Document:

<PAGE>   1
                                                                Exhibit 10.43(a)

                        AMENDMENT TO OPTION AGREEMENT 2

This Amendment to Option Agreement 2 between Altera Corporation and Taiwan
Semiconductor Manufacturing Co., Ltd., dated October 1, 1995 (the "Option
Agreement 2"), is effective as of December 1, 1999 (the "Effective Date") by
and between Altera Corporation, a company organized under the laws of
California, USA, with its current registered address at 101 Innovation Drive,
San Jose, CA  95134, USA ("Customer"), and Taiwan Semiconductor Manufacturing
Co., Ltd., a company organized under the laws of the R.O.C., with its
registered address at No. 121, Park Ave. 3, Science-Based Industrial Park,
Hsin-Chu, Taiwan, R.O.C. ("TSMC").

RECITALS

WHEREAS, Customer desires to change the registered address and amend the
Capacity Factor Table in the Option Agreement 2; and

WHEREAS, TSMC desires to supply wafers in accordance to the Option Agreement 2
and the amended Capacity Factor Table.

AGREEMENT

In consideration of mutual covenants and conditions, both parties agree that
all terms and conditions set forth in the Option Agreement 2 remain unchanged
except the following:

1.   Change the Customer Address from "2610 Orchard Parkway, San Jose, CA
     95134-2020, USA" to "101 Innovation Drive, San Jose, CA 95134, USA" in the
     introduction Paragraph and in Section 10, the Notice Section, of the Option
     Agreement 2.

2.   Replacement of Exhibit A, Capacity Factor Table, with the Exhibit A
     attached hereto.

IN WITNESS WHEREOF, the parties have executed this Amendment to Option
Agreement 2 effective as of the Effective Date:

Taiwan Semiconductor                 Altera Corporation
Manufacturing Co., Ltd.

/s/   RON NORRIS                     BY: /s/   DENIS BERLEN
   ------------------                   --------------------------
Ron Norris                           NAME:  Denis Berlen
Sr. Vice President                        ------------------------
Worldwide Sales and Marketing        TITLE: Chief Operating Officer
                                          ------------------------

                                  Page 1 of 2
<PAGE>   2
                                   EXHIBIT A

                                      [*]

[*]  CERTAIN INFORMATION IN THIS EXHIBIT HAS BEEN OMITTED AND FILED SEPARATELY
     WITH THE COMMISSION. CONFIDENTIAL TREATMENT HAS BEEN REQUESTED WITH RESPECT
     TO THE OMITTED PORTIONS.<PAGE>   1
                                                                     EXHIBIT 4.3

RULES OF THE

ALLERGAN PHARMACEUTICALS (IRELAND) LTD., INC.

SAVINGS RELATED SHARE OPTION SCHEME (2000)

Draft: April 2000

<PAGE>   2

1. DEFINITIONS

    1.1  In these Rules the following words and expressions shall have the
         following meanings:

         "Act" the Taxes Consolidation Act, 1997, as amended

         "Adoption Date" the date on which the Scheme is adopted by the Board.

         "Announcement Date" the date on which the annual or half-yearly results
         of the Parent Company are announced.

         "Appropriate Period" has the meaning given to it in paragraph 16(2) of
         Schedule 12A, to the Act.

         "Associated Company" has the meaning assigned to it by paragraph 1(1)
         of Schedule 12A to the Act.

         "Auditors" the auditors for the time being of the Parent Company or in
         the event of there being Joint Auditors such one of them as the Parent
         Company shall select.

         "Board" the Board of Directors of the Company or, as provided in Rule
         10.4, a duly constituted committee thereof.

         "Bonus Date" has the meaning assigned to it by paragraph 18 of Schedule
         12A, to the Act.

         "Company" ALLERGAN PHARMACEUTICALS (IRELAND) LTD., INC.

         "Control" has the meaning assigned to it in Section 432 to the Act.

         "Date of Grant" the date on which the Board grants an Option in
         accordance with Rule 4.

