This calculator does arithmetic (mod P), where P is an integer you set with "Set P". P is initialized to 11, so you can enter "5+8=", by pushing the corresponding buttons, to find the answer "2 (mod 11)". If you enter "13 Set P", subsequent arithmetic will be (mod 13), etc. Integers are limited in size only by the size of the display.
Input is also accepted from the keyboard. You can use the "Enter" key to prompt for the result, instead of the "=" key. "Set P" is accomplished with the "S" key. MEM and RCL (accomplished with "M" and "R" from the keyboard) enable you to save a number in memory and to recall it.
The operations include '^', exponentiation, as in 2^11=2 (mod 11). The division operation "x/y=" is a request for a number 'z' such that x=yz (mod P). There may be no such 'z', or there may be several. Try "2/6" and "3/6" in "mod 15", for example. "1/x" requests the multiplicative inverse of 'x' (mod P). This exists if and only if x and P are relatively prime.