A surprisingly interesting calculator, this TI-65. Its shape is reminiscent of other "Galaxy" series calculators, like the TI-66. Its buttons look like the buttons on more modern TI calculators, such as the TI-68. Its features, however, are unlike any TI machine I've seen before.

No, nothing dramatic here, just a curious mixture of the old and the new.

This is a keystroke programmable calculator, but a rare kind in TI's lineup; a machine with a fully merged programming model. Most 2-3 keystroke sequences count as a single instruction in program memory. The size of program memory is quite decent; 100 fully merged program steps can be used for many things.

The calculator also has many advanced programming features, including conditional branching, labels, and subroutines. It also has a DSZ loop instruction. It also has a built-in numerical integration capability that evaluates the integral of a function implemented in the form of a user program.

But perhaps the most unusual feature on a programmable TI calculator is an HP-55 style up-down timer (albeit with only a 1-second resolution). The timer is started using the TUp or TDn buttons; while the timer is running, it is possible to perform other calculations, or keeping the timer on the display, it is possible to save "lap times" into memory registers by simply pressing the appropriate register's number.

The following program example uses iteration to accurately compute the logarithm of the Gamma function using a modified version of Stirling's formula, for any argument, including negative arguments whose integer part is odd (e.g., -3.5):

2nd 53.53  00	LBL F1
    12.0   01	STO 0
    1      02	1
    12.1   03	STO 1
2nd 53.0   04	LBL 0
    5      05	5
3rd 42     06	x<m
    0      07	0
2nd 54.1   08	GTO 1
    13.0   09	RCL 0
    12.38  10	STO×
    1      11	1
    1      12	1
    12.59  13	STO+
    0      14	0
2nd 54.0   15	GTO 0
2nd 53.1   16	LBL 1
    13.0   17	RCL 0
    38     18	×
    32     19	LN
    49     20	-
    13.0   21	RCL 0
    49     22	-
    13.1   23	RCL 1
    32     24	LN
    59     25	+
    16     26	(
    2      27	2
    38     28	×
2nd 17     29	π
    28     30	÷
    13.0   31	RCL 0
    17     32	)
    33     33	√
    32     34	LN
    59     35	+
    16     36	(
    16     37	(
    16     38	(
    16     39	(
    9      40	9
    9      41	9
    34     42	1/x
    28     43	÷
    13.0   44	RCL 0
2nd 33     45	x2
    49     46	-
    1      47	1
    4      48	4
    0      49	0
    34     50	1/x
    17     51	)
    28     52	÷
    13.0   53	RCL 0
2nd 33     54	x2
    59     55	+
    1      56	1
    0      57	0
    5      58	5
    34     59	1/x
    17     60	)
    28     61	÷
    13.0   62	RCL 0
2nd 33     63	x2
    49     64	-
    3      65	3
    0      66	0
    34     67	1/x
    17     68	)
    28     69	÷
    13.0   70	RCL 0
2nd 33     71	x2
    59     72	+
    1      73	1
    17     74	)
    28     75	÷
    1      76	1
    2      77	2
    28     78	÷
    13.0   79	RCL 0
    39     80	=
2nd 52     81	RTN