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                                                                    Exhibit 3.20

                   CERTIFICATE OF DETERMINATION OF PREFERENCES
                                       OF
                            SERIES S PREFERRED STOCK
                                       OF
                                   DISC, INC.



     We, J. Richard Ellis and Henry Madrid, hereby certify that we are the 
President and the Chief Financial Officer, respectively, of DISC, INC., a
corporation organized and existing under the General Corporation Law of the
State of California, and further, DO HEREBY CERTIFY:

     That pursuant to the authority conferred upon the Board of Directors by the
Articles of Incorporation of the said Corporation, the said Board of Directors
on June 30, 1998 adopted the following resolution creating a series of 96,875
shares of Preferred Stock designated as Series S Preferred Stock, none of which
shares have been issued:

          "RESOLVED, that pursuant to the authority vested in the Board of 
Directors of the corporation by the Articles of Incorporation, the Board of
Directors does hereby provide for the issuance of a series of Preferred Stock,
no par value, of the Corporation, to be designated "Series S Preferred Stock,"
initially consisting of 96,875 shares and to the extent that the designations,
powers, preferences and relative and other special rights and the
qualifications, limitations and restrictions of the Series S Preferred Stock are
not stated and expressed in the Articles of Incorporation, does hereby fix and
herein state and express such designations, powers, preferences and relative and
other special rights and the qualifications, limitations and restrictions
thereof, as follows (all terms used herein which are defined in the Articles of
Incorporation shall be deemed to have the meanings provided therein):

          Section 1. Designation and Amount. The shares of such series shall be 
designated as "Series S Preferred Stock," no par value, and the number of shares
constituting such series shall be 96,875.

          Section 2. Dividends and Distributions.

               (A)  Subject to the prior and superior right of the holders of
any shares of Series C Preferred Stock ranking prior and superior to the shares
of Series S Preferred Stock with respect to dividends, and pari passu with the
rights of the holders of shares of Series D Preferred Stock, Series E Preferred
Stock, Series F Preferred Stock, Series G Preferred Stock, Series H Preferred
Stock, Series I Preferred Stock, Series J Preferred Stock, Series K Preferred
Stock, Series L Preferred Stock, Series M Preferred Stock, Series N Preferred
Stock, Series O Preferred Stock, Series P Preferred Stock, Series Q Preferred
Stock and Series R Preferred Stock with respect to dividends, subject to the
rights of any series of Preferred Stock which may hereafter come into existence,
the holders of shares of Series S Preferred Stock shall be entitled to receive
when, as and if declared by the 

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Board of Directors out of funds legally available for the purpose, dividends in
the same amount per share as declared on the Common Stock, treating such number
of shares of Series S Preferred Stock for this purpose as equal to the number of
shares of Common Stock into which it is then convertible. In the event any
dividends are declared or paid on the outstanding shares of Series D Preferred
Stock, Series E Preferred Stock, Series F Preferred Stock, Series G Preferred
Stock, Series H Preferred Stock, Series I Preferred Stock, Series J Preferred
Stock, Series K Preferred Stock, Series L Preferred Stock, Series M Preferred
Stock, Series N Preferred Stock, Series O Preferred Stock, Series P Preferred
Stock, Series Q Preferred Stock, or Series R Preferred Stock dividends shall
simultaneously be declared and paid on the outstanding shares of Series S
Preferred Stock, pari passu with the shares of Series D Preferred Stock, Series
E Preferred Stock, Series F Preferred Stock, Series G Preferred Stock, Series H
Preferred Stock, Series I Preferred Stock, Series J Preferred Stock, Series K
Preferred Stock, Series L Preferred Stock, Series M Preferred Stock, Series N
Preferred Stock, Series O Preferred Stock, Series P Preferred Stock, Series Q
Preferred Stock or Series R Preferred Stock, based upon the number of shares of
Common Stock into which shares of Series D Preferred Stock, Series E Preferred
Stock, Series F Preferred Stock, Series G Preferred Stock, Series H Preferred
Stock, Series I Preferred Stock, Series J Preferred Stock, Series K Preferred
Stock, Series L Preferred Stock, Series M Preferred Stock, Series N Preferred
Stock, Series O Preferred Stock, Series P Preferred Stock, Series Q Preferred
Stock, Series R Preferred Stock, and Series S Preferred Stock are then
convertible. In the event the Corporation shall at any time after the date of
the filing of this Certificate of Determination of Preferences (the "Rights
Declaration Date") (i) declare any dividend on Common Stock payable in shares of
Common Stock, (ii) subdivide the outstanding Common Stock, or (iii) combine the
outstanding Common Stock into a smaller number of shares, then in each such
case, the amount of Common Stock or other consideration to which holders of
shares of Series S Preferred Stock were entitled immediately prior to such event
under the preceding sentence shall be adjusted as set forth in Section 4(C)
hereof.

