Scrolls

This program demonstrates eight different software-driven text scroll routines in BASIC, sourced from the book “Mastering Your Timex” by Hartnell and Jones. The core technique stores the entire 22-row by 32-column display (704 characters) in a single string array variable, then shifts it by manipulating substrings before reprinting with PRINT AT 0,0. Vertical scrolls move the string by 32-character (one row) increments, while horizontal scrolls use a nested loop stepping through each row with STEP 32 to shift characters one position at a time. Wrap-around variants preserve the displaced characters by concatenating the “scrolled-off” portion back onto the opposite end of the string rather than replacing with spaces.

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Program Analysis

Program Structure

The program is organized into a menu front-end (lines 4–21) and eight self-contained scroll subroutines located at high line numbers (9500–9850). Each routine is reached by a direct GO TO from the menu dispatcher and returns to line 1 (which falls through to the menu at line 6) via GO TO 1. The high-numbered placement of the routines is a common BASIC practice to keep them out of the way of any future expansion of the main program body.

Display Buffer Strategy

Every routine allocates a 704-character string array a$ with DIM a$(704). This exactly models the 22-row × 32-column visible text area (22 × 32 = 704). The user fills this buffer via INPUT "Enter text: ";a$, and the entire buffer is blasted to the screen each iteration with PRINT AT 0,0;a$, relying on the interpreter to wrap the long string across rows automatically.

Scroll Algorithms

The eight routines fall into two families — non-wrapping (options 1–4) and wrapping (options 5–8) — each covering all four cardinal directions.

Option	Direction	Wrap	Key operation
1	Up	No	a$=a$(33 TO )+"                                "
2	Down	No	a$="                                "+a$( TO 672)
3	Left	No	Per-row: a$(f TO f+31)=a$(f+1 TO f+31)+" "
4	Right	No	Per-row: a$(f TO f+31)=" "+a$(f TO f+30)
5	Up	Yes	a$=a$(33 TO )+a$( TO 32)
6	Down	Yes	a$=a$(673 TO )+a$( TO 672)
7	Left	Yes	Per-row: a$(f TO f+31)=a$(f+1 TO f+31)+a$(f)
8	Right	Yes	Per-row: a$(f TO f+31)=a$(f+31)+a$(f TO f+30)

Vertical scrolls operate on the whole 704-character string in a single assignment, shifting by 32 characters per step. Horizontal scrolls use a FOR f=1 TO 673 STEP 32 inner loop to process each of the 22 rows individually, shifting by one character per step within each 32-character row slice.

Notable Techniques

• Single-string display buffer: Packing the entire screen into one string and printing it with a single PRINT AT 0,0 avoids the overhead of per-row print statements and keeps the scroll loop tight.
• Substring slice assignment: The BASIC interpreter supports in-place slice assignment (a$(f TO f+31)=...), allowing each row to be updated without rebuilding the entire string for horizontal scrolls.
• Wrapping via concatenation: Wrap routines reattach the displaced portion directly — for example, scroll-up-with-wrap at line 9695 appends the first row back at the tail: a$=a$(33 TO )+a$( TO 32).
• Menu input validation: Line 13 uses a compound IF a<1 OR a>8 THEN GO TO 12 to re-prompt on out-of-range input before the chain of equality tests at lines 14–21.

Bugs and Anomalies

• Off-by-one in horizontal scroll loop bound: The FOR f=1 TO 673 STEP 32 loop visits 22 values: 1, 33, 65, …, 673. With a 32-character slice f TO f+31, the last iteration addresses characters 673–704, which is correct. However, the loop runs FOR n=0 TO 32 (33 iterations) for left/right scrolls, one more than the 32-column width needed for a full single-pixel shift cycle — the final iteration is redundant but harmless.
• Re-declaration of a$ each routine: Every routine starts with DIM a$(704), which resets the array to spaces before the user fills it. This is intentional for isolation between menu selections but means any previous display content is discarded.
• GO TO 1 target: Line 1 does not exist; execution falls through to line 4 (the first REM), then continues to line 6 where BORDER, PAPER, and CLS reset the display before the menu redraws. This is an intentional fall-through, not a bug.