Chainsaw

This program implements a carnival shooting-gallery style game called “CHAIN-SAW” for the ZX81/TS1000. Three rows of a scrolling marquee — built from inverse-video characters — move across the screen in opposite directions, simulating a moving target belt; the top and bottom rows scroll left-to-right and right-to-left respectively. The player presses any key to fire a “bullet” (an asterisk) that travels upward from row 21, and the program checks whether it strikes a target on row 12 (the lower belt, string C$) or row 10 (the upper belt, string A$), awarding points equal to the character code of the hit symbol minus 156. The game ends after 10 shots, displaying total shots and score before restarting via RUN.

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Program Analysis

Program Structure

The program is organized into a main animation loop and two subroutines:

1. Initialization (lines 5–6): Score S and shots J are set to zero.
2. String setup (lines 10–30): Three 32-character display strings A$, B$, and C$ are defined.
3. Main loop (lines 40–80): Prints the three-row display, scrolls A$ and C$ in opposite directions, then calls the input/bullet subroutine before repeating.
4. Input and bullet subroutine (lines 100–220): Checks for a keypress; if pressed, animates a bullet travelling upward from row 21 to row 0, calling hit-detection subroutines at rows 12 and 10.
5. Hit detection — lower belt (lines 300–340): Increments shot count, scores if target at position 11 of C$ is not already blank, then blanks it.
6. Hit detection — upper belt (lines 400–430): Scores and blanks if target at position 11 of A$ is not already blank, then advances the scroll.
7. Game over (lines 500–550): Displays result, waits for ENTER, and restarts via RUN.

Display Strings and Scrolling

A$ (line 10) and C$ (line 30) are 32-character strings. Each character pair consists of a block graphic followed by an inverse-video digit or letter (0–9, A–F), making 16 pairs that fill the 32-column display exactly. B$ (line 20) is a static 32-character string spelling “CHAIN-SAW” in inverse video, centred with inverse spaces.

Scrolling is achieved by simple string slicing each frame:

• A$ scrolls left: A$(2 TO 32)+A$(1) — drops the first character and appends it to the end.
• C$ scrolls right: C$(32)+C$( TO 31) — moves the last character to the front.

Because the marquee characters are two-character pairs (graphic + label), scrolling by one character per frame causes the pairs to break up midway through a cycle, giving an asymmetric visual shimmer rather than a clean per-symbol scroll.

Bullet Animation

When a key is detected (line 110–120), the bullet loop (lines 140–190) runs from row 20 down to row 0 in STEP -1, printing an asterisk * at the current row and erasing the previous position one row below. The horizontal position I starts at 1 and increments each step, so the bullet travels diagonally upward-right rather than straight up. Hit-detection subroutines are called when N=12 and N=10, corresponding to the rows occupied by C$ and A$ respectively.

Scoring Mechanism

Scoring exploits the character codes of the block graphics used in the target strings. The formula CODE C$(11)-156 (line 320) and CODE A$(11)-156 (line 410) extract a numeric value from whichever symbol the bullet strikes at column position 11. The block graphic ▀ (ZX81 char 156) used in C$ and the block graphic ▖ (ZX81 char 154) used in A$ both yield small non-negative differences from 156, effectively giving a score of 0 or −2 respectively for a blank hit — though the blank-check guard (lines 310, 400) prevents scoring a previously-hit cell. Targets that have already been hit are replaced with the respective “blank” graphic.

Notable Techniques and Idioms

• Inverse video in strings: The %X escape encodes inverse-video characters directly in string literals, used extensively for the title bar and score display.
• Block graphics as target markers: \..
(▖) marks an active upper-belt target; \'\' (▀) marks an active lower-belt target; hitting changes the cell to the same graphic, effectively making it look identical but registering as spent — a subtle visual bug.
• FAST/SLOW around restart: Lines 520–540 bracket CLS with FAST/SLOW to clear the screen quickly without display flicker.

Bugs and Anomalies

Location	Issue
Lines 310 / 400	The “already hit” check compares C$(11) to \'\' (▀) and A$(11) to \.. (▖), but these are the same graphics used to represent active targets, so an un-hit cell and a previously-hit cell are visually and logically identical. Scoring is blocked by the guard but the player receives no visual feedback that a target was already spent.
Line 150	The erase step prints a space at AT N+1,I-1 using the previous row and column. When N=20 (first iteration), this prints at row 21, column 0 — overwriting the inverse-asterisk prompt printed at line 100.
Line 430	The upper belt A$ is scrolled an extra step inside the hit-detection subroutine (line 430), meaning a hit causes the belt to advance one extra character that frame, creating a slight lurch in the animation.
Lines 310–320 vs 400–410	Shot counter J is only incremented in the lower-belt subroutine (line 300/310), not in the upper-belt subroutine (line 400). Hitting the upper belt does not consume a shot, potentially allowing more than 10 scoring events.