<PAGE>

                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549

                                  SCHEDULE 13G

             INFORMATION STATEMENT PURSUANT TO RULES 13d-1 AND 13d-2
                  UNDER THE SECURITIES AND EXCHANGE ACT OF 1934
                             (Amendment No.   4   )*


                                  Unisys Corp.
- -------------------------------------------------------------------------------
                                (Name of Issuer)


                                  Common Stock
- -------------------------------------------------------------------------------
                         (Title of Class of Securities)


                                   909214108
- -------------------------------------------------------------------------------
                                 (CUSIP Number)



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

*The remainder of this cover page shall be filled out for a reporting 
person's initial filing on this form with respect to the subject class of 
securities, and for any subsequent amendment containing information which 
would alter disclosures provided in a prior cover page.

The information required on the remainder of this cover page shall not be 
deemed to be "filed" for the purpose of Section 18 of the Securities Exchange 
Act of 1934 ("Act") or otherwise subject to the liabilities of that section 
of the Act but shall be subject to all other provisions of the Act (however, 
see the Notes).


                        (Continued on following page(s))
                                Page 1 of 8 Pages


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CUSIP No. 909214108                    13G              Page  2  of  8  Pages  
- ----------------------                               --------------------------

- -------------------------------------------------------------------------------
1    NAME OF REPORTING PERSON
     S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON

               The TCW Group, Inc.
- -------------------------------------------------------------------------------
2    CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*
                                                                       (a)  / /
                                                                       (b)  /X/
- -------------------------------------------------------------------------------
3    SEC USE ONLY

- -------------------------------------------------------------------------------
4    CITIZENSHIP OR PLACE OF ORGANIZATION

          Nevada corporation
- -------------------------------------------------------------------------------
           NUMBER OF               5    SOLE VOTING POWER
            SHARES                                                         -0-
         BENEFICIALLY              --------------------------------------------
           OWNED BY                6    SHARED VOTING POWER
             EACH                                                          -0-
           REPORTING               --------------------------------------------
            PERSON                 7    SOLE DISPOSITIVE POWER
             WITH                                                          -0-
                                   --------------------------------------------
                                   8    SHARED DISPOSITIVE POWER
                                                                           -0-
- -------------------------------------------------------------------------------
9    AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
                                                                           -0-
- -------------------------------------------------------------------------------
10   CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES
     CERTAIN SHARES*
                                                                           / /
- -------------------------------------------------------------------------------
11   PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9)
     0.0% (see response to Item 4)
- -------------------------------------------------------------------------------
12   TYPE OF REPORTING PERSON*
                                      HC/CO
- -------------------------------------------------------------------------------



                    *SEE INSTRUCTIONS BEFORE FILLING OUT!


<PAGE>

CUSIP No. 909214108                    13G              Page  3  of  8  Pages  
- ----------------------                               --------------------------

- -------------------------------------------------------------------------------
1    NAME OF REPORTING PERSON
     S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON

               Robert Day
- -------------------------------------------------------------------------------
2    CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*
                                                                       (a)  / /
                                                                       (b)  /X/
- -------------------------------------------------------------------------------
3    SEC USE ONLY

- -------------------------------------------------------------------------------
4    CITIZENSHIP OR PLACE OF ORGANIZATION

          United States Citizen
- -------------------------------------------------------------------------------
           NUMBER OF               5    SOLE VOTING POWER
            SHARES                                                         -0-
         BENEFICIALLY              --------------------------------------------
           OWNED BY                6    SHARED VOTING POWER
             EACH                                                          -0-
           REPORTING               --------------------------------------------
            PERSON                 7    SOLE DISPOSITIVE POWER
             WITH                                                          -0-
                                   --------------------------------------------
                                   8    SHARED DISPOSITIVE POWER
                                                                           -0-
- -------------------------------------------------------------------------------
9    AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
                                                                           -0-
- -------------------------------------------------------------------------------
10   CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES
     CERTAIN SHARES*
                                                                           / /
- -------------------------------------------------------------------------------
11   PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9)
       0.0% (see response to Item 4)
- -------------------------------------------------------------------------------
12   TYPE OF REPORTING PERSON*7
                                      HC/IN
- -------------------------------------------------------------------------------



                    *SEE INSTRUCTIONS BEFORE FILLING OUT!