         "Eligible Employee" means every person who:

            i. (a) is an employee of any Participating Company, including a
                   full-time director

               (b) is chargeable to tax in the Republic of Ireland under
                   Schedule E in respect of that employment, and

               (c) who had on the date of the relevant issue of the relevant
                   invitations pursuant to Rule 2.1 been such an employee or
                   director continuously for a period of 6 months or,

                                      2

<PAGE>   3

            ii. is any other employee or director of a Participating Company,
                nominated by the Board to be an Eligible Employee,

            and provided, that no person shall be an Eligible Employee if that
            person is ineligible to participate in the Scheme by virtue of
            paragraph 8 , Schedule 12A to the Act.

         "Exchange Rate" on any date on which it falls to be determined, the
         average US Dollar for Irish Pound spot rate quoted by the Company's
         bankers (being an Irish clearing bank situated in Dublin) at 11.00 a.m.
         on such date.

         "Market Value" has the meaning assigned to it by Section 548 to the
         Act.

         "Maximum Bonus" means the bonus available in connection with a seven
         year savings period.

         "Option" a right to acquire Shares granted (or to be granted) in
         accordance with the Rules of the Scheme.

         "Option Price" the price at which each Share subject to an Option may
         be acquired on the exercise of that Option being subject to Rule 8, the
         higher of;

            i.  the nominal value of a share, or

            ii. the Market Value of a Share on the day that the invitation to
                apply for that Option was issued pursuant to Rule 2

            In determining the Option Price in Irish Pounds, the price in US
            Dollar shall be converted at the Exchange Rate on the day the
            invitation to apply for Options was issued.

         "Parent Company" ALLERGAN, INC.

         "Participating Company" means the Company and any other company of
         which the Company has control and which has been nominated by the Board
         as a Participating Company.

         "Savings Contract" a contract under a certified contractual savings
         scheme, within the meaning of Schedule 12B, to the Act.

                                       3

<PAGE>   4

         "The Scheme" means the Allergan Pharmaceuticals (Ireland) Ltd., Inc.
         Savings Related Share Option Scheme constituted and governed by these
         rules as from time to time amended.

         "Shares" fully paid up shares of common stock of the Parent Company
         which comply with the provisions of paragraphs 10 to 15, of Schedule
         12A, to the Act.

         "Standard Bonus" means the bonus available in connection with a five
         year savings period.

         "Subsisting Option" an Option which has neither lapsed nor been
         exercised.

    1.2  Words importing the singular shall include the plural, and vice versa,
         and words importing the masculine shall include the feminine.

    1.3  Any reference to any statute (or a particular Part, Chapter or Section
         thereof) shall mean and include any statutory modification or
         re-enactment thereof for the time being in force, and any regulations
         made thereunder.

2.  INVITATIONS TO APPLY FOR OPTIONS

    2.1  The Board may invite every Eligible Employee to apply for the grant of
         an Option to acquire shares in the Parent Company on any one occasion
         within each of the following periods:

         (i)   a period of eight weeks following the approval of the Scheme by
               the Revenue Commissioners

         (ii)  a period of six weeks following the Announcement Day provided
               that no invitation may be made after the tenth anniversary of the
               Adoption Date

    2.2  Each invitation shall specify

         (i)   the date, not being less than 14 days after the issue of the
               invitation, by which the application may be made

         (ii)  the Option Price at which the Shares may be acquired on the
               exercise of any Option granted in response to the application,
               and

         (iii) the maximum permitted aggregate monthly savings contribution,
               being the lesser of the maximum specified in paragraph 25 of
               Schedule 12A to the Act, and such sum (being a multiple of Pound
               Sterling1 and not less than Pound Sterling10) as the Board
               decides shall apply to every Eligible Employee in respect of that
               invitation.

