Invaders

Bug-Byte Software

Products: Invaders

Date: 1982

Type: Cassette

Platform(s): TS 1000

Invaders is a Space Invaders-style arcade game that uses direct memory PEEK and POKE operations to manipulate the display file for smooth character-cell animation. The player sprite and alien formations are positioned using absolute memory addresses calculated from the system variable at address 16398/16399 (the display file pointer, stored in E1), allowing the gun to move left and right along the bottom row while waves of aliens scroll downward. Five alien types are cycled through using a pattern-index variable D, each rendered with a different block-graphic character combination, and each worth a different point value (20–50 points). The game includes a high-score tracker, a multi-wave progression system, a bonus jet escape sequence, and a “DANGER” warning when aliens reach a critical row.

Program Analysis

Program Structure

The program is organized into several functional regions, loosely separated by line number ranges:

Lines 1–10: Initialization, high-score reset, title block, jump to setup.
Lines 80–99: Per-wave setup — reset score display, draw play field, branch to alien shape selector via GOTO 100*D.
Lines 100–510: Alien shape definitions (five variants) and animation frame cycling.
Lines 600–630: Subroutine to fill the play area rows with solid inverse blocks.
Lines 1500–1840: Keyboard input handler and player gun movement via POKE.
Lines 1900–2900: Main game loop — alien scroll, collision, bonus jet sequence, scoring.
Lines 7000–7100: Game-over handler with flashing death animation and score display.
Lines 7397–7450: Title/attract screen with animated “GET YOURSELF READY” text.
Lines 7500–7695: Wave-start subroutine — animates player gun appearing from top of screen.
Lines 8020–8600: Fire (laser) subroutine — traces bullet upward using POKEs, tests for hit.
Lines 9005–9950: Screen setup subroutine — reads display file base address into E1, draws border and title banner.

Display File Addressing

The core technique throughout is computing absolute display-file addresses to animate characters without using PRINT AT. The subroutine at line 9005 reads the display file start address from system variables 16398–16399 and stores it in E1. All subsequent POKE/PEEK operations use offsets from E1; for example, each row is 33 bytes wide (32 character columns plus one attribute/newline byte in the ZX81 file format), so adding 33 advances one row. The player gun position is tracked in Y1, initialized at E1+705 (row 21, roughly column 12), and incremented or decremented by 1.5 per keypress — an unusual fractional step that still produces integer POKE targets due to how BASIC truncates addresses.

Alien Shape Cycling

Five alien shapes are defined in A$ at lines 100, 200, 300, 400, and 500, each using different block graphic characters to simulate animation frames. The variable D (1–5) selects the current frame, and GOTO 100*D at line 99 is a computed branch — a classic BASIC optimization avoiding a chain of IF statements. After printing, line 510 advances D with wrap-around: LET D=D+1-5*(D=5), cycling D through 1→2→3→4→5→1.

Main Game Loop and Alien Scroll

The alien formation is drawn using PLOT/PRINT pairs. PLOT col,row positions the cursor, then PRINT A$ renders the alien string. Ten columns of aliens are drawn per pass (lines 2001–2091), with the loop variable S counting down from a difficulty-dependent starting row Q (10, 22, 22, 18, or 14 depending on D) toward row 4 in steps of 4. After each alien print, GOSUB 1500 checks for key input, giving the player opportunities to move and fire during the alien descent.

When aliens reach row 6 (S=6), a “DANGER” warning is printed and T1 is incremented. After three such warnings (T1=3, line 2091), the game jumps to the bomb-hit sequence at line 2900.

Laser Fire Mechanism

Firing is handled at line 8020. The subroutine traces a bullet upward from the gun position Y1 by POKEing character codes at successive addresses 66 bytes apart (two rows up per step), creating a moving dot effect. Hit detection at line 8125 checks whether the bullet’s column position falls within the alien string bounds by comparing (Y1-U) against a range derived from PEEK 16438 (the print position register). Line 8150 then checks whether the character at that position in A$ equals CHR$ 52 (the character “4”, used as the alien center pixel “O”) to confirm a hit rather than striking empty space.

