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                                                                EXHIBIT 4.12

                         FIRST AMENDMENT TO AMENDED AND
                       RESTATED REVOLVING CREDIT AGREEMENT

            This FIRST AMENDMENT TO SECOND AMENDED AND RESTATED REVOLVING CREDIT
AGREEMENT (this "First Amendment") is entered into as of January __, 1997 by and
between (a) HMT TECHNOLOGY CORPORATION (the "Borrower"), a Delaware corporation
having its principal place of business at 1055 Page Avenue, Fremont, California
94538, (b) THE FIRST NATIONAL BANK OF BOSTON, a national banking association,
BANQUE PARIBAS, acting through its Los Angeles branch, and the other lending
institutions listed on Schedule 1.1, (c) THE FIRST NATIONAL BANK OF BOSTON as
administrative and co-syndication agent for itself, the other lending
institutions listed on Schedule 1.1, and the other Bank Agents (as defined
herein), and (d) BANQUE PARIBAS, as co-syndication agent for itself, the other
lending institutions listed on Schedule 1.1, and the other Bank Agents.

                                    RECITALS

            A. Borrower, Banks, and Bank Agents are parties to that certain
Amended and Restated Revolving Credit Agreement dated as of August 28, 1996 (the
"Credit Agreement"), pursuant to which the Banks agreed to make available to
Borrower certain credit facilities.

            B. Borrower has requested that the Banks agree to various amendments
to the Credit Agreement which will permit, inter alia, the issuance of certain
convertible subordinated securities of the Borrower. Banks have agreed to make
such amendments, subject to the conditions and in reliance on the
representations and warranties set forth below.

            NOW, THEREFORE, for good and valuable consideration, the receipt and
adequacy of which are hereby acknowledged, Bank, Bank Agents and Borrower hereby
agree as follows:

                                    AGREEMENT

            1. DEFINED TERMS; SECTION REFERENCES. Initially capitalized terms
used but not defined in this First Amendment shall have the meanings assigned to
such terms in the Credit Agreement. All "Section " references herein are to
sections of the Credit Agreement unless otherwise specified.

            2. AMENDMENTS TO SECTION 1.1 OF THE CREDIT AGREEMENT. Section 1.01
of the Credit Agreement shall be revised in relevant part as follows:

            (a) The definition of "Replacement Subordinated Debt " shall be
deleted in its entirety and a new definition substituted therefor which reads as
follows:

            Replacement Subordinated Debt. Unsecured subordinated Indebtedness
            of the Borrower, in an original principal amount not to exceed Two
            Hundred Fifty Million Dollars ($250,000,000), as to which all of the
            following conditions are satisfied: (i) the first proceeds of such
            subordinated Indebtedness shall be used to pay the Subordinated Debt
            in full; (ii) the proceeds of such subordinated Indebtedness shall
            not be used to make payments to the holders of, and redemptions of,
            Borrower's Preferred Stock or to make any other Restricted Payments,
            provided, however, that


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            if the total proceeds of such subordinated Indebtedness exceed One
            Hundred Fifty Million Dollars ($150,000,000), the proceeds of such
            subordinated Indebtedness may be used to make payments to the
            holders of Borrower's Preferred Stock in accordance with the terms
            of such Preferred Stock; (iii) the terms and conditions governing
            such subordinated Indebtedness shall be substantially the same as
            those described in that certain preliminary offering memorandum
            relating to the issuance by Borrower of Two Hundred Million Dollars
            ($200,000,000) in convertible subordinated notes due 2004, dated on
            or about January 14, 1997, (iv) the coupon applicable to such
            subordinated Indebtedness shall not exceed eight percent (8%) per
            annum; and (v) such Indebtedness is incurred in a transaction that
            is consummated not later than April 1, 1997.

            3. AMENDMENTS TO SECTION 9.4 OF THE CREDIT AGREEMENT. Section 9.4 of
the Credit Agreement shall be amended in relevant part as follows:

            (a) The word "and" shall be deleted from the end of subsection (c)
of Section 9.4.

            (b) The period shall be deleted from the end of subsection (d) of
Section 9.4, and in its place shall be inserted "; and".

            (c) A new subsection (e) will be added to Section 9.4 of the Credit
Agreement, as follows:

            (e) payments to the holders of, and redemptions of, Preferred Stock
            of the Borrower funded by the proceeds of Replacement Subordinated
            Debt, (provided that such payments may not be made unless the total
            proceeds of the Replacement Subordinated Debt exceed One Hundred
            Fifty Million Dollars ($150,000,000)).

