Remaining-enemy speed-up

References

1. ^ abSpace 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
   

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

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

   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)
   

   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
   

   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
   

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

   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

2. ^ abGalaxian counts the aliens still in the swarm, leaving out those in flight, and sets HAVE_AGGRESSIVE_ALIENS when 3 or fewer remain. With the flag set, an attacking alien other than a flagship that reaches the bottom of the screen reappears at the top and attacks again instead of rejoining the swarm, once the player has spawned; before that, it rejoins. Flagships take a separate routine. The disassembly reads the Namco ROM, MAME's galaxian set.CodeScottTunstall/Galaxianrevised
   How we know

   5058INIT_INFLIGHT_ALIEN:
   5059145C: 36 00         ld   (hl),$00            ; clear flag in ALIEN_SWARM_FLAGS - effectively removing it from swarm            
   

   5615HANDLE_ALIEN_AGGRESSIVENESS:
   ⋮
   561916BD: 21 23 41      ld   hl,$4123            ; load HL with address of first alien (bottom rightmost) in ALIEN_SWARM_FLAGS
   562016C0: 11 06 00      ld   de,$0006            ; DE is an offset to add to HL after processing a row of aliens
   562116C3: 4B            ld   c,e                 ; Conveniently, E is also number of rows of aliens in swarm! (6) 
   562216C4: 3E 01         ld   a,$01               ; A is going to be used to total the number of aliens in the swarm 
   562316C6: 06 0A         ld   b,$0A               ; 10 aliens maximum per row
   562416C8: 86            add  a,(hl)              
   ⋮
   563316D4: 06 03         ld   b,$03               ; number of ports to write to maximum 
   563416D6: 3D            dec  a                   ; decrement total by 1 
   563516D7: 28 14         jr   z,$16ED             ; if total is zero, goto $16ED 
   ⋮
   564016DC: 10 F8         djnz $16D6
   ⋮
   564216DE: FE 02         cp   $02                 ;                  
   ⋮
   

   3747INFLIGHT_ALIEN_REACHED_BOTTOM_OF_SCREEN:
   ⋮
   3758;if the player has not spawned, the alien will return to the swarm.
   ⋮
   3772; alien reappears at top of screen and will keep attacking - it will not return to swarm. 
   

   the manual above is not correct with the Namco Galaxian ROM

   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

3. ^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
   ⋮
   

   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
   

   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
   

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

   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
   

   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

4. ^Each pass of the game loop, SpeedShots checks the number of aliens left; below 9 it sets the alien shots' step ($207E, added to a shot's Y each move) to $FB, -5 px, in place of the $FC (-4 px) that the game's RAM copy puts there, so the aliens' shots fall faster.CodeComputer Archeology, Space Invaders
   How we know

   150308D8: 3A 82 20        LD      A,($2082)           ; {ram.numAliens} Number of aliens on screen
   150408DB: FE 09           CP      $09                 ; More than 8?
   150508DD: D0              RET     NC                  ; Yes ... leave shot speed alone
   150608DE: 3E FB           LD      A,$FB               ; Normally FF (-4) ... now FB (-5)
   150708E0: 32 7E 20        LD      ($207E),A           ; {ram.alienShotDelta} Speed up alien shots
   

   14050825: CD F3 15        CALL    $15F3               ; {code.CountAliens} Count aliens (count to 2082)
   ⋮
   14120838: CD D8 08        CALL    $08D8               ; {code.SpeedShots} Adjust alien shot speed
   

   109305E5: 3A 7B 20        LD      A,($207B)           ; {ram.alienShotYr} Get shot's Y coordinate
   109405E8: 47              LD      B,A                 ; Hold it
   109505E9: 3A 7E 20        LD      A,($207E)           ; {ram.alienShotDelta} Get alien shot delta
   109605EC: 80              ADD     A,B                 ; Add to shots coordinate
   109705ED: 32 7B 20        LD      ($207B),A           ; {ram.alienShotYr} New shot Y coordinate
   

   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
   

   40831B70: 0F 0B 00 18 26 0F 0B 00 18 04 11 24 1B 25 FC 
   

   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