Bonus Jet Sequence

When the aliens are defeated (scroll completes without collision), the game triggers a bonus sequence (line 2200 onward). A jet icon (▝X▘) appears randomly at the left or right edge of row 20. The player must guide their gun to intercept it: the code traces a path across the display file from W+33 downward, animating movement with POKEs (characters 189, 190, 128), then checks alignment with the gun. A successful intercept awards 100 points and triggers an explosion animation (lines 2355–2365) using +, *, =, and inverse-block characters in a loop.

Scoring and High Score

Points are accumulated in S1. Each alien hit scores D*10 points (line 8500), so wave 1 aliens are worth 10 pts and wave 5 aliens worth 50 pts — matching the score table displayed on the attract screen (lines 7400–7404). The bonus jet awards a flat 100 pts. The high score is stored in HI (initialized to 500) and HS flags a new high score. When a new high score is achieved, lines 7057–7070 POKE a flashing “NEW HIGH SCORE” message directly into the display file using alternating normal and inverse character codes (toggled by 128*(INT(J/2)=J/2)).

Notable Techniques

GOTO 100*D — computed GOTO used as a jump table for five alien sprite variants.
LET D=D+1-5*(D=5) — Boolean arithmetic for modular cycling without IF.
Fractional gun step (Y1±1.5) — unusual but functionally equivalent to ±1 for POKE addressing due to integer truncation in the memory address context.
Display file manipulation via PEEK 16398/16399 — portable within the platform, since the display file can move depending on program size.
Row width of 33 bytes — correctly accounts for the ZX81 display file format where each row is 32 character bytes plus a NEWLINE token.
Attract screen animation (lines 7406–7414) — builds “GET YOURSELF READY” one character at a time across successive PRINT statements for a typewriter effect.
POKE 16442,4 at line 9123 — sets the system variable MARGIN, adjusting the left margin of the display.

Bugs and Anomalies

Line 1700 is referenced by line 1510 (GOTO 1700) but the actual movement code starts at line 1710. Line 1700 does not exist, so the branch falls through to 1710 — a well-known technique that works correctly here.
The empty loops at lines 7565–7570, 7595–7596, and 7645–7646 (FOR J=1 TO 5: NEXT J with no body) serve as deliberate timing delays.
B$ is set at line 98 but its length (13 characters) and the hit-detection comparison at line 8590 check for 14 characters ("██████████████") — a possible off-by-one that may prevent the all-clear condition from triggering reliably.
Line 2200 computes B=33*((42-S)/2)+(Y1-U); the factor (42-S)/2 depends on the last value of S from the scroll loop, which may not always be a clean integer, potentially misaligning the jet target address.

Content

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Invaders

Shoot down the marching aliens with your rapid fire laser cannon before they take over the planet forever. Shields are...

