Python implementations of the sort of six-bit, two-tap, XNOR-based polynomial counter (linear feedback shift register) used in several places within the Atari VCS's TIA graphics chip.
tia_counter.py defines two classes:
- TiaCounterV1 - Represents the register bits internally as a list of boolean values, manipulated "manually" using standard boolean operators
- TiaCounterV2 - Stores register contents in a single integer variable, manipulated using bit-wise operators
V2 is closer in spirit to how the TIA chip actually works, but V1 might be a bit more intuitive to understand.
tia_counter_demo.py demonstrates instantiating both classes and cycling through all their possible states. The output (reproduced below) shows that their states are identical to each other at each period.
Note that the states are also identical to those produced by the real TIA counter circuits, as outlined in Andrew Towers' Atari 2600 TIA Hardware Notes.
i List Counter Bit Counter Match?
-- ------------ ----------- ------
1 000000 0 000000 0 Yes
2 100000 32 100000 32 Yes
3 110000 48 110000 48 Yes
4 111000 56 111000 56 Yes
5 111100 60 111100 60 Yes
6 111110 62 111110 62 Yes
7 011111 31 011111 31 Yes
8 101111 47 101111 47 Yes
9 110111 55 110111 55 Yes
10 111011 59 111011 59 Yes
11 111101 61 111101 61 Yes
12 011110 30 011110 30 Yes
13 001111 15 001111 15 Yes
14 100111 39 100111 39 Yes
15 110011 51 110011 51 Yes
16 111001 57 111001 57 Yes
17 011100 28 011100 28 Yes
18 101110 46 101110 46 Yes
19 010111 23 010111 23 Yes
20 101011 43 101011 43 Yes
21 110101 53 110101 53 Yes
22 011010 26 011010 26 Yes
23 001101 13 001101 13 Yes
24 000110 6 000110 6 Yes
25 000011 3 000011 3 Yes
26 100001 33 100001 33 Yes
27 010000 16 010000 16 Yes
28 101000 40 101000 40 Yes
29 110100 52 110100 52 Yes
30 111010 58 111010 58 Yes
31 011101 29 011101 29 Yes
32 001110 14 001110 14 Yes
33 000111 7 000111 7 Yes
34 100011 35 100011 35 Yes
35 110001 49 110001 49 Yes
36 011000 24 011000 24 Yes
37 101100 44 101100 44 Yes
38 110110 54 110110 54 Yes
39 011011 27 011011 27 Yes
40 101101 45 101101 45 Yes
41 010110 22 010110 22 Yes
42 001011 11 001011 11 Yes
43 100101 37 100101 37 Yes
44 010010 18 010010 18 Yes
45 001001 9 001001 9 Yes
46 000100 4 000100 4 Yes
47 100010 34 100010 34 Yes
48 010001 17 010001 17 Yes
49 001000 8 001000 8 Yes
50 100100 36 100100 36 Yes
51 110010 50 110010 50 Yes
52 011001 25 011001 25 Yes
53 001100 12 001100 12 Yes
54 100110 38 100110 38 Yes
55 010011 19 010011 19 Yes
56 101001 41 101001 41 Yes
57 010100 20 010100 20 Yes
58 101010 42 101010 42 Yes
59 010101 21 010101 21 Yes
60 001010 10 001010 10 Yes
61 000101 5 000101 5 Yes
62 000010 2 000010 2 Yes
63 000001 1 000001 1 Yes
64 000000 0 000000 0 Yes