Enemy formation

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An enemy formation keeps enemies in a fixed arrangement that moves as a whole. Space Invaders draws all 55 aliens from one reference alien, so the rack moves as one block¹. Galaxian holds its swarm in place while aliens leave it to attack, a flagship with up to two escorts or single aliens, at most 7 in flight².

Games

Game	Year	How	Note
Space Invaders	1978	Marching rack	55 aliens drawn from one reference alien move as a single block
Galaxian	1979	Break-off	the swarm holds formation while single aliens and escorted flagships dive out of it

Every claim about this mechanic, by game (1)

Galaxian 1979	Galaxian's swarm holds its formation while aliens break off from it to attack. An array of 8 in-flight slots holds them: slot 1 for a flagship, slots 2 and 3 for its escorts, of which there are at most 2, and slots 4 to 7 for single attackers, so at most 7 aliens fly at once. No attack uses slot 0, which the disassembly's comment says is kept for other uses such as explosions. A single attack scans only $\min (⌊ (b + e) / 2 ⌋, 3) + 1$ of the last four slots, where $b$ and $e$ are the two difficulty values, so 1 to 4 single attackers can fly at once.	Code

References

^ a bSpace Invaders moves its rack of aliens one alien per frame. The interrupt at scanline 96 calls CursorNextAlien, which skips dead aliens to the next live one and waits until the end-of-screen interrupt's DrawAlien has drawn it. Only when the cursor passes the last of the 55 slots does MoveRefAlien step the reference alien, from which every other alien is drawn. A step of the rack therefore takes one 60 Hz frame per live alien: 55 frames at the start, 1 for the last alien. The listing reads MAME's invaders set, Midway's release.CodeComputer Archeology, Space Invaders

How we know

7>>> binary 0000:roms/invaders.h + roms/invaders.g + roms/invaders.f.diag + roms/invaders.e

content/Arcade/SpaceInvaders/Code.md line 7 at 1adc2a7 · upstream ↗

28If I understand this right then the system gets RST 8 when the beam is *near* the middle of the screen and RST 10

content/Arcade/SpaceInvaders/Hardware.md lines 28–33 at 1adc2a7 · upstream ↗

120; Main game-play timing loop
⋮
1240078: CD 00 01        CALL    $0100               ; {code.DrawAlien} Draw the current alien (or exploding alien)

content/Arcade/SpaceInvaders/Code.md lines 120–124 at 1adc2a7 · upstream ↗

138; Continues here at scanline 96
⋮
15400AB: CD 41 01        CALL    $0141               ; {code.CursorNextAlien} Advance cursor to next alien (move the alien if it is last one)

content/Arcade/SpaceInvaders/Code.md lines 138–155 at 1adc2a7 · upstream ↗

248CursorNextAlien:
⋮
2600146: 3A 00 20        LD      A,($2000)           ; {ram.waitOnDraw} Still waiting on ...
2610149: A7              AND     A                   ; ... this alien to be drawn?
262014A: C0              RET     NZ                  ; Yes ... leave cursor in place
⋮
2670154: 3C              INC     A                   ; Have we drawn all aliens ...
2680155: FE 37           CP      $37                 ; ... at last position?
2690157: CC A1 01        CALL    Z,$01A1             ; {code.MoveRefAlien} Yes ... move the bottom/right alien and reset index to 0
270015A: 6F              LD      L,A                 ; HL now points to alien flag
271015B: 46              LD      B,(HL)              ; Is alien ...
272015C: 05              DEC     B                   ; ... alive?
273015D: C2 54 01        JP      NZ,$0154            ; {} No ... skip to next alien

content/Arcade/SpaceInvaders/Code.md lines 248–273 at 1adc2a7 · upstream ↗

319MoveRefAlien:
320; The "reference alien" is the bottom left. All other aliens are drawn relative to this
321; reference. This routine moves the reference alien (the delta is set elsewhere) and toggles
⋮
32601A5: 21 06 20        LD      HL,$2006            ; Set current alien ...
32701A8: 36 00           LD      (HL),$00            ; ... index to 0
⋮
33101AE: CD D9 01        CALL    $01D9               ; {code.AddDelta} Move alien

content/Arcade/SpaceInvaders/Code.md lines 319–331 at 1adc2a7 · upstream ↗

3910; The game starts with 55 aliens. The aliens are move/drawn one per interrupt which means it

content/Arcade/SpaceInvaders/Code.md lines 3909–3911 at 1adc2a7 · upstream ↗

Checked against the evidence by its author, an AI model, 2026-10-25.

