pre_text: 5. Goodwill and Purchased Intangible Assets (b) Purchased Intangible Assets The following table presents the amortization of purchased intangible assets (in millions):
table: Years Ended July 27, 2019 July 28, 2018 July 29, 2017
table: Amortization of purchased intangible assets:
table: Cost of sales . $624 $640 $556
table: Operating expenses
table: Amortization of purchased intangible assets 150 221 259
table: Restructuring and other charges — — 38
table: Total  . $774 $861 $853
question: Analyse this data from a financial earnings document. What was the change in cost of sales between 2017 and 2018?