The first Soviet programmable calculator, the B3-21, came in two versions. More common is the one with a red LED display; however, machines were also produced using a green vacuum fluorescent display tube.

Why, I wonder? Based on the fact that most subsequent Soviet models used vacuum fluorescent displays, and based further on the fact that the LED modules in my B3-21s appear to be Western made, a possible explanation presents itself: in an economy starved for hard currency, solutions were favored that could replace Western imports with domestically produced parts.

Apart from the display, my newest B3-21 is identical in all respects to the red LED version. This even includes the way the display is utilized. Perhaps uniquely among VFD machines, the "green" B3-21 reserves a full digit position for the decimal point, and accordingly, the decimal point is placed in the center of the lower half of the 7-segment digit, as in HP LED display modules.

When I first wrote about the B3-21, it seemed to me that its somewhat ineffective programming model (a most annoying "feature" is the lack of an automatic stack lift before register retrievals) makes it impossible to write a useful implementation of my favorite example, the Gamma function. This is not so. Stirling's formula can, in fact, be used to write a program that computes the logarithm of the Gamma function 6+ digits of precision for all positive and negative arguments. This program uses a simple iteration to compute the result for arguments smaller than 5; consequently, for negative arguments with a large magnitude, the algorithm can be slow. For positive and small negative numbers, however, the algorithm is reasonably fast (this is a mighty slow machine!) and accurate.

01 06	^
02 21	P 2
03 14	1
04 41	P 4
05 16	x-y
10 58	8
11 86	-
12 69	x<0
13 32	32 [F 3]
14 42	F 4
15 26	×
20 41	P 4
21 16	x-y
22 14	1
23 96	+
24 21	P 2
25 06	^
30 58	BP
31 06	06 [^]
32 22	F 2
33 06	^
34 13	ln
35 26	×
40 86	-
41 31	P 3
42 23	π
43 24	2
44 26	×
45 06	^
50 22	F 2
51 36	÷
52 65	√
53 06	^
54 42	F 4
55 36	÷
60 13	ln
61 06	^
62 32	F 3
63 86	-
64 31	P 3
65 34	3
70 04	0
71 45	1/x
72 06	^
73 22	F 2
74 55	x^2
75 36	÷
80 56	/-/
81 14	1
82 96	+
83 06	^
84 22	F 2
85 36	÷
90 14	1
91 24	2
92 36	÷
93 06	^
94 32	F 3
95 96	+
−0 78	C/П