Checked by a second AI model, 2026-10-21.

Checked by a second AI model, 2026-10-25.

In the claim ledger

^ a bGalaxian's swarm holds its formation while aliens break off from it to attack. An array of 8 in-flight slots holds them: slot 1 for a flagship, slots 2 and 3 for its escorts, of which there are at most 2, and slots 4 to 7 for single attackers, so at most 7 aliens fly at once. No attack uses slot 0, which the disassembly's comment says is kept for other uses such as explosions. A single attack scans only

\min (⌊ (b + e) / 2 ⌋, 3) + 1

of the last four slots, where

b

and

e

are the two difficulty values, so 1 to 4 single attackers can fly at once.CodeScottTunstall/Galaxianrevised

How we know

50791472: DD 21 D0 42   ld   ix,$42D0            ; pointer to INFLIGHT_ALIENS_START+sizeof(INFLIGHT_ALIEN)
50801476: CD 5C 14      call $145C               ; call INIT_INFLIGHT_ALIEN to make flagship take flight and leave the swarm 
⋮
5084147D: FD 21 F0 42   ld   iy,$42F0            ; pointer to INFLIGHT_ALIENS_START+(sizeof(INFLIGHT_ALIEN) * 2)
50851481: 06 03         ld   b,$03               ; we're scanning 3 entries in red aliens row max             
50861483: 0E 02         ld   c,$02               ; But we only want 2 red aliens as an escort.  
⋮
5094148E: CD 9B 14      call $149B               ; call TRY_INIT_ESCORT_INFLIGHT_ALIEN
50951491: 11 20 00      ld   de,$0020            ; sizeof(INFLIGHT_ALIEN)
50961494: FD 19         add  iy,de               ; bump IY to point to next member of INFLIGHT_ALIENS array
50971496: 0D            dec  c                   ; reduce count of red aliens left to check for use as escort
50981497: C0            ret  nz                  ; return if we have all the escort we need
50991498: 06 01         ld   b,$01

galaxian.asm lines 5079–5100 at a16b402 · upstream ↗

FLAGSHIP_ESCORT_COUNT EQU $422A ; Number of red aliens escorting the flagship. Max value of 2.

galaxian.asm#L517 · upstream ↗

47841352: 2A 1A 42      ld   hl,($421A)          ; load H with DIFFICULTY_BASE_VALUE and L with DIFFICULTY_EXTRA_VALUE
47851355: 7C            ld   a,h                 ; 
47861356: 85            add  a,l                 ; add DIFFICULTY_EXTRA_VALUE to DIFFICULTY_BASE_VALUE 
47871357: 1F            rra                      ; divide by 2.
47881358: FE 04         cp   $04                 ; is result < 4?
4789135A: 38 02         jr   c,$135E             ; yes, goto $135E.
4790135C: 3E 03         ld   a,$03               ; Clamp maximum number of INFLIGHT_ALIEN slots to scan to 3.
4791135E: 3C            inc  a                   ; ensure that slots to scan is in range of 1..4
⋮
4796135F: 47            ld   b,a                  ; save number of slots to scan in B
47971360: 21 91 43      ld   hl,$4391             ; point HL to last INFLIGHT_ALIEN.IsDying flag in INFLIGHT_ALIENS array
47981363: 11 E1 FF      ld   de,$FFE1             ; load DE with -31, which is sizeof(INFLIGHT_ALIEN)-1

galaxian.asm lines 4784–4800 at a16b402 · upstream ↗

Checked against the evidence by its author, an AI model, 2026-10-25.

