HP HP-21 - Automatic Display Switching

20 Controlling the Display
In scientific notation display, the HP-21 contains a ten-digit
number and a two-digit exponent of ten inside the calculator,
even though it displays only out to seven digits after the decimal
point. So although [DsP] 7, [DsP]8, and [DSP]9 may affect the
rounding of the display, the calculator maintains full accuracy
internally.
For example, if you key in 1.000000094 and specify full scien-
tific notation display ([psp] 7), the calculator rounds off to the
seventh digit after the decimal point: 1.000000094
A
Calculator rounds to this digit in 7.
Press Display
1.000000094
7 1.0000001 00
In 8 , the calculator rounds off to the eighth digit after the
decimal point, but you see only out to seven digits after the
decimal: 1.000000094
You see to here . . _TT_ .. l?ut the calculator rounds to here
in 8.
Press Display
8 1.0000000 00
You can see that if you had keyed in 1.000000095, 8 would
also have caused the seventh and final displayed digit to be
rounded to a one (1).
Automatic Display Switching
The HP-21 features automatic overflow and underflow that
switch the display to full scientific whenever the number is too
large or too small for fixed decimal point display. For example,
solve (.05)%:
Press Display
CLX 0.0000000 00 7 from previous
example.
By Normal fix 2 display.
05
05 1