Cash and cash equivalents primarily consist of deposits held at major banks, Tier-1 commercial paper debt securities, money market funds and other securities with original maturities of 90 days or less. Marketable securities consist of Tier-1 commercial paper debt securities, corporate debt securities and certain other securities.

The amortized principal amounts of our cash, cash equivalents and marketable securities approximated their fair values at May 31, 2021 and 2020. We use the specific identification method to determine any realized gains or losses from the sale of our marketable securities classified as available-for-sale. Such realized gains and losses were insignificant for fiscal 2021, 2020 and 2019. The following table summarizes the components of our cash equivalents and marketable securities held, substantially all of which were classified as available-for-sale:

	May 31,
(in millions)	2021		2020
Money market funds	$	12,263	$	18,587
Corporate debt securities and other		9,470		6,625
Commercial paper debt securities		11,712		5,640
Total investments	$	33,445	$	30,852
Investments classified as cash equivalents	$	16,989	$	25,034
Investments classified as marketable securities	$	16,456	$	5,818

As of May 31, 2021 and 2020, substantially all of our marketable securities investments mature within one year. Our investment portfolio is subject to market risk due to changes in interest rates. As described above, we limit purchases of marketable debt securities to investment-grade securities, which have high credit ratings and also limit the amount of credit exposure to any one issuer. As stated in our investment policy, we are averse to principal loss and seek to preserve our invested funds by limiting default risk and market risk.

Restricted cash that was included within cash and cash equivalents as presented within our consolidated balance sheets as of May 31, 2021 and 2020 and our consolidated statements of cash flows for the years ended May 31, 2021, 2020 and 2019 was nominal.