    2.3  Each invitation shall be accompanied by a proposal form for a Savings
         Contract, and an application form, which will provide for the applicant
         to state

                                      4

<PAGE>   5

         (i)   the weekly or bi-weekly savings contribution (being a multiple of
               Pound Sterling1 and not less than Pound Sterling10) which he
               wishes to make under the related Savings Contract

         (ii)  that his proposed weekly or bi-weekly savings contribution, when
               added to any savings contributions then being made under any
               other Savings Contract linked to an Option granted under the
               Scheme or any other savings related share Option scheme approved
               by the Revenue Commissioners, will not exceed the maximum
               permitted aggregate monthly savings contributions specified in
               the invitation

         (iii) whether, for the purposes of determining the number of Shares
               over which an Option is to be granted, the repayment under the
               Savings Contracts to be taken as including the maximum bonus, the
               standard bonus or no bonus,

         and to authorise the Board to enter on the Savings Contract proposal
         form such monthly savings contribution, not exceeding the maximum
         stated on the application form as shall be determined subject to rule 3
         below.

    2.4  Each application shall be deemed to be for an Option over the largest
         whole number of Shares which can be bought at the Option Price with the
         expected repayment under the related Savings Contract at the
         appropriate Bonus Date.

3.  SCALING DOWN

    3.1  If the Board receives valid applications for Options over an aggregate
         number of Shares which exceeds the limit determined pursuant to Rule
         5.1 below in respect of that invitation, then the following steps shall
         be carried out successively to the extent necessary to eliminate the
         excess

         (i)   each election for the maximum bonus to be included in the
               repayment under the Savings Contract shall be deemed to be an
               election for only the standard bonus to be so included.

         (ii)  each election for the standard bonus to be included in the
               repayment under the Savings Contract shall be deemed to be an
               election for no bonus to be so included.

         (iii) the excess over Pound Sterling10 of the monthly savings
               contribution chosen by each applicant shall be reduced pro rata
               to the extent necessary

         Each application shall be deemed to have been modified or withdrawn in
         accordance with the application of the foregoing provisions and the
         Board shall complete each Savings Contract proposal form to reflect any
         reduction in the monthly savings contribution resulting therefrom.

4.  GRANT OF OPTIONS

    4.1  No later than the thirtieth day or if Rule 3 applies forty second day
         following the day on which the invitations were issued pursuant to Rule
         2, the Board shall

                                      5

<PAGE>   6

         grant to each applicant who is still an Eligible Employee and is not
         precluded from participation in the Scheme by virtue of paragraph 8 of
         Schedule 12A to the Act, an Option over the number of Shares for which,
         pursuant to Rule 2.4 and subject to Rule 3, he is deemed to have
         applied.

         As soon as possible after Options have been granted the Board shall
         issue an Option certificate in respect of each Option in such form, not
         inconsistent with these Rules, as the Board may determine.

    4.2  No Option may be transferred, assigned or charged, and any purported
         transfer, assignment or charge shall cause the Option to lapse
         forthwith. Each Option certificate shall carry a statement to this
         effect.

5.  LIMITATIONS ON GRANTS

    5.1  The Board may, before issuing invitations on any occasion, determine a
         limit on the number of Shares, which are to be available in respect of
         that invitation in order to ensure that Shares remain available for
         subsequent invitations.

    5.2  No Option shall be granted to an Eligible Employee if the monthly
         savings contribution under the related Savings Contract, when added to
         the monthly savings contributions then being made under any other
         Savings Contract, would exceed the maximum specified in paragraph 25 of
         Schedule 12A to the Act.

6.  EXERCISE OF OPTIONS

    6.1  Subject to Rule 9 any Subsisting Option may be exercised in whole or in
         part at any time following the earliest of the following events

         (i)   the relevant Bonus Date if, on the day of exercise, the Option
               holder is an employee or director of a Participating Company

         (ii)  the death of the Option holder

         (iii) the Option holder ceasing to be a director or employee of any
               Participating Company by reason of injury, disability, redundancy
               within the meaning of the Redundancy Payments Acts, 1967 to 1991
               or retirement on reaching pensionable age within the meaning of
               section 2 of the Social Welfare (Consolidation) Act, 1993

