The financial registers at left work the same way as the HP 12C calculator, including the round-up of "n". Each financial register matches the button at its right. For example, fill the PMT register if you know value of payment, or fill the other registers and press PMT if you want to calculate PMT.
For the arithmetic operations (plus, minus etc.) fill the values into the yellow registers, and the result will appear in the blue register.
The "m" register is the number of payments per year. It is useful for operations where time and interest rate are expressed in a yearly basis. For example: m=12, n=3 and i=12 is equivalent to m=1, n=36 and i=1. When "m" is greater than 1, "n" is no longer rounded up. The default m=1 makes the other registers behave the same way as HP 12C.
BEGIN: If enabled, the first payment is made at the beginning of the period. Matches the "begin" flag in HP 12C. It is important for calculations that involve non-zero PMTs.
CFP: If enabled, the fractional part of "n" will be calculated as compound interest (more fair but seldom employed).