238 Part 3÷ The instruction-set processor level: variations in the processor
Section 4÷ Desk calculator computers: keyboard processors with small memories
b, B, c, C, d, D, e, F, f and F. The lowercase designation is obtained by first entering the corresponding uppercase letter and the depressing the "/" key, for example, c º C/.
The registers D, E, and F or their splits have the additional capability of storing either instructions or constants to be used within programs. Thus they can store 1 signed 22-digit number, 2 signed 11-digit numbers, 1 signed 11-digit number, and 11 instructions, or 24 instructions. Programs of up to 120 instructions can be stored internally (Fig. 1). When registers D, E, and F and their splits are not used for instructions, they are free to store constants or intermediate results.
The relationship of memory, keyboard, printer and magnetic card is shown in Fig. 1. Registers are referenced explicitly. Pro grams do not use explicit addresses in instruction. Thus, special marker characters are placed in the instructions to serve as jump reference addresses (program labels).
Fig. 1. Programma 101 functional block diagram. (Courtesy of Olivetti Underwood Corporation.)
Fig. 2. Programma 101. (Courtesy of Olivetti Underwood Corporation.)
The calculator parts are described briefly below. The parts correspond to both the numbers (Fig. 2) and the lettered keyboard (Fig. 3). The following parts are, in effect, the console. Some of the keys are used for control of the calculator, and some can be used either as programmed instructions or as commands which are executed directly. The following section discusses their instruction function.
The on-off key (1). This is a dual-purpose switch for both the on and off positions. (Note: The OFF position automatically clears all stored data and instructions.)
The error (red) light (2). This lights when the computer is turned on and whenever the computer detects an operational error, e.g., exceeding capacity, division by zero.
The general reset key (3). This key erases all data and instructions from the computer and turns off the error light.
The correct-performance (green) light (4). This light indicates the computer is functioning properly. A steady light indicates that the computer is ready for an operator decision; a flickering light indicates that the computer is executing programmed instructions and that the keyboard is locked.
The decimal wheel (5). This determines the number of decimal places (0, 1 15) to which computations will be carried out in the A register and the decimal places in the printed output, except for results from the R register. Up to 22 decimal digits may be developed in, and printed from, the R register.