There is an unlimited stack of numbers. The last 4 entries are displayed as "T", "Z", "X" and "Y". The number currently being entered is shown on the line marked ">".
Thanks to the jepler_udecimal library, the calculator uses 14-digit numbers and allows exponents from -99 to +99.
Enter a simple number by pressing digit keys followed by ENT, such as "1234". This places the number at the last entry of the stack and moves the other entries deeper in the stack.
The "decimal point" key doubles as the exponent key, so to enter the number "6.02e23" enter "6.02.23". For "1E3" type "1..3".
To perform an operation, enter the required operands and then press the operation key. If you were in the middle of entering a number when you press an operation key, the number is automatically Entered.
For example, to add "1" and "2", you can type: "1 ENT 2 +". "1" and "2" are removed, and "3" is placed on the stack.
Some keys have two functions. To access the alternate function, press and release ALT, then press the other key.
When you press Paste (ALT 0), the "X" number is entered via USB HID on the attached computer. The number remains on the stack.
Here are all the functions you can access within the calculator. The left hand side is the normal key, and the right hand side is the alternate function.