Sidescroll

This program demonstrates a one-line horizontal text scrolling technique by continuously printing a 32-character window of a long string at screen row 10. It uses BASIC’s substring notation `A$(X TO X+31)` to extract successive 32-character slices and prints them at a fixed screen position using `PRINT AT`, creating the illusion of smooth scrolling. A `PAUSE 10` between frames controls the scroll speed. The program halts automatically when the window reaches the end of the string, detected by checking `(X+31)=Z` against the string’s length.

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Program Analysis

Program Structure

The program is organized into a simple sequential loop with a setup phase, a display loop, and a termination condition:

1. Line 5 — Sets display attributes: white ink, black paper, black border.
2. Line 10 — Defines the scroll string A$, a long message with deliberate padding of trailing spaces.
3. Line 20 — Initializes the window pointer X=1 and caches the string length in Z.
4. Lines 30–50 — Main scroll loop: print a 32-character window, advance the pointer, check for end condition, pause, then repeat.
5. Line 9997 — Redundant STOP guard (never reached in normal execution).
6. Line 9998 — SAVE with auto-run at line 1 (but execution begins at line 5, so the auto-run target is effectively a no-op start).

Scrolling Technique

The core of the effect is on line 30:

PRINT AT 10,0;A$(X TO X+31);" ";

This extracts a 32-character substring starting at position X and prints it at column 0 of row 10. Because the PRINT AT statement overwrites exactly those character cells and the trailing " " suppresses a newline scroll, the display updates in-place each iteration. Incrementing X by 1 on each pass shifts the visible window one character to the left, producing the scrolling illusion.

Termination Condition

Line 40 checks (X+31)=Z to detect when the right edge of the 32-character window has reached the last character of the string. At that point execution stops. Note this uses equality rather than >=, so it depends on the loop always advancing by exactly 1 — which it does. The large block of trailing spaces in A$ ensures the message fully scrolls off before the stop condition is triggered.

Key BASIC Idioms

• LET Z=LEN A$ cached at line 20 avoids recalculating string length on every iteration, a minor but sensible optimization.
• The trailing semicolon on the PRINT statement suppresses the automatic newline and prevents the display from scrolling vertically.
• PAUSE 10 (approximately 0.2 seconds at 50 Hz) throttles the scroll speed; removing or reducing it would make the scroll nearly unreadable.
• A$(X TO X+31) uses the standard slice notation to extract a fixed-width window from the string.

Potential Bugs and Anomalies

Issue	Detail
Auto-run line mismatch	SAVE "SIDESCROLL" LINE 1 targets line 1, which does not exist; execution would begin at line 5 (the first actual line), so this is harmless in practice.
Off-by-one risk	If A$ length is not carefully chosen such that (X+31)=Z is ever exactly true, the loop would run past the end of the string and trigger a substring error. The generous trailing spaces in A$ guard against this in practice but the logic is fragile for arbitrary strings.
No wraparound	The scroll is one-shot only; there is no mechanism to loop the message continuously.