a result, the overall net effect on our gross margin from inventory provisions and sales of items previously written down was an unfavorable impact of 0.6%, 0.1% and 0.8% in fiscal years 2015, 2014 and 2013, respectively.



A discussion of our gross margin results for each of our reporting segments is as follows:



GPU Business. The gross margin of our GPU business increased during fiscal year 2015 when compared to fiscal year 2014 due to richer product mix resulting from strong sales of high-end GeForce GTX GPU products based on our Maxwell architecture and the volume increase in our Tesla accelerated computing products. The increase in fiscal year 2014 when compared to fiscal year 2013 was primarily due to a combination of a richer product mix of our high-end GeForce GTX GPU, Tesla high performance computing, and Quadro professional workstation products. Lower inventory provisions for excess inventory in fiscal year 2014 also contributed to the increase.



Tegra Processor Business. The gross margin of our Tegra Processor business decreased during fiscal year 2015 when compared to fiscal year 2014, and during fiscal year 2014 when compared to fiscal year 2013. These decreases were driven primarily by a combination of an overall decline in margins of our Tegra products and a less rich mix between tablet products, which have had higher gross margins, and smartphone and automotive module products, which have had comparably lower gross margins.



Operating Expenses

 

Research and Development



Research and development expenses remained relatively flat during fiscal year 2015 compared to fiscal year 2014. Compensation and benefits increased by $56.5 million resulting from employee additions, employee compensation increases and related costs, including stock-based compensation expense. Offsetting this increase was a $38.9 million decrease in engineering development expenses.



Research and development expenses increased by 16% in fiscal year 2014 compared to fiscal year 2013. Compensation and benefits increased by $101.9 million resulting from employee additions, employee compensation increases and related costs. The growth in engineering employees also drove an increase in facilities and IT support expense of $34.6 million, purchases of computer and software supplies of $14.1 million and depreciation and amortization of $11.0 million. In addition, engineering development expenses increased by $23.2 million, primarily related to the ramp up of Tegra products.



Sales, General and Administrative



Sales, general and administrative expenses increased by 10% in fiscal year 2015 compared to fiscal year 2014. Compensation and benefits increased by $53.7 million resulting from employee additions, employee compensation increases and related costs, including stock-based compensation expense. Facilities costs increased by $10.3 million as we expanded our offices internationally and leased an office building within the boundaries of our main Santa Clara campus. Offsetting these increases were a decrease in outside professional fees of $8.8 million as well as more favorable international taxes and government subsidies.



Sales, general and