         (iv)  the Option holder ceasing to be a director or employee of any
               Participating Company by reason only that

               (a)  that office or employment is in a company which the Company
                    ceases to have Control, or

               (b)  that office or employment relates to a business or part of
                    business which is transferred to a person who is neither an
                    associated company nor a company of which the Company has
                    Control

                                      6

<PAGE>   7

         (v)   the relevant Bonus Date, where an Option holder holds an office
               or employment in a company which is not a Participating Company
               but which is

               (a) an Associated Company of the Company, or

               (b) a company of which the Company has control

         (vi)  the Option holder ceasing to be a director or employee of any
               Participating Company more than three years after the Date of
               Grant of the relative Option for any reason determined by the
               Board provided that reasons shall then apply to all Eligible
               Employees.

    6.2  An Option shall lapse on the earliest of the following events:

         (i)   except where the Option holder has died, the expiry of six months
               following the Bonus Date

         (ii)  where the Option holder died during the six months following the
               Bonus Date the first anniversary of the Bonus Date

         (iii) where the Option holder has died before the Bonus Date, the first
               anniversary of his death

         (iv)  unless the Option holder has died, the expiry of six months after
               the Option has become exercisable by virtue of paragraph (iii) of
               Rule 6.1

         (v)   the expiry of six months after the Option has become exercisable
               by virtue of paragraph (iv) or (vi) of Rule 6.1 or on the
               happening of any of the events referred to in Rule 7.

         (vi)  the Option holder ceasing to be a director or employee of any
               Participating Company in circumstances in which the Option does
               not become exercisable

         (vii) the Option holder becoming bankrupt.

    6.3  No person shall be treated for the purposes of this Rule 6 as ceasing
         to be employed by a Participating Company until he is no longer
         employed by the Parent Company, any Associated Company or a company of
         which the Parent Company has Control.

    6.4  If an Option holder continues to be employed by a Participating Company
         after the date on which he reaches pensionable age (within the meaning
         of section 2, Social Welfare (Consolidation) Act, 1993), he may
         exercise any Subsisting Option within six months following that date.

                                      7

<PAGE>   8

7.  TAKE-OVERS AND LIQUIDATIONS

    7.1  If any person obtains Control of the Parent Company as a result of
         making

         (i)   a general offer to acquire the whole of the issued ordinary share
               capital of the Company which is made on condition such that if it
               is satisfied the person making the offer will have Control of the
               Parent Company, or

         (ii)  a general offer to acquire all the shares of the Company which
               are the same class as the Shares

         then any Subsisting Option shall lapse at the time when the person
         making the offer has obtained Control of the parent Company and any
         condition subject to which the offer is made has been satisfied.

    7.2  If under United States legislation, the court sanctions a compromise or
         arrangement proposed for the purposes of or in connection with a scheme
         for the reconstruction of the Parent Company or its amalgamation with
         any other company or companies, any Subsisting Option shall lapse at
         the time when the court sanctions the compromise or arrangements.

    7.3  If any person becomes bound or entitled to acquire shares in the Parent
         Company, under United States legislation, any Subsisting Option shall
         lapse at the time when that person becomes so bound or entitled.

    7.4  If the Parent Company passes a resolution for voluntary winding up, any
         Subsisting Option shall lapse on the passing of the resolution.

    7.5  For the purposes of this Rule 7 a person shall be deemed to have
         obtained Control of a Parent Company if he and others acting in concert
         with him have together obtained Control of it.

    7.6  Where in accordance with this Rule 7 Subsisting Options lapse, the
         individual by whom the Options were held, or as the case may be, his
         personal representatives, shall be entitled to withdraw the full amount
         of the contributions under the related Savings Contract which have not
         previously been used by way of repayment in respect of the exercise of
         an Option.

