EXHIBIT 5.1

June 25, 2001

Monolithic System Technology, Inc.
1020 Stewart Drive
Sunnyvale, CA 94085

                            REGISTRATION ON FORM S-1

Dear Ladies and Gentlemen:

    We have acted as counsel to Monolithic System Technology, Inc., a Delaware
corporation (the "Company"), in connection with its registration statement on
SEC Form S-1 (the "Registration Statement") of the public offering of 5,000,000
shares of common stock, par value $.01 per share and up to 750,000 shares of
such common stock issuable upon exercise of the underwriters' over-allotment
option (the "Common Stock").

    In this regard, we have examined the Company's Restated Certificate of
Incorporation and Bylaws, each as amended to date, and records of meetings of
and written consents by the stockholders and the directors of the Company. In
addition, we have made such examinations of matters of law as we deemed
appropriate for purposes of this opinion. As to certain factual matters we deem
relevant to this opinion, we have relied upon a certificate of officers of the
Company and have not sought to independently verify the matters stated therein.

    Based upon the foregoing, it is our opinion that, on issuance of the Common
Stock subsequent to the time that the Securities and Exchange Commission
declares the Registration Statement to be effective, the 5,750,000 shares of
Common Stock will be validly issued, fully paid and non-assessable, and no
personal liability will attach to the holders of such shares by reason of the
ownership thereof.

    This opinion is rendered solely in connection with the registration of the
shares of Common Stock under the Registration Statement.

    We consent to being named in the Registration Statement and to the inclusion
of a copy of this opinion letter as an exhibit to the Registration Statement. In
giving this consent, however, we do not thereby admit that we are an "expert"
within the meaning of the Securities Act of 1933, as amended.


                                                
                                                 Very truly yours,

                                                 McCUTCHEN, DOYLE, BROWN & ENERSEN, LLP

                                                 By:  /s/ ALAN B. KALIN
                                                      ------------------------------------------------
                                                      Alan B. Kalin
                                                      A Member of the Firm