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Source Code
REM █[I][N][V][A][D][E][R][S]█
GOTO 5
SAVE "INVADER[S]"
REM █[P][R][O][G][R][A][M]█[L][O][A][D][E][D]█
LET S1=0
LET HS=0
LET HI=500
GOSUB 9000
GOTO 7400
LET D=1
LET S1=0
PRINT AT 1,16;S1;"...."
GOSUB 600
LET T1=0
LET B$="█████████████"
GOTO 100*D
LET A$="██[H]O[H]██[H]O[H]█[H]O[H]"
GOTO 510
LET A$="██▗O▖██▗O▖█▗O▖"
GOTO 510
LET A$="██▚O▞██▚O▞█▚O▞"
GOTO 510
LET A$="██▛O▜██▛O▜█▛O▜"
GOTO 510
LET A$="██▀O▀██▀O▀█▀O▀"
LET D=D+1-5*(D=5)
GOSUB 7500
GOTO 1900
FOR J=8 TO 21
PRINT AT J,0;"████████████████████████████████"
NEXT J
RETURN
IF INKEY$ ="P" THEN GOTO 1800
IF INKEY$ ="O" THEN GOTO 1700
IF INKEY$ ="A" THEN GOTO 8000
RETURN
POKE Y1,128
LET Y1=Y1-1.5
IF PEEK Y1<>128 THEN LET Y1=E1+694
POKE Y1,19
RETURN
POKE Y1,128
LET Y1=Y1+1.5
IF PEEK Y1<>128 THEN LET Y1=E1+723
POKE Y1,18
RETURN
LET Q=10
IF D=1 THEN LET Q=26
IF D=2 THEN LET Q=22
IF D=3 THEN LET Q=18
IF D=4 THEN LET Q=14
FOR S=Q TO 4 STEP -4
PLOT 0,S
PRINT A$
GOSUB 1500
PLOT 4,S
PRINT A$
GOSUB 1500
PLOT 8,S
PRINT A$
GOSUB 1500
PLOT 12,S
PRINT A$
GOSUB 1500
PLOT 16,S
PRINT A$
GOSUB 1500
PLOT 20,S
PRINT A$
GOSUB 1500
PLOT 24,S
PRINT A$
GOSUB 1500
PLOT 28,S
PRINT A$
GOSUB 1500
PLOT 32,S
PRINT A$
GOSUB 1500
PLOT 34,S
PRINT A$
IF T1=3 THEN GOTO 2900
IF S=6 THEN PRINT AT 14,10;"[D][A][N][G][E][R]"
IF S=6 THEN LET T1=T1+1
IF S=6 THEN GOTO 2001
NEXT S
LET B=33*((42-S)/2)+(Y1-U)
LET W=E1+B
PRINT AT 14,10;"[1][0][0][P][T][S]"
PRINT AT 10,10;"[R][U][N]█[T][O]█[T][H][E]█[J][E][T][.][.][.]"
LET O=INT (RND*2)
IF O=1 THEN GOTO 2210
PRINT AT 20,1;"▝X▘"
GOTO 2225
PRINT AT 20,28;"▝X▘"
POKE W+33,148
GOSUB 1500
POKE W+66,148
GOSUB 1500
IF PEEK (W+99)<>128 THEN GOTO 7000
IF Y1>(W+99) THEN LET I=1
IF Y1<(W+99) THEN LET I=-1
PRINT AT 14,10;"██████"
FOR J=I TO I*30 STEP I
IF PEEK (W+99+J)<>128 THEN GOTO 7000
POKE W+99+J,189
POKE W+99+J,190
POKE W+99+J,128
GOSUB 1500
IF PEEK (Y1-33)=61 THEN GOTO 2300
NEXT J
LET R=2
IF O=1 THEN LET R=29
POKE W+99+J,190
POKE W+R,128
FOR J=20 TO 9 STEP -1
PRINT AT J,R-1;"▝X▘"
PRINT AT J,R-1;"███"
NEXT J
PRINT AT J,R-1;"███"
FOR N=1 TO 4
PRINT AT J,R;" "
PRINT AT J,R;"[+]"
PRINT AT J,R;"[*]"
PRINT AT J,R;"[=]"
PRINT AT J,R;"█"
NEXT N
LET S1=S1+100
PRINT AT 1,16;S1;"."
GOTO 90
POKE Y1-100,173
POKE Y1-99,52
POKE Y1-98,173
GOTO 2200
PRINT AT 10,10;"████[D][A][M][N][E][D]█[.][.][.]█"
FOR J=1 TO 30
POKE Y1,0
POKE Y1,149
POKE E1,103
POKE Y1,128
POKE E1,128
NEXT J
PRINT AT 1,16;S1
IF S1>HI THEN LET HS=1
IF HS THEN LET HI=S1
PRINT AT 1,0;"████[*][*][*]█[H][I][G][H]█[S][C][O][R][E]█.....█[*][*][*]████"
PRINT AT 1,19;HI
IF HS=0 THEN GOTO 7080
FOR J=1 TO 10
LET I=0+128*(INT (J/2)=J/2)
POKE E1+41,45+I
POKE E1+42,46+I
POKE E1+43,44+I
POKE E1+44,45+I
POKE E1+45,0+I
