I’ve been playing with the Atari 2600 ever since I saw Halo 2600. It inspired me to delve once more into the realm of retro game development, and I’ve been gathering some notes related to it. Here’s a bit of summarized information I’ve collected:
Drawing the screen
The screen has 262 total scanlines of 160 pixels each. Each pixel is one “color-clock” of the TIA (Television Interface Adapter), and there are 228 color-clocks for each scanline. The first 160 color-clocks of a scanline are for the display, while the remaining 68 are for horizontal blank (retrace). For every clock on the Atari’s 6502 (actually a 6508 but it hardly matters), 3 color-clocks on the TIA occur. Of the 262 scanlines, not all are used for the display. Below is a detailed look at how many scanlines are used where.
Scanline information
- 3 scanlines of vertical synchronization
- 37 scanlines of vertical blank
- 192 scanlines of display
- 30 scanlines of overscan
An introduction to the TIA
| Register | Brief description |
|---|---|
WSYNC |
A write halts the 6502 until the next scanline |
COLUBK |
Sets the color of the background |
COLUPF |
Sets the color of the playfield |
PF0 |
Top 4 bits of the playfield (reversed) |
PF1 |
Middle 8 bits of the playfield |
PF2 |
Bottom 8 bits of the playfield (reversed) |
CTRLPF |
Controls the state of the playfield and the ball |
Anyways, more to come later, such as a description of this mystical “playfield.”