Formation Descent
The formation drops one row at each edge — classic Space Invaders descent with an invasion fail condition.
The formation moves left and right, but never gets closer. Without descent, the player can sit still and wait for targets to drift into their line of fire. This unit adds the classic Space Invaders mechanic: when the formation reaches a screen edge, it drops one row before reversing direction. Each bounce brings the enemies closer. If they reach the bottom, the game is over.
The Program
5 REM === FORMATION DESCENT ===
10 BORDER 0: PAPER 0: INK 7: CLS
15 GO SUB 800
20 LET sc = 0: LET hi = 0: LET li = 3
22 LET wv = 1: LET ne = 8: LET sp = 4
25 DIM e(8): DIM r(8): DIM h(8)
30 FOR i = 1 TO ne
32 LET e(i) = (i - 1) * 2: LET r(i) = 0: LET h(i) = 1
34 NEXT i
36 LET fx = 6: LET fy = 2: LET fd = 1: LET tk = 0
40 LET px = 15: LET py = 20
42 LET ba = 0: LET bx = 0: LET by = 0
50 GO SUB 600
55 PRINT AT py, px; INK 7; CHR$ 152
57 GO SUB 400
60 REM === game loop ===
70 LET k$ = INKEY$
80 IF k$ = "o" OR k$ = "O" THEN IF px > 1 THEN PRINT AT py, px; " ": LET px = px - 1: PRINT AT py, px; INK 7; CHR$ 152
90 IF k$ = "p" OR k$ = "P" THEN IF px < 30 THEN PRINT AT py, px; " ": LET px = px + 1: PRINT AT py, px; INK 7; CHR$ 152
100 IF k$ = " " THEN IF ba = 0 THEN LET bx = px: LET by = py - 1: LET ba = 1: PRINT AT by, bx; INK 7; CHR$ 148
110 REM move bullet
115 IF ba = 0 THEN GO TO 170
120 PRINT AT by, bx; " "
130 LET by = by - 1
140 IF by < 1 THEN LET ba = 0: GO TO 170
150 LET ht = 0
155 FOR i = 1 TO ne
156 IF h(i) <= 0 THEN GO TO 158
157 IF bx = fx + e(i) AND by = fy + r(i) THEN LET ht = i: LET i = ne
158 NEXT i
160 IF ht > 0 THEN LET ba = 0: LET h(ht) = 0: PRINT AT fy + r(ht), fx + e(ht); INK 2; CHR$ 149: BEEP 0.02, -5: PAUSE 5: PRINT AT fy + r(ht), fx + e(ht); " ": LET sc = sc + 10: GO SUB 600: GO TO 170
165 PRINT AT by, bx; INK 7; CHR$ 148
170 REM move formation on tick
175 LET tk = tk + 1
180 IF tk < sp THEN GO TO 60
185 LET tk = 0
190 GO SUB 400
195 IF fy > 19 THEN GO TO 700
200 GO TO 60
400 REM === move formation ===
402 FOR i = 1 TO ne
404 IF h(i) <= 0 THEN GO TO 410
406 LET sx = fx + e(i): LET sy = fy + r(i)
408 IF sx >= 1 AND sx <= 30 AND sy >= 1 AND sy <= 21 THEN PRINT AT sy, sx; " "
410 NEXT i
412 LET fx = fx + fd
414 IF fx + 14 > 30 OR fx < 1 THEN LET fd = -fd: LET fy = fy + 1
416 FOR i = 1 TO ne
418 IF h(i) <= 0 THEN GO TO 425
420 LET sx = fx + e(i): LET sy = fy + r(i)
422 IF sx >= 1 AND sx <= 30 AND sy >= 1 AND sy <= 21 THEN PRINT AT sy, sx; INK 4; CHR$ 144
425 NEXT i
427 RETURN
600 REM === draw HUD ===
605 PRINT AT 0, 0; INK 7; BRIGHT 1; "SC:"; sc; " HI:"; hi; " LV:"; li; " W"; wv; " "
610 RETURN
700 REM === game over ===
705 CLS
710 PRINT AT 8, 10; INK 2; BRIGHT 1; "GAME OVER"
715 PRINT AT 10, 8; INK 7; "INVADED!"
720 PRINT AT 12, 7; INK 7; "Score: "; sc
730 PRINT AT 18, 6; INK 7; "Press any key"
735 PAUSE 0
800 REM === define UDGs ===
805 FOR i = 0 TO 13
810 FOR j = 0 TO 7
815 READ a
820 POKE USR CHR$ (65 + i) + j, a
825 NEXT j
830 NEXT i
835 RETURN
840 REM UDG A: drone
842 DATA 36, 126, 219, 255, 255, 102, 66, 0
844 REM UDG B: scout
846 DATA 24, 60, 126, 255, 219, 24, 36, 0
848 REM UDG C: tank
850 DATA 126, 255, 255, 255, 255, 255, 126, 0
852 REM UDG D: bomber
854 DATA 66, 231, 255, 126, 60, 90, 36, 0
856 REM UDG E: bullet
858 DATA 16, 56, 16, 16, 0, 0, 0, 0
860 REM UDG F: explosion
862 DATA 36, 153, 66, 129, 66, 153, 36, 0
864 REM UDG G: double shot
866 DATA 84, 170, 84, 170, 84, 170, 84, 0
868 REM UDG H: shield
870 DATA 60, 126, 255, 255, 255, 126, 60, 0
872 REM UDG I: player ship
874 DATA 16, 56, 56, 124, 254, 254, 130, 0
876 REM UDG J: enemy bomb
878 DATA 16, 16, 56, 16, 0, 0, 0, 0
880 REM UDG K: explosion 2
882 DATA 129, 66, 36, 0, 36, 66, 129, 0
884 REM UDG L: shield icon
886 DATA 60, 126, 126, 126, 60, 24, 0, 0
888 REM UDG M: drone frame 2
890 DATA 66, 102, 255, 255, 219, 126, 36, 0
892 REM UDG N: scout frame 2
894 DATA 36, 24, 219, 255, 126, 60, 24, 0
How It Works
Line 424 adds the descent trigger. When the formation hits either edge and fd flips, the base row fy also increases by 1. The formation drops one row every time it reverses direction.
Line 426 checks for invasion. If fy + nr (the base row plus the number of enemy rows) exceeds row 19, the enemies have reached the play area bottom. This triggers an immediate game over regardless of how many lives remain. The aliens have invaded.
The Invasion Rule
The invasion fail state creates tension that scoring alone cannot provide. No matter how many enemies the player has destroyed, no matter how high the score, if the formation reaches the bottom, the game ends. This forces aggressive play — the player must shoot quickly to thin the formation before it descends too far.
In Space Invaders, this mechanic creates an elegant feedback loop: as the player destroys enemies, fewer remain, but the fewer enemies there are, the faster the game runs (fewer objects to process). In Blockstorm, the speed is set by the wave data, not the enemy count. But the descent pressure is the same.
Descent Speed
The formation descends one row every time it reverses direction. How quickly it descends depends on how wide the play area is and how far the formation travels before hitting an edge. A narrow formation reaches the edges more often and descends faster. A wide formation has less room to move and also descends quickly. The sweet spot is a formation width that gives the player enough time to clear a few enemies before the formation drops close.
Try This
Count the bounces. Add a variable that increments every time the formation reverses direction. Display it in the HUD. How many bounces does it take for the formation to reach the bottom from its starting position?
Start higher. Change the initial fy to 1 instead of 2. The formation starts one row higher, giving the player slightly more time. Does one extra row make a noticeable difference?