Is the cost basis for employee stock options exercised based on FMV?

Back in 2014 I exercised employee stock options at a price of $18.73.  The FMV at the time was $56.41 and I had to pay AMT based on the FMV even though I did not sell my shares that year.  I sold some shares in 2019 which used up the majority of the AMT credit carryforward.  This year I sold the remainder of the shares.  Is the cost basis the FMV price of $56.41 or the exercise price of $18.73 since my AMT credit carryforward is zero now or is that not a factor?  Thanks!