any such breach. Such remedies shall not be deemed
to be the exclusive remedies for a breach by you of this letter agreement but
shall be in addition to all other remedies available at law or equity to the
Company. In the event of litigation relating to this letter agreement, if a
court of competent jurisdiction determines that you or any of your
Representatives have breached this letter agreement, then you shall be liable
and pay to the Company the reasonable legal fees incurred by the Company in
connection with such litigation, including any appeal therefrom.

     This letter agreement supercedes all prior agreements between you and the
Company with respect to Evaluation Material provided after the date hereof.

     This letter agreement shall be governed by and construed in accordance with
the laws of the State of California.

     Please confirm your agreement with the foregoing by signing and returning
one copy of this letter to the undersigned, whereupon this letter agreement
shall become a binding agreement between you and the Company.


ACCEPTED AND AGREED:


IKOS Systems, Inc.                     Mentor Graphics Corporation


By:_________________________________   By:_________________________________

                                       Title:______________________________



                                       4

EX-99.D.3

                                                                  Exhibit (d)(3)

                         [LATHAM & WATKINS LETTERHEAD]


                                December 18, 2001

VIA FACSIMILE AND U.S. MAIL

Diane Holt Frankle
Gray Cary Ware & Freidenrich
400 Hamilton Avenue
Palo Alto, California 94301

        Re: Mentor Graphics Corporation Offer to Purchase All Outstanding Shares
            of IKOS Systems, Inc. Common Stock

Dear Diane:

        I appreciated our opportunity to speak Monday morning concerning the
proposed confidentiality and standstill agreement between IKOS Systems, Inc. and
Mentor Graphics Corporation.

        As we discussed, the proposed form of confidentiality and standstill
agreement mirrors the agreements IKOS employed during its blind bidding
process earlier this year. At that time, it appears that IKOS sought to
provide a level playing field for the companies participating. However, the
proposed form of confidentiality and standstill agreement cannot provide
a level playing field following the execution by IKOS of the merger agreement
with Synopsys.

        Nevertheless, you have indicated that IKOS cannot, under the merger
agreement with Synopsys, even speak with Mentor Graphics (much less provide any
confidential information) absent execution of a confidentiality and standstill
agreement in the form proposed. While Mentor Graphics does not believe that this
provision in the merger agreement is enforceable, Mentor Graphics believes that
its $11.00 per share all cash proposal is clearly superior for IKOS'
stockholders than the Synopsys agreement and wants to be able to discuss with
IKOS an acquisition of IKOS.

        For this reason, Mentor Graphics suggests that it execute the proposed
form of confidentiality and standstill agreement subject to a waiver letter
which provides that certain provisions of the confidentiality and standstill
agreement will be waived and not enforced against any of Mentor Graphics,
Synopsys and any third party accepting the terms of the confidentiality and
standstill agreement and waiver letter. The provisions that would be waived and
not enforced are ones included for the benefit of IKOS during its blind bidding
process and not for the benefit of any other bidder. In the present situation,
where IKOS has a merger agreement with




LATHAM & WATKINS

Diane Holt Frankle
December 18, 2001
Page 2


Synopsys, these provisions prevent, rather than foster, the level playing field
that will produce the best result for IKOS' stockholders. Moreover, these
provisions prevent the IKOS directors from fulfilling their fiduciary duty to
secure the maximum price available in connection with the sale of the company
in light of the Mentor Graphics offer. The waiver letter would provide that:

        -       IKOS would waive and not enforce the standstill provisions, as
                to any party, with respect to any tender or exchange offer to
                acquire all outstanding IKOS shares at $11.00 per share or
                higher, including shares acquired in a second-step merger;

        -       IKOS would waive and not enforce the standstill provisions, as
                to any party, with respect to influencing IKOS' management,
                calling and taking actions with respect to a special
                stockholders' meeting, advancing stockholder proposals,
                nominating individuals as IKOS directors, soliciting proxies on
                any matter and the handling of similar matters;

        -       IKOS would waive and not enforce the confidentiality and
                standstill agreement provisions, as to any party, with respect
                to contacting third parties;

        -       All standstill provisions (but not the confidentiality
                provisions) would be waived and not enforced following January
                8, 2002;

        -       IKOS would waive and not enforce the confidentiality provisions,
                as to any party, with respect to disclosing confidential
                information if required to do so by law;

        -       In light of the fact that Mentor Graphics is already in the
                emulation market, reasonable limitations on enforcement of the
                non-solicitation of IKOS employees acceptable to both IKOS and
                Mentor Graphics would be included; and

        -       A "most favored nations" clause, applicable to any party, would
                be included with respect to the provisions of the
                confidentiality and standstill agreement, as modified by the
                waiver letter, and any bidding procedures.

        Mentor Graphics' proposals seek to create as level a playing field as
possible within the confines of IKOS' views on its merger agreement with
Synopsys. I look forward to



LATHAM & WATKINS

Diane Holt Frankle
December 18, 2001
Page 3


hearing from you so that the parties can seek to reach a negotiated acquisition
agreement that would be in the best interests of IKOS' stockholders and Mentor
Graphics can complete its tender offer by January 8, 2002.

                                    Very truly yours,

                                    /s/ Christopher L. Kaufman

                                    Christopher L. Kaufman
                                    of LATHAM & WATKINS