788 **Part 4½ Family Range, Compatibility,
and Evolution**
**Section 3½ Evolution of HP Calculators**

numbers are stored in "floating point arithmetic." A floating point number is expressed with the decimal point following the first digit and an exponent representing the number of places the decimal point should be moved¾ to the right if the exponent is positive, or to the left if the exponent is negative.

The FIXED POINT mode displays numbers in the way they are most commonly
written. The DECIMAL DIGITS wheel allows setting the number of digits displayed
to the right of the decimal point anywhere from 0 to 9. Figure 2 shows
a display of three numbers with the DECIMAL DIGITS wheel set at 5. The
number in the Y register, 5.336 845 815 ´
10^{5} = 533 684.5815, is too big to be displayed in FIXED POINT
without reducing the DECIMAL DIGITS setting to 4 or less. If the number
is too big for the DECIMAL DIGITS setting, the register involved reverts
automatically to floating point to avoid an apparent overflow. In FIXED
POINT display, the number displayed is rounded, but full significance is
retained in storage for calculations.

To improve readability, 0's before the displayed number and un-entered 0's following the number are blanked. In FLOATING POINT, digits to the right of the decimal are grouped in threes.

*Pull-out Instruction Card*

A pull-out instruction card, Fig. 3, is located at the front of the calculator under the keyboard. The operation of each key is briefly explained and key codes are listed. Some simple examples are provided to assist those using the machine for the first time or to refresh the memory of an infrequent user. Most questions regarding the operation of the Model 9100A are answered on the card.

*Data Entry*

The calculator keyboard is shown in Fig. 4. Numbers can be entered into
the X register using the digit keys, the p key
or the ENTER EXP key. The ENTER EXP key allows powers of 10 to be entered
directly which is useful for very large or very small numbers. 6.02 ´
10^{23} is entered [6][.][0][2][ENTER EXP][2][3].
If the ENTER EXP key is the first key o a number entry, a 1 is automatically
entered into the mantissa. Thus only two keystrokes [ENTER
EXP][6] suffice to enter 1,000,000. The CHG SIGN key changes the
sign of either the mantissa or the exponent depending upon which one is
presently being addressed. Numbers are entered in the same way, regardless
of whether the machine is in FIXED POINT

or FLOATING POINT. Any key, other than a digit key, decimal point, CHG SIGN or ENTER EXP, terminates an entry; it is not necessary to clear before entering a new number. CLEAR X sets the X register to 0 and can be used when a mistake has been made in a number entry.