EXHIBIT 5.1

                       [letterhead of Cooley Godward llp]

                               September 12, 2000

Applied Micro Circuits Corporation
6290 Sequence Drive
San Diego, CA 92121

   Ladies and Gentlemen:

   You have requested our opinion with respect to certain matters in connection
with the filing by Applied Micro Circuits Corporation, a Delaware corporation
(the "Company"), of a Registration Statement on Form S-4 (the "Registration
Statement") with the Securities and Exchange Commission, including a related
prospectus/proxy statement filed with the Registration Statement (the
"Prospectus"), covering the registration of up to 25,035,752 shares of Common
Stock of the Company (the "Shares"), proposed to be issued in connection with
the merger between MMC Networks, Inc., a Delaware corporation ("MMC Networks"),
and Mercury Acquisition Corp., a Delaware corporation and a wholly owned
subsidiary of the Company ("Merger Sub"), contemplated by the Agreement and
Plan of Merger dated as of August 28, 2000 (the "Merger Agreement"), among the
Company, Merger Sub and MMC Networks .

   In connection with this opinion, we have examined the Registration Statement
and related Prospectus, the Company's Amended and Restated Certificate of
Incorporation and Bylaws, as amended, and such other records, documents,
certificates, memoranda and other instruments as we deem necessary as a basis
for this opinion. We have assumed the genuineness and authenticity of all
documents submitted to us as originals, the conformity to originals of all
documents submitted to us as copies thereof and the due execution and delivery
of all documents where due execution and delivery are a prerequisite to the
effectiveness thereof.

   On the basis of the foregoing, and in reliance thereon, we are of the
opinion that the Shares, when issued in accordance with the terms and
conditions of the Merger Agreement, will be validly issued, fully paid and
nonassessable.

   We consent to the reference to our firm under the caption "Legal Matters" in
the Prospectus included in the Registration Statement and to the filing of this
opinion as an exhibit to the Registration Statement.

                                          Very truly yours,

                                          Cooley Godward LLP

                                          /s/ D. Bradley Peck
                                          -------------------------------------
                                          D. Bradley Peck