Chapter 20 The HP Model 9100A computing calculator 247

operations must be performed repeatedly or an iterative technique must be used. A program library supplied with the Model 9100A provides a set of representative programs from many different fields. If a program cannot be found in the library to solve a particular problem, a new program can easily be written since no special experience or prior knowledge of a programming language is necessary.

Any key on the keyboard can be remembered by the calculator as a program step except STEP PRGM. This key is used to 'debug' a program rather than as an operation in a program. Many individual program steps, such as 'sin x' or 'to polar' are comparatively powerful. and avoid the need of sub-routines for these functions and the programming space such sub-routines require. Registers 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, a, b, c, d can store 14 program steps each. Steps within the registers are numbered 0 through d just as the registers themselves are numbered. Programs can start at any of the 196 possible addresses. However 0-0 is usually used for the first step. Address d-d is then the last available, after which the program counter cycles back to 0-0.

Registers f and e are normally used for storage of constants only, one constant in each register. As more constant storage is required, it is recommended that registers d, then c, then b, etc., are used starting from the bottom of the list. Lettered registers are used first, for the frequently recalled constants, because constants stored in them are more easily recalled. A register can be used to store one constant or 14 program steps, but not both.

The bank on the far right of the keyboard, Fig. 4, contains program oriented keys. [GO TO () ()] is used to set the program counter. The two sets of parentheses indicate that this key should be followed by two more key depressions indicating the address of the program step desired. As a program step, 'GO TO' is an unconditional branch instruction, which causes the program to branch to the address given by the next two program steps. The 'IF' keys in this group are conditional branch instructions. With [IF x<y] [IF x =y], and [IF x>y] the numbers contained in the X and Y registers are compared. The indicated condition is tested and, if met, the next two program steps are executed. If the first is alphameric, the second must be also, and the two steps are interpreted as a branching address. When the condition is not met, the next two steps are skipped and the program continues. [IF FLAG] is also a very useful conditional branching instruction which tests a 'yes' or 'no' condition internally stored in the calculator. This condition is set to 'yes' with the SET FLAG from the keyboard when the calculator is in the display mode or from a program as a program step. The flag is set to a 'no' condition by either asking IF FLAG in a program or by a CLEAR instruction from the keyboard or from a program.

Data can be entered for use in a program when the machine is in the display mode. (The screen is blank while a program is running.) A program can be stopped in several ways. The [STOP] key will halt the machine at any time. The operation being performed will be completed before returning to the display mode. As a program step, STOP stops the program so that answers can be displayed or new data entered. END must be the last step in a program listing to signal the magnetic card reader; when encountered as a program step it stops the machine and also sets the program counter to 0-0.

As a program step, PAUSE causes a brief display during program execution. Nine cycles of the power line frequency are counted-the duration of the pause will be about 150 ms for a 60 Hz power line or 180 ms for a 50 Hz power line. More pauses can be used in sequence if a longer display is desired. While a program is running the PAUSE key can be held down to stop the machine when it comes to the next PAUSE in the program. PAUSE provides a particularly useful way for the user and the machine to interact. It might, for instance, be used in a program so that the convergence to a desired result can be observed.

Other means of input and output involve peripheral devices such as an X-Y Plotter or a Printer. The PRINT key activates the printer, causing it to print information from the display register. As a program step, PRINT will interrupt the program long enough for the data to be accepted by the printer and then the program will continue. If no printer is attached, PRINT as a program step will act as a STOP. The FMT key, followed by any other keystroke, provides up to 62 unique commands to peripheral equipment. This flexibility allows the Model 9100A to be used as a controller in small systems.

A simple program to calculate N! demonstrates how the Model 9100A is programmed. Figure 6 (top) shows a flow chart to compute N! and Fig. 6 (bottom) shows the program steps. With this program, 60! takes less than 1/2 second to compute.

After a program is written it can be entered into the Model 9100A from the keyboard. The program counter is set to the address of