Target Practice

This is a ZX81/TS1000 shooting-gallery style game where a scrolling target string moves across the screen and the player steers a cursor left or right with keys 5 and 8, firing with key B. The program opens with a machine-code call via RAND USR 16514 before entering the main game loop. Targets in the string are marked with special inverse characters (X, M, H) worth different point values, and a running high-score is maintained across rounds using variable U. The score calculation at line 5040 multiplies hits by 6453 and adds a small random offset, and the SAVE command at line 5520 with an inverse character in the filename acts as an auto-run marker.

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Program Analysis

Program Structure

The program is divided into clearly separated functional blocks:

1. Initialisation (lines 1–5): Sets FAST mode, calls machine code, restores SLOW mode, and zeroes the high-score variable U.
2. Round setup subroutine (lines 1000–1500): Initialises the scrolling target string A$, resets score accumulator S, and draws the playfield borders.
3. Main game loop (lines 10–700): A FOR K=1 TO 70 loop advances the scroll, reads keys, and dispatches to the fire subroutine.
4. Fire subroutine (lines 5000–5500): Checks the character under the cursor, awards points, animates a shot, recalculates score Q, and conditionally drifts the cursor position.
5. Save block (lines 5510–5530): Dead code reached only by fall-through; saves and re-runs the program.

Machine Code Usage

Line 3 executes RAND USR 16514. Address 16514 (0x4082) is in the ZX81/TS1000 system variable area rather than in free RAM, suggesting this call patches or reads a system variable rather than running a standalone routine. No POKE sequence loads machine code before this call, so it relies on whatever bytes happen to reside there — likely a short return or a known ROM entry point used as an initialisation trick by the original author.

Target String and Scrolling

The target string A$ (line 1000) is 33 characters wide and contains three types of inverse-video targets embedded in a field of inverse spaces:

Inverse char	Meaning	Base bonus
%X	Standard target	+1 (from generic hit check)
%M	Medium target	+1 (generic) +1 (specific) = +2
%H	High-value target	+1 (generic) +2 (specific) = +3

Scrolling is achieved at line 200 with the classic Sinclair BASIC string-rotation idiom: LET A$=A$(2 TO )+A$(1), which shifts every character one position left and wraps the first character to the end, cycling the pattern indefinitely over the 70-iteration round.

Key Handling and Player Control

Three keys are polled inside the main loop using INKEY$:

• "5" — moves cursor column Y left (decrements).
• "8" — moves cursor column Y right (increments).
• "B" — triggers the fire subroutine at line 5000.

Keys 5 and 8 match the standard ZX81 left/right convention. There is no boundary clamping on Y from the movement keys, though the fire subroutine at line 5070 only randomises drift if Y>2 AND Y<30.

Scoring

Hit accumulator S is incremented in the fire subroutine (lines 5000–5003). The final displayed score Q is computed as S*6453+INT(RND*3). Multiplying by 6453 inflates raw hit counts into large, impressive-looking scores; the random offset of 0–2 adds minor unpredictability. The score is only printed (line 5050) when it exceeds 5000, i.e. roughly after one confirmed hit. High score U is updated and displayed at lines 520–530 at round end.

Shot Animation

When B is pressed, the subroutine at line 5000 runs a FOR A=18 TO 1 STEP -3 loop (lines 5008–5030), printing an inverse asterisk %* then immediately overwriting it with an inverse space % at each row, creating a crude upward-travelling bullet animation without any PAUSE delay — relying on the inherent SLOW-mode display overhead for timing.

Round Transition and High Score Persistence

After 70 iterations the program prints “END OF ROUND” in inverse video (line 510), updates the high score, waits 60 iterations of an empty loop (lines 540–550), briefly flashes a “STAND BY” message, then uses LET Z=RND*RND (line 560) as a dummy delay before looping back to start a new round via GOTO 10. The high score variable U persists across rounds because GOTO 10 does not re-execute line 5 where U=0 was set.

Notable Techniques and Anomalies

• The REM at line 1 contains block graphics and inverse characters, which on the ZX81 is a common location for machine-code bytes; the content here appears decorative rather than functional given USR 16514 does not point into it.
• The cursor display at line 100 uses "% %*% " — an inverse space, an asterisk, and a plain space — as the player’s gun sight marker, printed relative to Y-1.
• Lines 5510–5530 (CLEAR / SAVE / RUN) are unreachable during normal play; they would only execute if control somehow fell through past RETURN at line 5500, making them effectively a save-and-restart utility embedded in the listing.
• The IF A$(Y)<>"% " THEN LET S=S+1 check at line 5000 awards a generic point for any non-space hit, and the subsequent specific checks for %H and %M stack additional points on top, so an %H hit correctly scores 3 total.