Bitworks RPN Calculator app for iPhone and iPad

The Bitworks RPN Calculator is a Reverse Polish Notation (RPN) calculator that works like popular HP calculators of the 1970s and 1980s. RPN notation also known as Postfix notation is a way of describing arithmetic operations such that every operator follows all of its operands. In operations with two operands both numeric operands are entered before the operation key is pressed. For example
to add 5 and 7, you type:

5 ENTER 7 +

An internal stack with four registers stores the operands until they are operated on. All four registers are visible in the display. The bottom two of these registers are known as x and y. As data values are entered they are pushed up onto the stack using the ENTER key. Operations such as add, subtract, multiply and divide pop two operands off the stack and push the result back into the x register. The result of an operation is always shown in the bottom register labeled "x" which has a larger display. Some keys show operations performed on x and y registers such as x squared or y to power of x.

∏ ─ Enters PI into x
int ─ Truncate x to its integer value
frac ─ Convert x to its fractional part following decimal point
1/x ─ Reciprocal of x
EE ─ Enter exponent while entering a value
+ ─ Add x + y
- ─ Subtract y - x
× ─ Multiply y by x
÷ ─ Divide y by x
Δ% ─ Delta percent computed from 100*(x-y)/y
x² ─ Multiply x by x
√x ─ Square root of x
  x
y ─ y to the power of x
LAST x ─ Restore x to value before the last operation
ROLL⇓ ─ Rotates the stack down moving x to top of stack
x⇔y ─ Exchanges bottom two registers on stack
CLR x ─ Clear the x register to 0
⌫ ─ Delete last digit during data entry
ENTER⇑ ─ Enter the current value. Indicates end of entry of a number before entering the next.
± -- Change sign of x register but if in data entry and EE exponent has been entered then sign changes on exponent portion of number not mantissa. To change sign of mantissa press this key before pressing EE.

0-9 -- In data entry mode (indicated by green text) digit is appended to current value in x register. If display is gray or black it means not currently in data entry mode and 1st digit pressed replaces contents of x register.

The color of the value in the x register indicates its state as follows:
BLACK ─ The result of an operation.
GRAY ─ A value that has just been ENTERd (pushed from x to y). A gray value is replaced by next data entry key.
GREEN ─ Data entry is active. Digits will be appended to current value.