               (B)  The Corporation shall declare a dividend or distribution on
the Series S Preferred Stock as provided in paragraph (A) above prior to
declaring a dividend payable on shares of Common Stock.

          Section 3. Voting Rights. The holders of shares of Series S Preferred 
Stock shall have the following voting rights:

               (A)  Each holder of Series S Preferred Stock is entitled to a
number of votes equal to the number of shares of Common Stock into which the
holder's Series S Preferred Stock is then convertible. Except as provided by
law, the Common Stock and Series S Preferred Stock (and any series of Preferred
Stock which may be subsequently authorized which is convertible into shares of
Common Stock and which has voting rights equal to the number of shares of Common
Stock into which such series of Preferred Stock is then convertible) shall vote
together as a single class on all matters to come before the shareholders for
approval. In the 


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event the Corporation shall at any time after the Rights Declaration Date (i)
declare any dividend on Common Stock payable in shares of Common Stock, (ii)
subdivide the outstanding Common Stock, or (iii) combine the outstanding Common
Stock into a smaller number of shares, then in each such case the number of
votes per share to which holders of shares of Series S Preferred Stock were
entitled immediately prior to such event shall be adjusted as set forth in
Section 4(C) hereof.

               (B)  Except as otherwise provided herein or by law, the holders
of shares of Series S Preferred Stock and the holders of shares of Common Stock
(and any series of Preferred Stock which may be subsequently authorized which is
convertible into shares of Common Stock and which has voting rights equal to the
number of shares of Common Stock into which such series of Preferred Stock is
then convertible) shall vote together as one class on all matters submitted to a
vote of shareholders of the Corporation.

               (C)  Except as required by law or under Section 8 hereof, holders
of Series S Preferred Stock shall have no special voting rights and their
consent shall not be required (except to the extent they are entitled to vote
with holders of Common Stock as set forth herein) for taking any corporate
action.

          Section 4. Conversion Rights.

               (A)  Each holder of Series S Preferred Stock may, at any time, in
such holder's sole discretion, convert all or any part of such holder's shares
of Series S Preferred Stock into fully paid and nonassessable shares of Common
Stock at the rate of ten (10) shares of Common Stock for each share of Series S
Preferred Stock surrendered for conversion.

               (B)  Such conversion may be effected by surrender of such
holder's certificate or certificates for the shares of Series S Preferred Stock
to be converted, duly endorsed, at the principal office of the Corporation, with
a written notice stating (i) that such holder elects to convert all or a
specified number of shares of Series S Preferred Stock into shares of Common
Stock, and (ii) the name in which such holder desires a certificate for the
shares of Common Stock to be issued. Promptly thereafter, the Company shall
issue and deliver to such holder a certificate for the number of shares of
Common Stock to which such holder shall be entitled. Such conversion shall be
deemed to have been made at the close of business on the date of such surrender,
and such holder shall be treated for all purposes as the record holder of such
shares of Common Stock on that date.

               (C)  In the event the Corporation shall at any time after the
Rights Declaration Date (i) declare any dividend on Common Stock payable in
shares of Common Stock, (ii) subdivide the outstanding Common Stock, or (iii)
combine the outstanding Common Stock into a smaller number of shares, then, in
each case, the number of shares of Common Stock issuable upon the conversion of
each share of Series S Preferred Stock shall be adjusted by multiplying such
amount by a fraction, the numerator of which is the number of shares of Common
Stock outstanding 


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immediately after such event and the denominator of which is the number of
shares of Common Stock that are outstanding immediately prior to such event.

               (D)  In the event the Corporation shall at any time or from time
to time after the Rights Declaration Date make or issue, or fix a record date
for the determination of holders of Common Stock entitled to receive, a dividend
or other distribution payable in securities of the Corporation or any of its
subsidiaries, or of any other corporation or third party, other than in shares
of Common Stock, then, in each such event, provisions shall be made so that the
holders of Series S Preferred Stock shall receive, upon the conversion thereof,
securities of the Corporation or any of its subsidiaries or of any other
corporation or third party which they would have received had their stock been
converted into Common Stock on the date of such event.

          Section 5. Reacquired Shares. Any shares of Series S Preferred Stock 
purchased or otherwise acquired by the Corporation in any manner whatsoever
shall be retired and canceled promptly after the acquisition thereof. All such
shares shall upon their cancellation become authorized but unissued shares of
Preferred Stock and may be reissued as part of a new series of Preferred Stock
to be created by resolution or resolutions of the Board of Directors, subject to
the conditions and restrictions on issuance set forth herein.

          Section 6. Liquidation, Dissolution or Winding Up.

