The HP-20S is a member of Hewlett-Packard's current line of calculators. When I first held one in my hands, I was very pleasantly surprised; despite the low cost, the HP-20S feels like a quality Hewlett-Packard device, and it has all the features most folks will ever need from a programmable scientific calculator.

Like its famous cousins from the past, the HP-20S is a keystroke programmable calculator. This method of programming has been largely abandoned these days in favor of various calculator programming languages. A pity, I say; when I program a calculator, I usually have no need for an elaborate, structured, or object-oriented programming language, what I want to do is capture short key sequences; which is precisely what keystroke programming is all about.

One feature that Hewlett-Packard did abandon with the HP-20S is the RPN (Reverse Polish Notation) entry logic. No doubt they were responding to market forces; for most calculator users, RPN remains an incomprehensible mystery. Still, it's a pity; once you know how to use it, RPN is much more efficient than algebraic entry, especially when used for keystroke programming. Oh well.

Algebraic logic notwithstanding, the HP-20S is a pleasure to use. Although this is not the purpose of my calculator-related Web pages, I heartily recommend the HP-20S to everyone looking for a low-cost, high-quality calculating device.

As a matter of interest, the HP-20S is very similar in terms of its capabilities to Hewlett-Packard's first programmable handheld device, the HP-65. The HP-20S has 10 memory registers whereas the HP-65 has nine; both calculators have 99 steps of program memory. The HP-20S has no magnetic card reader, but it has continuous memory, and it also contains several programs in its built-in program library. Of course there is a huge difference in price; whereas the HP-65 cost 800 dollars in 1975, the HP-20S can be purchased for under 40 dollars today.

Needless to say, I have written a Gamma function implementation for this calculator as well. Due to the limited size of program memory (99 steps) it was necessary to make use of registers for storing constants. The program uses 8 out of the calculator's ten registers (6 of which must be preset before running the program), and 92 out of 100 program steps. It usually yields a Gamma function value with ten digits of precision.

M3: 2.50662827511
M4: 83.8676043424
M5: 1168.92649479
M6: 8687.24529705
M7: 36308.2951477
M8: 80916.6278952
M9: 75122.6331530

01 -   61 41 A	LBL A
02 -   21 1	STO 1
03 -   1	1
04 -   21 0	STO 0
05 -   61 41 1	LBL 1
06 -   0	0
07 -   31	INPUT
08 -   22 1	RCL 1
09 -   61 42	x<=y?
10 -   51 41 2	GTO 2
11 -   21 45 0	STO÷ 0
12 -   1	1
13 -   21 75 1	STO+ 1
14 -   51 41 1	GTO 1
15 -   61 41 2	LBL 2
16 -   71	C
17 -   22 1	RCL 1
18 -   55	×
19 -   22 3	RCL 3
20 -   75	+
21 -   22 4	RCL 4
22 -   74	=
23 -   55	×
24 -   22 1	RCL 1
25 -   75	+
26 -   22 5	RCL 5
27 -   74	=
28 -   55	×
29 -   22 1	RCL 1
30 -   75	+
31 -   22 6	RCL 6
32 -   74	=
33 -   55	×
34 -   22 1	RCL 1
35 -   75	+
36 -   22 7	RCL 7
37 -   74	=
38 -   55	×
39 -   22 1	RCL 1
40 -   75	+
41 -   22 8	RCL 8
42 -   75	+
43 -   22 9	RCL 9
44 -   45	÷
45 -   22 1	RCL 1
46 -   74	=
47 -   45	÷
48 -   1	1
49 -   21 75 1	STO+ 1
50 -   22 1	RCL 1
51 -   45	÷
52 -   1	1
53 -   21 75 1	STO+ 1
54 -   22 1	RCL 1
55 -   45	÷
56 -   1	1
57 -   21 75 1	STO+ 1
58 -   22 1	RCL 1
59 -   45	÷
60 -   1	1
61 -   21 75 1	STO+ 1
62 -   22 1	RCL 1
63 -   45	÷
64 -   1	1
65 -   21 75 1	STO+ 1
66 -   22 1	RCL 1
67 -   45	÷
68 -   33	(
69 -   22 1	RCL 1
70 -   75	+
71 -   1	1
72 -   74	=
73 -   55	×
74 -   73	.
75 -   5	5
76 -   21 75 1	STO+ 1
77 -   22 1	RCL 1
78 -   14	yx
79 -   33	(
80 -   22 1	RCL 1
81 -   65	-
82 -   5	5
83 -   74	=
84 -   13	LN
85 -   65	-
86 -   22 1	RCL 1
87 -   74	=
88 -   12	ex
89 -   55	×
90 -   22 0	RCL 0
91 -   74	=
92 -   61 26	RTN