The Museum of HP Calculators


HP-91 Features

Features in optional ROMs are listed as Option.
Features in HP mag card or tape Application Pacs are listed as +.
Other software is not included.
Display and Keyboard HP-91
Display type LED
Display format 1 Line
FIXed number format Yes
SCIentific number format Yes
ENGineering number format Yes
STanDard / ALL number format -
Alpha messages and prompts -
Softkeys labeled by mag. card or overlay -
Softkeys labeled by display menus -
Softkeys with neither labeling system -
Status annunciators -
Redefinable keyboard -
Logic HP-91
Logic system RPN
Stack levels 4
Last x Yes
Undo -
Precedence -
Parentheses -
Storage Addition (STO +) Yes
Storage Arithmetic (STO +, −, ×, ÷) Yes
Storage Exponentiation (STO ^) -
Recall Arithmetic -
Named variables -
Multiple data types -
Directories -
Math HP-91
+, −, ×, ÷ Yes 
1/x Yes
Yes
x2 Yes
LOG x Yes
10x Yes
LN x Yes
ex Yes
yx Yes
π Yes
% Yes
% change Yes
INTeger truncation and ABSolute value -
Trigonometric functions (SIN, COS etc.) Yes
Hyperbolic trig. (SINH, COSH etc.) -
# of trig. modes (degrees, radians, grads) 3
Advanced Math HP-91
Solver / root finder -
Numeric integration -
Complex numbers -
Matrix operations -
Matrix writer -
Equation writer -
Symbolic operations -
Fractions -
Plotting and graphics (display) -
Plotting and graphics (printer / plotter) -
Statistical HP-91
Mean, standard deviation (# of variables) 2
Σ of n, x, x2 Yes
Σ of n, x, x2, y Yes
Σ of n, x, x2, y, xy Yes
Σ of n, x, x2, y, y2, xy Yes
Σ of lists -
Random number generator -
Linear regression, estimate  Yes
Curve fitting -
Correlation coefficient -
Normal distribution -
Factorial Yes
Gamma -
Combinations and permutations -
Scientific and Time HP-91
Polar / rectangular conversions Yes
Base conversions (2, 8, 10, 16) -
Arithmetic in bases 2, 8 and 16 -
Bit operations (AND, OR, XOR, ...) -
Selectable word size (1-64 bits) -
Selectable complement mode (1's, 2's, unsigned) -
Decimal hrs / hrs.min.sec. conversions Yes
Hrs.min.sec arithmetic Yes
Time of day clock -
Timer -
Dynamic calculations (timer × rate) -
Alarms -
Unit Conversions HP-91
Centimeters / inches -
Millimeters / inches -
Meters / feet -
Kilograms / pounds (mass) -
Newtons / pounds (force) -
British thermal units / joules -
Liters / gallons -
° Celsius / ° Fahrenheit -
Other -
User-defined units -
Time Value of Money HP-91
Number of payments (n) -
Interest rate (i) -
Present Value (PV) -
Payment (PMT) -
Future Value (FV) -
Cash Flow Analysis HP-91
Net present value (NPV) -
Internal rate of return (IRR) -
Amortization HP-91
Accumulated Interest (ACC INT) -
Balance (BAL) -
Depreciation HP-91
Straight line (SL) -
Sum of the years digits (SOYD) -
Declining balance (DB) -
Bonds and calendar functions HP-91
Bond price -
Bond yield -
Date arithmetic / calendar functions -
Memory and Programming HP-91
Programming language(s) -
RAM / Max # of variables or addressable registers 16 registers
Max # of program steps -
RAM / Max vars or regs with full optional RAM -
Max # of prog steps with full optional RAM -
Merged program steps  -
Manual register / program memory allocation  -
Automatic register / program memory allocation -
Overlapped register and program memory -
Continuous memory -
Flags -
Conditional tests -
Subroutine levels -
Indexed looping (increment by +/- 1, end at 0) -
Indexed looping (choice of increment and end) -
Multiple index / loop registers -
Line number addressing -
Branching by relative line number -
Global program label addressing -
Local program label addressing -
Indirect / computed addressing and control -
Multiple indirection registers -
Pause -
Alpha messages and prompts in programs -
Alpha program listings -
Alpha string manipulation -
Program step insert / delete -
I/O and Expansion Features HP-91
ROM modules -
RAM modules or expansion option -
General expansion slots / connector -
Magnetic card reader/writer -
Tape drive -
Disk drive -
Upgradeable firmware -
Optical wand -
Serial port -
Infrared input -
Infrared output -
Printer Yes
Networking -
Other -
General HP-91
Sounds -
Self test -
Hard case (in addition to soft case) -
Additional Features -
Batteries Rechargeable
Length (inches) 9"
Width 8"
Height 2.5"
Weight (oz or lbs) 2.5 lbs
Year of introduction 1976
Features in optional ROMs are listed as Option.
Features in HP mag card or tape Application Pacs are listed as +.
Other software is not included.

Unlim means limited only by available memory.
HP-91 notes:

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