Atari Video Computer System / 2600

Atari Video Computer System / 2600

· The original version of the VCS was designed to have a game built-in (Combat for the Atari version and Air-Sea Battle for the Tele-Games version). There’s a space on the motherboard for the ROM chip, which does work – however, there’s no circuit to switch between the ROM socket and a cart.

· The original VCS was designed to output sound via 2 internal speakers, instead of a TV (via the RF box).  The molding on the top-half of the system’s casing has mounting areas for the speakers, as well as slits on the top.  This also explains why the TIA has 2 separate sound channel outputs, why Combat and Air-Sea Battle are the only 2 games that Atari made which were programmed for stereo sound, and why neither manual for those original releases describes how to use the cart with the system.

· BUG: The black lines that are seen in some games (on the left side of the screen) are the result of using an HMOVE command trick.  If an HMOVE is initiated immediately after HBlank starts (which is the case when HMOVE is used as documented), the [HMOVE] signal is latched and used to delay the end of the HBlank by exactly 8 CLK, or two counts of the HSync Counter. This is achieved in the TIA by resetting the HB (HBlank) latch on the [LRHB] (Late Reset H-Blank) counter decode rather than the normal [RHB] (Reset H-Blank) decode. The extra HBlank time shifts everything except the Playfield right by 8 pixels, because the position counters will now resume counting 8 CLK later than they would have without the HMOVE. This is also the source of the HMOVE 'comb' effect; the extended HBlank hides the normal playfield output for the first 8 pixels of the line. Some companies (such as Activision) got around this by using the trick on every scanline, which effectively blacked out the screen area where the lines appear.  For more information about the TIA and how it works, check out:

· BUG: Using the Activision work-around method of avoiding the HMOVE lines doesn’t work on some PAL 2600 JR (all-black) models.

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