Features in optional ROMs are listed as Option. 

Display and Keyboard  HP91 
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  HP91 
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  HP91 
+, , ×, ÷  Yes 
1/x  Yes 
sq. root  Yes 
x^{2}  Yes 
LOG x  Yes 
10^{x}  Yes 
LN x  Yes 
e^{x}  Yes 
y^{x}  Yes 
pi  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  HP91 
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  HP91 
Mean, standard deviation (# of variables)  2 
Summation of n, x, x^{2}  Yes 
Summation of n, x, x^{2}, y  Yes 
Summation of n, x, x^{2}, y, xy  Yes 
Summation of n, x, x^{2}, y, y^{2}, xy  Yes 
Summation of lists   
Random number generator   
Linear regression, estimate  Yes 
Curve fitting   
Correlation coefficient   
Normal distribution   
Factorial  Yes 
Gamma   
Combinations and permutations   
Scientific and Time  
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 (164 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  
Centimeters / inches   
Millimeters / inches   
Meters / feet   
Kilograms / pounds (mass)   
Newtons / pounds (force)   
British thermal units / joules   
Liters / gallons   
° Celsius / ° Fahrenheit   
Other   
Userdefined units   
Time Value of Money  HP91 
Number of payments (n)   
Interest rate (i)   
Present Value (PV)   
Payment (PMT)   
Future Value (FV)   
Cash Flow Analysis  HP91 
Net present value (NPV)   
Internal rate of return (IRR)   
Amortization  HP91 
Accumulated Interest (ACC INT)   
Balance (BAL)   
Depreciation  HP91 
Straight line (SL)   
Sum of the years digits (SOYD)   
Declining balance (DB)   
Bonds and calendar functions  HP91 
Bond price   
Bond yield   
Date arithmetic / calendar functions   
Memory and Programming  HP91 
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  HP91 
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  HP91 
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. 
HP91 notes: 
Go to the feature comparison page
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