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As with most other calculator programming languages of the 1980s, a FOCAL program is a linear list of instructions. Each instruction (or step) roughly correspond to a key (or key combination) press, and thus correspond to what the user would do if he was to perform the computation himself on the calculator. This paradigm made FOCAL programming relatively easy for the newcomer, but program maintenance could be a nightmare. Besides that, the use of GOTO instructions—including going to a program step based on a value in a data register ("GTO IND"), a technique known as computed goto—, easily led to spaghetti code.
Program steps are numbered starting from 1, but this numbering has no intrinsic meaning, and can change as new instructions are added or removed in the middle of a program. A special instruction LBL is used to create a label, that can be used by the user to invoke the program, or by the program itself, as target of a GTO (unconditional go-to) or XEQ (execute) instruction.
Here is a very simple FOCAL program:
001 LBL "DOUBLE" 002 2 003 * 004 END
- The LBL "DOUBLE" instruction is a label, indicating that the user can invoke the program by XEQ "DOUBLE". The name "DOUBLE" can also be assigned to any key on the user keyboard, effectively adding a new function to the calculator.
- The next two steps correspond to what the function does, mainly multiply the X register by 2 (the HP-41 uses Reverse Polish Notation.)
- The END instruction indicates the end of the program.
Note that contrary to many other programmable calculators, each FOCAL step could be stored in memory as more than one byte. While the calculator memory could hold 2233 bytes of program code, no reported program actually reached this number of steps.
- HP-41C/CV/CX Programming – from The Museum of HP Calculators (MoHPC)