Revised after a check by a second AI model, 2026-10-21.

Checked by a second AI model, 2026-10-25.

In the claim ledger

^When the rack bumps an edge, RackBump reverses it and drops it 8 px: it sets the next step's drop from $200E, which the game's RAM copy fills with $F8 (-8), and MoveRefAlien clears the drop after one step. Its step to the right is 2 px, or 3 px when one alien is left; its step to the left is always 2 px.CodeComputer Archeology, Space Invaders

How we know

31631597: 3A 0D 20        LD      A,($200D)           ; {ram.rackDirection} Get rack direction
3164159A: A7              AND     A                   ; Moving right?
3165159B: C2 B7 15        JP      NZ,$15B7            ; {} No ... handle moving left
⋮
317015A5: 06 FE           LD      B,$FE               ; Delta X of -2
317115A7: 3E 01           LD      A,$01               ; Rack now moving right
⋮
317315A9: 32 0D 20        LD      ($200D),A           ; {ram.rackDirection} Set new rack direction
317415AC: 78              LD      A,B                 ; B has delta X
317515AD: 32 08 20        LD      ($2008),A           ; {ram.refAlienDXr} Set new delta X
317615B0: 3A 0E 20        LD      A,($200E)           ; {ram.rackDownDelta} Set delta Y ...
317715B3: 32 07 20        LD      ($2007),A           ; {ram.refAlienDYr} ... to drop rack by 8
⋮
318315BE: CD F1 18        CALL    $18F1               ; {} Get moving-right delta X value of 2 (3 if just one alien left)
318415C1: AF              XOR     A                   ; Rack now moving left
⋮

content/Arcade/SpaceInvaders/Code.md lines 3163–3185 at 1adc2a7 · upstream ↗

368418F1: 06 02           LD      B,$02               ; Rack moving right delta X
368518F3: 3A 82 20        LD      A,($2082)           ; {ram.numAliens} Number of aliens on screen
368618F6: 3D              DEC     A                   ; Just one left?
368718F7: C0              RET     NZ                  ; No ... use right delta X of 2
368818F8: 04              INC     B                   ; Just one alien ... move right at 3 instead of 2

content/Arcade/SpaceInvaders/Code.md lines 3684–3689 at 1adc2a7 · upstream ↗

39301E4: 06 C0           LD      B,$C0               ; Number of bytes
39401E6: 11 00 1B        LD      DE,$1B00            ; RAM mirror in ROM
39501E9: 21 00 20        LD      HL,$2000            ; Start of RAM
39601EC: C3 32 1A        JP      $1A32               ; {code.BlockCopy} Copy [DE]->[HL] and return

content/Arcade/SpaceInvaders/Code.md lines 393–396 at 1adc2a7 · upstream ↗

40721B00: 01 00 00 10 00 00 00 00 02 78 38 78 38 00 F8 00

content/Arcade/SpaceInvaders/Code.md line 4072 at 1adc2a7 · upstream ↗

32601A5: 21 06 20        LD      HL,$2006            ; Set current alien ...
⋮
32801AA: 23              INC     HL                  ; Point to DeltaX
32901AB: 4E              LD      C,(HL)              ; Load DX into C
33001AC: 36 00           LD      (HL),$00            ; Set DX to 0
33101AE: CD D9 01        CALL    $01D9               ; {code.AddDelta} Move alien

content/Arcade/SpaceInvaders/Code.md lines 326–331 at 1adc2a7 · upstream ↗

Checked against the evidence by its author, an AI model, 2026-10-25.

Checked by a second AI model, 2026-10-21.

Checked by a second AI model, 2026-10-25.

In the claim ledger

Sources

Code

Cantrell, Christopher. 2026. “Computer Archeology: Space Invaders.” Commit 1adc2a7. topherCantrell/computerarcheology @ 1adc2a7 · licence, all uses
Tunstall, Scott. 2025. “Galaxian: annotated reverse engineering.” Commit a16b402. ScottTunstall/Galaxian @ a16b402 · licence, all uses

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