Features in optional ROMs are listed as Option. 

Display and Keyboard  HP81 
Display type  LED 
Display format  1 Line 
FIXed number format  Yes* 
SCIentific number format  Modified* 
ENGineering number format   
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  HP81 
Logic system  RPN 
Stack levels  4 
Last x   
Undo   
Precedence   
Parentheses   
Storage Addition (STO +)   
Storage Arithmetic (STO +, , ×, ÷)   
Storage Exponentiation (STO ^)   
Recall Arithmetic   
Named variables   
Multiple data types   
Directories   
Math  HP81 
+, , ×, ÷  Yes 
1/x   
sq. root  Yes 
x^{2}   
LOG x  Yes 
10^{x}   
LN x   
e^{x}   
y^{x}  Yes 
pi   
%  Yes 
% change  Yes 
INTeger truncation and ABSolute value   
Trigonometric functions (SIN, COS etc.)   
Hyperbolic trig. (SINH, COSH etc.)   
# of trig. modes (degrees, radians, grads)   
Advanced Math  HP81 
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  HP81 
Mean, standard deviation (# of variables)  1 
Summation of n, x, x^{2}  Yes 
Summation of n, x, x^{2}, y   
Summation of n, x, x^{2}, y, xy   
Summation of n, x, x^{2}, y, y^{2}, xy   
Summation of lists   
Random number generator   
Linear regression, estimate  Yes 
Curve fitting  Yes 
Correlation coefficient   
Normal distribution   
Factorial   
Gamma   
Combinations and permutations   
Scientific and Time  
Polar / rectangular conversions   
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   
Hrs.min.sec arithmetic   
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  HP81 
Number of payments (n)  Yes 
Interest rate (i)  Yes 
Present Value (PV)  Yes 
Payment (PMT)  Yes 
Future Value (FV)  Yes 
Cash Flow Analysis  HP81 
Net present value (NPV)   
Internal rate of return (IRR)  Yes 
Amortization  HP81 
Accumulated Interest (ACC INT)  Yes 
Balance (BAL)  Yes 
Depreciation  HP81 
Straight line (SL)  Yes 
Sum of the years digits (SOYD)  Yes 
Declining balance (DB)  Yes 
Bonds and calendar functions  HP81 
Bond price  Yes 
Bond yield  Yes 
Date arithmetic / calendar functions  Yes 
Memory and Programming  HP81 
Programming language(s)   
RAM / Max # of variables or addressable registers  20 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  HP81 
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   
Networking   
Other   
General  HP81 
Sounds   
Self test   
Hard case (in addition to soft case)  Yes 
Additional Features   
Batteries   
Length (inches)  15.4" 
Width  10.9" 
Height  5.6" 
Weight (oz or lbs)  13.2 lbs 
Year of introduction  1973 
Features in optional ROMs are listed as Option. 
HP81 notes:

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