LXVII is a faithful simulation, but not exact emulator, of the HP-67. It has the same number memory registers and program steps as the original.

The stack depth is the same (4 cells), the button and prefix key layout is also the same, (which unfortunately makes its on-screen footprint a bit larger than it was intended) it displays the same key-codes per program step, it even has a slider switch for selecting between user and program mode.

The impetus for keeping that level of compatibility was to give the owner of an HP-67 the ability to immediately use the application without needing to worry about differences from what he or she is used to.

New features were created with the intent that they extend existing features in a matter that was compatible with existing behavior.

Here are some key features of "LXVII":

· Unlike most other calculator programs, LXVII is document based. This means you can have multiple calculators, running multiple programs, active at the same time. LXVII takes advantage of Cocoa threads, so it also takes advantage of as many CPUs as are available on the system.
· The document based architecture allows users to redefine LXVII's functionality so that each document is essentially a new calculator, tailored to the user's specific task.
· A paper tape panel like that found on the hp-97 desktop calculator is provided for printing out memory registers, intermediate stack values and program listings.
· A pull-out drawer can display the current values of the stack, the last-X register, the program counter, all memory locations and all program steps in memory. All values except the program steps can be edited directly within the drawer interface.
· All buttons have an assigned single-keystroke key so that the user can get to all calculator functions without the need of the mouse. To help remember the keystrokes, all buttons and button labels have active tooltips specifying the button's functions.
· The 5 programmable buttons on the first row (A-E) can be assigned labels and tooltip messages that are saved with the document.