246 Part 3 The instruction-set processor level: variations in the processor

them. Special keys located in a block to the left of the digit keys are used to identify the lettered registers.

To store a number from the X register the key [x® ()] is used. The parenthesis indicates that another key depression, representing the storage register, is necessary to complete the transfer. For example, storing a number from the X register into register 8 requires two key depressions: [x® ()] [8]. The X register remains unchanged. To store a number from Y register the key [y® ()] is used.

The contents of the alpha registers are recalled to X simply by pressing the keys a, b, c, d, e, and f. Recalling a number from a numbered register requires the use of the [x® ¬ y] key to distinguish the recall procedure from digit entry. This key interchanges the number in the Y register with the number in the register indicated by the following keystroke, alpha or numeric, and is also useful in programs since neither number involved in the transfer is lost.

The CLEAR key sets the X, Y, and Z display registers and the f and e registers to zero. The remaining registers are not affected. The f and e registers are set to zero to initialize them for use with the [ACC+] and [ACC-] keys as will be explained. In addition the CLEAR key clears the FLAG and the ARC and HYPER conditions, which often makes it a very useful first step in a program.

Vectors and complex numbers are easily handled using the keys in the column on the far left of the keyboard. Figure 5 defines the variables involved. Angles can be either in degrees or radians. To convert from rectangular to polar coordinates, with y in Y and x in X, press [TO POLAR]. Then the display shows q in Y and R in X. In converting from polar to rectangular coordinates, q is placed in Y, and R in X, [TO RECT], is pressed and the display shows yin Y and x in X.

ACC + and ACC - allow addition or subtraction of vector components in the f and e storage registers. ACC + adds the contents of the X and Y register to the numbers already stored in f and e respectively; ACC - subtracts them. The RCL key recalls the numbers in the f and e registers to X and Y.

A light to the left of the CRT indicates that an illegal operation has been performed. This can happen either from the keyboard or when running a program. Pressing any key on the keyboard will reset the light. When running a program, execution will continue but the light will remain on as the program is completed. The illegal operations are:

The Model 9100A does all calculations using floating point arithmetic with a twelve digit mantissa and a two digit exponent. The two least significant digits are not displayed and are called guard digits.

The algorithms used to perform the operations and generate the functions were chosen to minimize error and to provide an extended range of the argument. Usually any inaccuracy will be contained within the two guard digits. In certain cases some inaccuracy will appear in the displayed number. One example is where the functions change rapidly for small changes in the argument, as in tan x where x is near 90° . A glaring but insignificant inaccuracy occurs when an answer is known to be a whole number, but the least significant guard digit is one count low: 2.000 000 000 ~ 1.999 999 999.

Accuracy is discussed further in the 'Internal Programming' section in this chapter. But a simple summary is: the answer resulting from any operation or function will lie within the range of true values produced by a variation of + 1 count in the tenth digit of the argument.

Problems that require many keyboard operations are more easily solved with a program. This is particularly true when the same