POKE E1+46,56+I
POKE E1+47,40+I
POKE E1+48,52+I
POKE E1+49,55+I
POKE E1+50,42+I
NEXT J
FOR J=1 TO 21
PRINT AT 20,J;"█[G][A][M][E]█[O][V][E][R]"
NEXT J
LET HS=0
PRINT AT 0,0;
GOSUB 9000
PRINT AT 10,10;"[H]O[H][.][.][.][2][0][P][T][S]"
PRINT AT 12,10;"▗O▖[.][.][.][3][0][P][T][S]"
PRINT AT 14,10;"▚O▞[.][.][.][4][0][P][T][S]"
PRINT AT 16,10;"▛O▜[.][.][.][5][0][P][T][S]"
PRINT AT 18,10;"▀O▀[.][.][.][1][0][P][T][S]"
PRINT AT 1,16;S1
PRINT AT 20,15;"[G][Y]"
PRINT AT 20,14;"[G][E][D][Y]"
PRINT AT 20,13;"[G][E][T][A][D][Y]"
PRINT AT 20,12;"[G][E][T]█[E][A][D][Y]"
PRINT AT 20,11;"[G][E][T]█[Y][R][E][A][D][Y]"
PRINT AT 20,10;"[G][E][T]█[Y][O]█[R][E][A][D][Y]"
PRINT AT 20,9;"[G][E][T]█[Y][O][U][F]█[R][E][A][D][Y]"
PRINT AT 20,8;"[G][E][T]█[Y][O][U][R][L][F]█[R][E][A][D][Y]"
PRINT AT 20,7;"[G][E][T]█[Y][O][U][R][S][E][L][F]█[R][E][A][D][Y]"
GOSUB 600
PRINT AT 12,11;"[P][.][.][.][L][E][F][T]"
PRINT AT 15,11;"[O][.][.][.][R][I][G][H][T]"
PRINT AT 18,11;"[A][.][.][.][F][I][R][E]"
FOR J=20 TO 1 STEP -1
PRINT AT 20,J;"[G][A][M][E]█[B][E][G][I][N][S]█"
NEXT J
GOTO 80
PRINT AT 1,16;S1;"."
LET Y1=E1+705
LET X=E1+248
POKE X,0
POKE X,149
POKE X,189
POKE X,155
POKE X,128
IF X-E1=578 THEN GOTO 7550
LET X=X+33
GOTO 7510
PRINT AT 17,16;"▝X▘"
FOR J=1 TO 5
NEXT J
POKE X+33,148
POKE X+66,148
POKE X+99,148
POKE X+132,148
FOR J=1 TO 5
NEXT J
POKE X+33,189
POKE X+33,148
POKE X+66,189
POKE X+66,148
POKE X+99,189
POKE X+99,148
POKE X+132,189
POKE X+132,148
POKE X+131,189
POKE X+131,128
POKE X+127,189
POKE X+127,190
POKE X+127,61
FOR J=1 TO 5
NEXT J
POKE X+132,128
POKE X+99,128
POKE X+66,128
POKE X+33,128
POKE X-1,128
POKE X+1,128
FOR J=1 TO 6
POKE X,189
POKE X,148
POKE X,149
POKE X,128
NEXT J
RETURN
LET Y=Y1
POKE Y-66,173
POKE Y-132,173
POKE Y-66,128
POKE Y-132,128
POKE Y-198,173
POKE Y-264,173
POKE Y-198,128
POKE Y-264,128
POKE Y-330,173
POKE Y-396,173
POKE Y-330,128
POKE Y-396,128
POKE Y-462,173
POKE Y-462,128
LET T=(PEEK 16438)/2
IF (Y1-U)<(T+2) OR (Y1-U)>(T+13) THEN RETURN
LET A=(Y1-U)-T
IF A$(A)<>CHR$ 52 THEN RETURN
IF S<7 THEN GOTO 2200
LET S1=S1+(D*10)
POKE E1,103
LET A$(A-1 TO A+1)="███"
POKE E1,128
IF A$="██████████████" THEN GOTO 90
RETURN
LET E1=PEEK 16398+256*PEEK 16399
LET U=E1+693
PRINT "████████████████████████████████"
PRINT "██████[*][*][*]█[S][C][O][R][E]█.....█[*][*][*]███████"
PRINT "████████████████████████████████"
PRINT "████████████████████████████████"
PRINT "█████▌▐ ▐▐ █▌▐ ▖▐ ▝▌▗▟▗ ▌ ▟█████"
PRINT "█████▌▐ ▞▐▌▐ █ ▘▐▐▌▌ █ ▄▙▖▐█████"
PRINT "█████▌▐ ▌▐█ ▐█ ▖▐ ▗▌▝▜▐ ▌ ▐█████"
FOR J=7 TO 21
PRINT AT J,0;"████████████████████████████████"
NEXT J
POKE 16442,4
PRINT "▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒"
RETURN

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