How does Space Invaders on the Atari 2600 display all the aliens?

Question

The Atari 2600 has two 8-bit wide player sprites, and it appears to be these sprites which are used to draw these aliens.

There are a few different ways to produce many copies of one sprite; controlled by nusiz0 and nusiz1. If you don't want the sprite stretched, then there's a choice between:

• one copy of the sprite
• two copies of the sprite, close together, far apart, or medium.
• three copies of the sprite, close together, or medium

I suppose it's possible they used player 1 and player 2, both together, three copies of each, to get the six aliens on a row. But what about when an alien gets shot down? Some kind of routine needs to work out how to configure each sprite, with number of copies and distance between changing every time. Has that ever been disassembled? I'd be interested to figure it out.

Answer 1

This answer elaborates a bit on the answers by @Raffzahn and @thrust26. The answer by @thrust26 is correct. The alien sprites are not positioned by repeated RESP0 or RESP1 triggers as the code below shows. The book "Racing the beam" mentions the following in a section about sprites:

Rick Maurer, the programmer for the VCS port of Space Invaders, discovered that strobing HMOVE while a line was being drawn would reposition objects immediately.

This suggests that this technique was used in Space Invaders. However, the only HMOVE triggers that appear in the code occur directly after a WSYNC, so definitely not multiple times per scan line.

The aliens are drawn using the two player sprites. Both NUSIZ0 and NUSIZ1 are set to $06 which means single pixel resolution and three copies each at a medium spacing. The copies of both sprites interlace to form a row of six in a pattern "A B A B A B". The appearance of each "copy" is changed by loading different sprite patterns in the GRP0 and GRP1 registers by carefully timed code:

           lda  ($f8),y   ; Load pattern for alien six in X.
           tax  
           lda  ($ee),y   ; Load pattern for alien one in GRP0
           sta  GRP0
           lda  ($f0),y   ; Load pattern for alien two in GRP1
           sta  GRP1
           lda  ($f2),y   ; Update pattern for alien three in GRP0
           sta  GRP0
           lda  ($f4),y   ; Update pattern for alien four in GRP1
           sta  GRP1
           lda  ($f6),y   ; Update pattern for alien five in GRP0
           sta  GRP0
           txa            ; Update pattern for alien six in GRP1
           sta  GRP1

The Y register counts down from 9 to 0 (inclusive) to draw ten scan lines for each alien in a row. An alien pattern can easily be modified by adjusting the low byte of any of the pointers in $ee to $f8. Interestingly, these scan lines are drawn without any intermediate WSYNC trigger, purely based on careful instruction timing.

Answer 2

It's rather simple. Neither the multiple sprite mode, nor any secret trick is used.

The TIA doesn't have any directly-accessible register for storing a sprite position ahead of time. Sprites are drawn when they are enabled on the actual line (Y coordinate) and whenever RESPx is triggered (X coordinate). To display the 6 aliens, RESPx gets triggered 6 successive times. To not draw an alien the corresponding player is just cleared for that activation — or, during an explosion, replaced by the appropriate sprite.

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You really want to read the by now classic book Racing the Beam. A must-read for anyone interested in TIA workings — and way less work than crawling through disassemblies and guessing what happened :))

Answer 3

Late to the party, but I think wrong information should be corrected.

The Space Invader sprites are simply displayed using 3 copies of each of the two sprites. The RESPx trick was first shown in Galaxian three years later.