            4. NEW SECTION 9.13 OF THE CREDIT AGREEMENT. A new Section 9.13
shall be added to the Credit Agreement, which shall provide as follows:

            9.13 DESIGNATED SENIOR DEBT UNDER THE INDENTURE. If the Borrower
            creates any Designated Senior Debt, as such term is defined in the
            Indenture, the instruments evidencing or governing such Designated
            Senior Debt will provide that the holders or the representative of
            any such Designated Senior Debt shall have no ability to exercise
            rights under the Indenture to block payment to the holders of
            Replacement Subordinated Debt without first obtaining the written
            consent of the Agent.

            5. Amendments to Section 13.1 of the Credit Agreement. Section 13.1
of the Credit Agreement shall be amended in relevant part as follows:

            (a) The word "or" shall be deleted from the end of subsection (o) of
Section 13.1.

            (b) The period shall be deleted from the end of subsection (p) of
Section 13.1, and in its place shall be inserted "; or".


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            (c) A new subsection (q) will be added to Section 13.1 of the Credit
            Agreement, as follows:

            (q) (i) the subordination provisions of the indenture or any
            agreement or instrument governing the Subordinated Debt or
            Replacement Subordinated Debt shall for any reason be revoked or
            invalidated, or otherwise cease to be in full force and effect.

            6. CONDITIONS PRECEDENT TO THE EFFECTIVENESS OF THIS FIRST
AMENDMENT. Each Bank's obligations under this First Amendment are conditioned
upon, and this First Amendment shall not be effective until, satisfaction in
full of each of the following:

            (a) Agent shall have received this First Amendment, duly executed on
behalf of Borrower and Majority Banks by the appropriate Person and in form and
substance satisfactory to Agent and its counsel;

            (b) Borrower shall have paid to Agent all amounts then due and
payable including, without limitation, all fees and expenses incurred by Agent
in connection with this First Amendment, and all other amounts then payable
pursuant to Section 16 of the Credit Agreement which shall have been presented
for payment;

            (c) All of the representations and warranties of Borrower contained
herein, in the Credit Agreement and in each other Loan Document shall be true
and correct in all material respects on and as of the effective date of this
First Amendment, as though made on and as of that date (except to the extent
that such representations and warranties expressly relate to an earlier date or
reflect changes brought about by this First Amendment);

            (d) Borrower shall have delivered to Agent certified copies of
resolutions of its Board of Directors authorizing Borrower to execute and
deliver this First Amendment and an incumbency certificate, each in form and
substance satisfactory to Agent in its sole and absolute discretion;

            (e) No Event of Default or Unmatured Event of Default shall have
occurred and be continuing or would result from the consummation of the
transactions contemplated in this First Amendment;

            (f) All other documents, certificates, consents and opinions
reasonably required by Bank Agent in connection with the transactions
contemplated by this First Amendment shall have been executed and delivered in
form and substance satisfactory to Bank Agent; and

            (g) Each of the Banks shall have received a copy of the most current
draft of the Indenture proposed to be executed in connection with the
Replacement Subordinated Debt. .

            7. REPRESENTATIONS AND WARRANTIES. In order to induce Bank to enter
into this First Amendment, Borrower makes the following representations and
warranties:

            (a) The representations and warranties contained in the Credit
Agreement (and in the Schedules thereto) and each of the other Loan Documents
(and in the Schedules thereto) are true, correct and complete in all material
respects at and as of the effective date of this First Amendment (except to the
extent that such representations and warranties expressly relate to an earlier
date or reflect changes brought about by this First Amendment);


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            (b) This First Amendment and all other agreements and documents
executed by Borrower in connection herewith have been duly executed and
delivered by Borrower and constitute the legal, valid and binding obligations of
Borrower, enforceable against Borrower in accordance with their terms, except as
enforcement may be limited by bankruptcy, insolvency, reorganization, moratorium
or similar laws relating to the enforcement of the rights of creditors
generally, or the exercise of judicial discretion with respect to equitable
remedies.

            8. REFERENCES. All references in the Credit Agreement to "this
Restated Agreement", "hereof", "herein", "hereto", or words of similar import,
and all references in all other Loan Documents to "the Credit Agreement" shall
be, and shall be deemed to be for all purposes, references to the Credit
Agreement as amended.

            9. CREDIT AGREEMENT AND OTHER LOAN DOCUMENTS OTHERWISE NOT AFFECTED.
Except as expressly amended pursuant to this First Amendment, the Credit
Agreement and each of the other Loan Documents shall remain unchanged and in
full force and effect and are hereby ratified and confirmed in all respects.
Each Bank continues to reserve any and all rights and remedies under the Credit
Agreement and each of the other Loan Documents, and no failure, delay or
discontinuance on the part of any Bank in exercising any right, power or remedy
thereunder shall operate as a waiver thereof, nor shall any single or partial
exercise of any such right, power or remedy preclude any other or further
exercise thereof or the exercise of any other right, power or remedy. This First
Amendment and the Credit Agreement shall be read together, as one document.