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                                                        Page  4  of  8  Pages  
Item 1(a).    Name of Issuer:

              Unisys Corp.

Item 1(b).    Address of Issuer's Principal Executive Offices:

              Township Line & Union Meeting Roads
              Blue Bell, PA 19424
             
Item 2(a).    Name of Persons Filing:
Item 2(b).    Address of Principal Business Office:
Item 2(c).    Citizenship:

              The TCW Group, Inc.
              865 South Figueroa Street
              Los Angeles, CA 90017
              (Nevada Corporation)

              Robert Day 
              200 Park Avenue, Suite 2200
              New York, New York 10166
              (United States Citizen)

Item 2(d).    Title of Class of Securities:

              Common Stock

Item 2(e).    CUSIP Number:

              909214108


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                                                        Page  5  of  8  Pages  

Item 3.        If this statement is filed pursuant to Rules 
               13d-1(b), or 13d-2(b), check whether the person 
               filing is a:

          (a)   [ ]  Broker or Dealer registered under Section 15 of 
                     the Act:

                         Not applicable

          (b)   [ ]  Bank as defined in Section 3(a)(6) of the Act:

                         Not applicable

          (c)   [ ]  Insurance Company as defined in Section 3(a)(19) 
                     of the Act:
                  
                         Not applicable

          (d)   [ ]  Investment Company registered under Section 8 of the
                     Investment Company Act:

                         Not applicable

          (e)   [ ]  Investment Adviser registered under Section 203 of the
                     Investment Advisers Act of 1940:

                         Not applicable

          (f)   [ ]  Employee Benefit Plan, Pension Fund which is 
                     subject to the provisions of the Employee 
                     Retirement Income Security Act of 1974 or 
                     Endowment Fund (SEE 13d-1(b)(1)(ii)(F)):

                         Not applicable

          (g)   [X]  Parent Holding Company, in accordance with Rule
                     13d-1(b)(ii)(G) (SEE Item 7):

                         The TCW Group, Inc.
                         Robert Day (individual who may be deemed to 
                              control The TCW Group, Inc. and other 
                              holders of the Common Stock of the 
                              issuer)

          (h)   [ ]  Group, in accordance with Rule 13d-1(b)(1)(ii)(H):

                         Not applicable.

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                                                        Page  6  of  8  Pages  

Item 4.       Ownership **

          THE TCW GROUP, INC.

               (a)  Amount beneficially owned: -0-

               (b)  Percent of class: 0.0% 

               (c)  Number of shares as to which such person has:

                    (i)   Sole power to vote or to direct the vote:         
                            -0-

                    (ii)  Shared power to vote or to direct the vote:   none.

                    (iii) Sole power to dispose or direct the disposition of: 
                            -0-

                    (iv)  Shared power to dispose or to direct the disposition 
                    of: none.

          ROBERT DAY 

               (a)  Amount beneficially owned: -0-

               (b)  Percent of class: 0.0%

               (c)  Number of shares as to which such person has:

                    (i)   Sole power to vote or to direct the vote:        
                            -0-

                    (ii)  Shared power to vote or to direct the vote: none.

                    (iii) Sole power to dispose or direct the disposition of: 
                            -0-

                    (iv)  Shared power to dispose or to direct the disposition 
                    of: none.

- -----------------------------
** The filing of this Schedule 13G shall not be construed as an admission that 
the reporting person or any of its affiliates is, for the purposes of Section 
13(d) or 13(g) of the Securities Exchange Act of 1934, the beneficial owner 
of any securities covered by this Schedule 13G.  In addition, the filing of 
this Schedule 13G shall not be construed as an admission that the reporting 
person or any of its affiliates is the beneficial owner of any securities 
covered by this Schedule 13G for any other purposes than Section 13(d) of the 
Securities Exchange Act of 1934.


<PAGE>
                                                        Page  7  of  8  Pages  

Item 5.        Ownership of Five Percent or Less of a Class.

                    Applicable.

Item 6.        Ownership of More than Five Percent on Behalf of Another Person.

                    Not applicable.

Item 7.        Identification and Classification of the Subsidiary 
               Which Acquired the Security Being Reported on by the 
               Parent Holding Company.

               SEE Exhibit A.

Item 8.        Identification and Classification of Members of the Group.

               Not applicable. SEE Exhibits A and B.

Item 9.        Notice of Dissolution of Group.

               Not applicable.