               (A)  Upon any liquidation (voluntary or otherwise), dissolution
or winding up of the Corporation, following the first priority liquidation
preference of the Series C Preferred Stock in the amount of $5.00 per share plus
any declared but unpaid dividends, and pari passu with the liquidation
preference of the Series D Preferred Stock in the amount of $5.00 per share plus
any declared but unpaid dividends ("Series D Liquidation Preference"), the
liquidation preference of the Series E Preferred Stock in the amount of $4.00
per share plus any declared but unpaid dividends ("Series E Liquidation
Preference"), the liquidation preference of the Series F Preferred Stock in the
amount of $8.00 per share plus any declared but unpaid dividends ("Series F
Liquidation Preference"), the liquidation preference of the Series G Preferred
Stock in the amount of $20.00 per share plus any declared but unpaid dividends
("Series G Liquidation Preference"), the liquidation preference of the Series H
Preferred Stock in the amount of $38.30 per share plus any declared but unpaid
dividends ("Series H Liquidation Preference"), the liquidation preference of the
Series I Preferred Stock in the amount of $4.19 per share plus any declared but
unpaid dividends ("Series I Liquidation Preference"), the liquidation preference
of the Series J Preferred Stock in the amount of $2.98 per share plus any
declared but unpaid dividends ("Series J Liquidation Preference"), the
liquidation preference of the Series K Preferred Stock in the amount of $3.19
per share plus any declared but unpaid dividends ("Series K Liquidation
Preference"), the liquidation preference of the Series L Preferred Stock in the
amount of $2.76 per share plus any declared but unpaid dividends ("Series L
Liquidation Preference"), the liquidation preference of the Series M Preferred
Stock in the amount of $2.23 per share plus any declared 


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but unpaid dividends ("Series M Liquidation Preference"), the liquidation
preference of the Series N Preferred Stock in the amount of $.90 per share plus
any declared but unpaid dividends ("Series N Liquidation Preference"), the
liquidation preference of the Series O Preferred Stock in the amount of $1.06
per share of Series O Preferred Stock plus any declared but unpaid dividends
(the "Series O Liquidation Preference"), the liquidation preference of the
Series P Preferred Stock in the amount of $.82 per share of Series P Preferred
Stock plus any declared but unpaid dividends (the "Series P Liquidation
Preference"), the liquidation preference of the Series Q Preferred Stock in the
amount of $.62 per share of Series Q Preferred Stock plus any declared but
unpaid dividends (the "Series Q Liquidation Preference"), the liquidation
preference of the Series R Preferred Stock in the amount of $.58 per share of
Series R Preferred Stock plus any declared but unpaid dividends (the "Series R
Liquidation Preference") no distribution shall be made to the holders of shares
of stock ranking junior (either as to dividends or upon liquidation, dissolution
or winding up) to the Series S Preferred Stock unless, prior thereto, the
holders of shares of Series S Preferred Stock shall have received an amount
equal to $.32 per share of Series S Preferred Stock plus any declared but unpaid
dividends ("Series S Liquidation Preference").

               (B)  In the event, however, that there are not sufficient assets
available to permit payment in full of the Series D Liquidation Preference, the
Series E Liquidation Preference, the Series F Liquidation Preference, the Series
G Liquidation Preference, the Series H Liquidation Preference, the Series I
Liquidation Preference, the Series J Liquidation Preference, the Series K
Liquidation Preference, the Series L Liquidation Preference, the Series M
Liquidation Preference, the Series N Liquidation Preference, the Series O
Liquidation Preference, the Series P Liquidation Preference, the Series Q
Liquidation Preference, the Series R Liquidation Preference, the Series S
Liquidation Preference and the liquidation preferences of all other series of
Preferred Stock, if any, which rank on a parity with the Series S Preferred
Stock, then such remaining assets shall be distributed ratably to the holders of
such parity shares in proportion to their respective liquidation preferences.

          Section 7. No Redemption. The shares of Series S Preferred Stock shall
not be redeemable.

          Section 8. Amendment. The Articles of Incorporation of the Corporation
shall not be further amended in any manner which would (a) alter or change the
powers, preferences or special rights or privileges of the Series S Preferred
Stock so as to affect them adversely or (b) grant to any other class of shares
any rights superior to those of the Series S Preferred Stock without the
affirmative vote of the holders of a majority or more of the outstanding shares
of Series S Preferred Stock, voting separately as a class.


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          Section 9. Fractional Shares. Series S Preferred Stock may be issued 
in fractions of a share which shall entitle the holder, in proportion to such
holder's fractional shares, to exercise voting rights, receive dividends,
participate in distributions and to have the benefit of all other rights of
holders of Series S Preferred Stock."


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     The undersigned declare under penalty of perjury that the matters set forth
in the foregoing Certificate are true of their own knowledge.

     Executed at Milpitas, California on December 31, 1998.


                                           /s/ J. Richard Ellis
                                           -------------------------------------
                                           J. Richard Ellis
                                           President and Chief Executive Officer


                                           /s/ Henry Madrid
                                           -------------------------------------
                                           Henry Madrid
                                           Chief Financial Officer


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