            10. BINDING EFFECT. This First Amendment shall be binding upon,
inure to the benefit of and be enforceable by Borrower and each Bank and their
respective successors and assigns, as permitted pursuant to the Credit
Agreement.

            11. TIME OF THE ESSENCE. Time and exactitude of each of the terms,
obligations, covenants and conditions of this First Amendment are hereby
declared to be of the essence.

            12. GOVERNING LAW. THIS FIRST AMENDMENT IS A CONTRACT UNDER THE LAWS
OF THE STATE OF CALIFORNIA AND SHALL FOR ALL PURPOSES BE CONSTRUED IN ACCORDANCE
WITH AND GOVERNED BY SUCH LAWS (EXCLUDING THE LAWS APPLICABLE TO CONFLICTS OR
CHOICE OF LAW).

            13. COUNTERPARTS. This First Amendment may be executed in several
counterparts and by each party on a separate counterpart, each of which when
executed and delivered shall be an original, and all of which together shall
constitute one instrument. In proving any matter with respect to this First
Amendment it shall not be necessary to produce or account for more than one such
counterpart signed by the party against whom enforcement is sought.

            14. CONFIDENTIALITY. Each Bank Agent and each Bank agrees to take,
and to cause its respective affiliates to take, reasonable precautions in
accordance with its safe and sound banking practice and its customary
procedures, to maintain the confidentiality of all non-public information
relating to the Borrower or any Subsidiary provided to such Bank Agent or Bank
in connection with this First Amendment, and none of the Bank Agents, the Banks,
nor any of their respective affiliates shall use any such information other than
in connection with or in enforcement of the Credit Agreement, as amended by this
First Amendment, and the other Loan Documents or in connection with other
business now or hereafter existing or contemplated with


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the Borrower or any Subsidiary, except to the extent such information (i) was or
becomes generally available to the public other than as a result of disclosure
by the Bank Agents or the Banks, or (ii) was or becomes available on a
non-confidential basis from a source other than the Borrower, so long as such
source is not bound by a confidentiality agreement with the Borrower known to
such Bank; provided, however, that the provisions of this Section 14 shall not
restrain the Bank Agents or the Banks from divulging such information (A) at the
request or pursuant to any requirement of any governmental authority to which a
Bank Agent or Bank is subject or in connection with an examination of such Bank
Agent or Bank by any such authority; (B) pursuant to subpoena or other court
process; (C) when required to do so in accordance with the provisions of any
applicable legal requirement; (D) to the extent reasonably required in
connection with any litigation or proceeding to which any Bank Agent or Bank or
any of their respective affiliates may be party; (E) to the extent reasonably
required in connection with the exercise by a Bank Agent or Bank of any right or
remedy hereunder or under any other Loan Documents; (F) to a Bank Agent's or
Bank's independent auditors and other professional advisors; (G) to any
participant or assignee, actual or potential, provided that such Person agrees
in writing to keep such information confidential to the same extent required of
the assigning Bank Agent or Bank hereunder; (H) as expressly permitted under the
terms of any other document or agreement regarding confidentiality to which the
Borrower or any Subsidiary is party or is deemed party with any Bank Agent or
Bank, or any affiliate of such Bank Agent or Bank; and (I) to its affiliates,
which shall be deemed to be bound by the provisions of this Section 14.

            IN WITNESS WHEREOF, the parties hereto have duly executed this First
Amendment, as of the date first above written.

                                    HMT TECHNOLOGY CORPORATION

                                        /s/ Peter S. Norris             
                                    By:_________________________________________
                                     Title:


                                    BANQUE PARIBAS, individually and as
                                    Co-Syndication Agent

                                        /s/ Nanci Meyer
                                    By:_________________________________________
                                     Title:

                                        /s/ Stanley P. Berkman
                                    By:_________________________________________
                                     Title:


                                    THE FIRST NATIONAL BANK OF BOSTON,
                                    individually and as Agent and
                                    Co-Syndication Agent
                            
                                        /s/ Joseph L. Massimo
                                    By:_________________________________________
                                     Title:


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                                    UNION BANK OF CALIFORNIA, N.A.


                                        /s/ Patrick J. Clemens
                                    By:_________________________________________
                                     Title:



                                    BANK OF AMERICA NATIONAL TRUST AND
                                    SAVINGS ASSOCIATION


                                        /s/ Christopher Gernhard
                                    By:_________________________________________
                                     Title:


                                    FLEET NATIONAL BANK


                                         /s/ Matthew M. Glauninger
                                    By:_________________________________________
                                     Title:


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