Java capable browser required Even before I acquired a working HP-25 specimen, I was already able to experiment with this wonderful small calculator thanks to an amazing Java simulator. To use, wait for the R/S button to appear on the left (you're downloading about 140 kB of data), and then click on it. If your browser supports Java, the calculator will appear. (This HP-25 simulator was written by Larry Leinweber and placed in the public domain. Calculator image is courtesy of The Museum of HP Calculators.) This version of the simulator is preprogrammed with the program shown below; however, you can use it for any other purpose if you clear the program (f PRGM) and registers (f REG).

Introduced shortly after the HP-65, the HP-25 was another source of amazement to its fans. Despite the much lower price, it was also a programmable calculator, with 49 program steps and 8 memory registers. Program steps were fully merged, so the HP-25's 49 steps were nearly equivalent to the 100 steps of the HP-65.

The simulator is sufficiently reliable and accurate to do some programming. This is how I was able to develop yet another implementation of the Gamma function. This program is of a somewhat reduced precision, which was the only way to make it fit within the calculator's limited program memory; in fact, this implementation is very similar to the one I wrote for the TI-57, showing that these calculators are pretty much in the same league.

To use the program, load it into memory and also initialize registers 2-7. Then, enter the argument and hit the R/S button.

M2=√2π
M3=68.82784822
M4=755.9596084
M5=4151.488796
M6=11399.36541
M7=12520.43913

01     23 00	STO  0
02     23 01	STO  1
03     24 02	RCL  2
04  23 61 00	STO× 0
05     24 03	RCL  3
06  23 51 00	STO+ 0
07     24 01	RCL  1
08  23 61 00	STO× 0
09     24 04	RCL  4
10  23 51 00	STO+ 0
11     24 01	RCL  1
12  23 61 00	STO× 0
13     24 05	RCL  5
14  23 51 00	STO+ 0
15     24 01	RCL  1
16  23 61 00	STO× 0
17     24 06	RCL  6
18  23 51 00	STO+ 0
19     24 07	RCL  7
20     24 01	RCL  1
21        71	÷
22  23 51 00	STO+ 0
23        05	5
24        31	ENTER
25        01	1
26  23 51 01	STO+ 1
27        41	-
28     24 01	RCL  1
29  23 71 00	STO÷ 0
30        21	x-y
31     15 61	x!=0
32     13 25	GTO 25
33     24 01	RCL  1
34        73	.
35        05	5
36        51	+
37        31	ENTER
38        32	CHS
39     15 07	ex
40        21	x-y
41        31	ENTER
42        31	ENTER
43        05	5
44        41	-
45     14 03	yx
46        61	×
47     24 00	RCL 0
48        61	×