8.  VARIATION OF SHARE CAPITAL

    In the event of any variation in the share capital of the Parent Company by
    way of capitalisation or rights issue or any consolidation, sub-division or
    reduction or otherwise, the number of Shares subject to any Option and the
    Option Price for each of those shares shall be adjusted in such manner as
    the Auditors confirm to be fair and reasonable provided that

         (i)   the aggregate amount payable on the exercise of an Option in full
               is neither materially changed nor increased beyond the expected
               repayment under the Savings Contract at the appropriate Bonus
               Date.

                                      8

<PAGE>   9

         (ii)  no adjustment shall be made without the prior written approval of
               the Revenue Commissioners, and

         (iii) following the adjustment the Shares continue to satisfy the
               conditions specified in Paragraphs 11 to 15 inclusive, Schedule
               12A to the Act,

         provided that the Option Price for a Share is not reduced below its
         nominal value.

9.  MANNER OF EXERCISE OF OPTIONS

    9.1  No Option shall be exercised by an individual at any time when he is,
         or by the personal representatives of an individual who at the date of
         his death was, precluded by Paragraph 8, of Schedule 12A to the Act
         from participating in the Scheme.

    9.2  No Option shall be exercised at any time when the shares which may
         thereby be acquired are not Shares as defined in Rule 1.1.

    9.3  An Option shall only be exercised over the number of Shares which may
         be purchased with the sum obtained by way of repayment under the
         related Savings Contract.

    9.4  An Option shall be exercised by the Option holder, or as the case may
         be, his personal representatives, giving notice to the Company in
         writing of the number of Shares in respect of which he wishes to
         exercise the Option accompanied by the appropriate payment (which shall
         not exceed the sum obtained by way of repayment under the related
         Savings Contract) and the relevant Option certificate, and shall be
         effective on the date of its receipt by the Company.

    9.5  Shares shall be transferred pursuant to a notice of exercise within
         thirty days of the date of exercise. Save for any rights determined by
         reference to a date preceding the date of transfer, such Shares shall
         rank pari passu with the other Shares of the same class in issue at
         that date of transfer.

    9.6  When an Option is exercised only in part, it shall lapse to the extent
         of the unexercised balance.

    9.7  For the purposes of Rules 9.3 and 9.4 above, any repayment under the
         related Savings Contract shall exclude the repayment of any
         contribution the due date for payment of which falls more than one
         month after the date on which repayment is made.

10. ADMINISTRATION AND AMENDMENT

    10.1 The Scheme shall be administered by the Board whose decision on all
         disputes shall be final.

                                      9

<PAGE>   10

    10.2 The Board may from time to time amend these Rules provided that

         (i)   no amendment shall materially affect an Option holder as regards
               an Option granted prior to the amendment being made

         (ii)  no amendment shall be made which would make the terms on which
               Options may be granted materially more generous

         (iii) no amendment shall be made without the prior written approval of
               the Revenue Commissioners.

    10.3 The costs of establishing and operating the Scheme shall be borne by
         the Participating Companies in such proportions as the Board shall
         determine.

    10.4 The Board may establish a committee consisting of not less than three
         persons to whom any or all of its powers in relation to the Scheme may
         be delegated. The Board may at any time dissolve the committee, alter
         its constitution or direct the manner in which it shall act.

    10.5 Any notice or other communication under or in connection with the
         Scheme may be given by the Company either personally or by post and to
         the Company either personally or by post to the secretary; items sent
         by post shall be pre-paid and shall be deemed to have been received
         seventy two hours after posting.

    10.6 The Parent Company shall at all times keep available sufficient
         authorised and unissued Shares and / or Treasury Shares to satisfy the
         exercise to the full extent still possible of all Options which have
         neither lapsed nor been fully exercised, taking account of any other
         obligations of the Company to issue Shares.

11. GENERAL

    11.1 All eligible employees shall be eligible to participate in the Scheme
         on similar terms.

                                      10

Source: [{"source": "alea-institute/alea-institute/kl3m-data-edgar-agreements/train-00013-of-00352.parquet"}, [{"source": "alea-institute/alea-institute/kl3m-data-edgar-agreements/train-00013-of-00352.parquet"}]]