Quick question on Amort HTM accounting, difference, question, remember

malbec

I have been plowing through EOCs and my brain managed to squeeze out this easy bit of info that can be crucial.  I forgot how to amortize a bond when finding out how much to effect the IS when accounting for HTM securities… Interest payment +/- Amort amount.   I know you can find it by PV of of Cash Flows but I remember using an easier way (for me) by finding the difference between the amount received using the coupon payment and the amount received using the yield… I just don’t remember the exact formula. I think it was par x coupon - Yield x (thats where I forget)… i know it is a simple thing to calculate but my brain just froze up… any ideas?