Item 10.       Certification.

          Because this statement is filed pursuant to Rule 13d-1(b), 
          the following certification is included:


          By signing below I certify that, to the best of my 
          knowledge and belief, the securities referred to above 
          were acquired in the ordinary course of business and 
          were not acquired for the purpose of and do not have the
          effect of changing or influencing the control of the issuer of 
          such securities and were not acquired in connection with 
          or as a participant in any transaction having such purpose 
          or effect.


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                                                        Page  8  of  8  Pages  

                                    SIGNATURE


               After reasonable 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 this 12th day of February, 1997.


                                        The TCW Group, Inc.

                                                  
                                        By:  /s/ Mohan V. Phansalkar
                                            ------------------------
                                            Mohan V. Phansalkar
                                            Authorized Signatory 


                                        Robert Day

                                                  
                                        By:  /s/ Mohan V. Phansalkar
                                            ------------------------
                                            Mohan V. Phansalkar
                                            Under Power of Attorney 
                                            dated January 30, 1996, 
                                            on File with Schedule 13G
                                            Amendment Number 1 for 
                                            Matrix Service Co. dated 
                                            January 30, 1996.







<PAGE>

                                    EXHIBIT A


                 RELEVANT SUBSIDIARIES OF PARENT HOLDING COMPANY

PART A: TCW ENTITIES

PARENT HOLDING COMPANY:

               The TCW Group, Inc.

               Robert Day (an individual who may be deemed to 
               control The TCW Group, Inc.)


RELEVANT SUBSIDIARIES THAT ARE PERSONS DESCRIBED IN RULE 13d-1(b):

         (i)   Trust Company of the West, a California corporation 
         and a bank as defined in Section 3(a)(6) of the Securities 
         Exchange Act of 1934. 

         (ii)  TCW Asset Management Company, a California 
         corporation and an Investment Adviser registered under 
         Section 203 of the Investment Advisers Act of 1940. 

         (iii) TCW Funds Management, Inc., a California corporation 
         and an Investment Adviser registered under Section 203 of 
         the Investment Advisers Act of 1940. 

Note:          No Common Stock of Unisys Corp. is held directly by The TCW
               Group, Inc.  Other than the indirect holdings of The TCW Group,
               Inc. no Common Stock of Unisys Corp. is held directly or
               indirectly by Robert Day, an individual who may be deemed to
               control The TCW Group, Inc. 
     
PART B: NON TCW ENTITIES

PARENT HOLDING COMPANY:

               Robert Day (an individual who may be deemed to 
               control the holders described below which are not 
               subsidiaries of The TCW Group, Inc.)

RELEVANT SUBSIDIARIES THAT ARE PERSONS DESCRIBED IN 
RULE 13d-1(b):

               Oakmont Corporation,
 a California corporation and an 
               Investment Adviser registered under Section 203 of 
               the Investment Advisers Act of 1940.

               Cypress International Partners Limited, a British 
               Virgin Islands corporation and an Investment Adviser 
               registered under Section 203 of the Investment 
               Advisers Act of 1940.

                                       A-1




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                                    EXHIBIT B

                             JOINT FILING AGREEMENT


              The undersigned acknowledge and agree that the foregoing 
statement on Schedule 13G is filed on behalf of each of the undersigned and 
that all subsequent amendments to this statement on Schedule 13G shall be 
filed on behalf of each of the undersigned without the necessity of filing 
additional joint acquisition statements.  The undersigned acknowledge that 
each shall be responsible for the timely filing of such amendments, and for 
the completeness and accuracy of the information concerning him or it 
contained therein, but shall not be responsible for the completeness and 
accuracy of the information concerning the other, except to the extent that 
he or it knows or has reason to believe that such information is accurate.


Dated: February 12, 1997



                                        The TCW Group, Inc.

                                                  
                                        By:  /s/ Mohan V. Phansalkar
                                            ------------------------
                                            Mohan V. Phansalkar
                                            Authorized Signatory 


                                        Robert Day

                                                  
                                        By:  /s/ Mohan V. Phansalkar
                                            ------------------------
                                            Mohan V. Phansalkar
                                            Under Power of Attorney 
                                            dated January 30, 1996, 
                                            on File with Schedule 13G
                                            Amendment Number 1 for 
                                            Matrix Service Co. dated 
                                            January 30, 1996.





                                       B-1