OVERS 1

This program demonstrates the OVER 1 print attribute by overlaying two user-supplied characters on the same screen position using a CHR$ 8 (backspace/cursor-left) control code to reposition the cursor. The user enters any two characters; the first is printed, the cursor backs up one space, OVER 1 is engaged, and the second character is XOR-composited on top of the first. The result and the individual characters are then displayed as a labeled equation. Input validation checks both for a space (stop condition) and for exactly two characters before proceeding.

From page 70 of Roger Valentine’s The Timex Sinclair 2068.

Program Analysis

Program Structure

The program is a short interactive loop with a title display, input, validation, and a single output line before cycling back. There is no initialization beyond the title print at line 10.

The OVER 1 Demonstration Technique

Line 50 is the heart of the program. It performs the following sequence in a single PRINT statement:

1. Print q$(1) — the first character, advancing the cursor one cell right.
2. Print CHR$ 8 — move the cursor one cell left, back over the printed character.
3. Activate OVER 1 — switch to XOR pixel-compositing mode.
4. Print q$(2) — the second character is XOR-overlaid onto the first in the same cell.
5. Print the remainder of the line as a plain-text label showing the two source characters.

CHR$ 8 is used instead of TAB or AT so the overlay demo fits naturally within a streaming PRINT statement without needing to know the current cursor column. OVER 1 remains in effect only for the immediately following item (q$(2)); subsequent items in the same statement revert to normal because no explicit OVER 0 is needed — the attribute is item-scoped within a PRINT statement.

Input Validation

Two guard conditions protect the main display line. Line 30 treats a lone space as a sentinel exit value via CHR$ 32, which avoids the user needing to type a command to quit. Line 40 rejects any input whose length differs from 2, looping back to the prompt with GO TO 20. Together, these ensure q$(1) and q$(2) are always safe to subscript at line 50.

Notable BASIC Idioms

• Embedded double-quotes in the title string at line 10 are represented by doubled "" pairs inside the string literal, which is standard Sinclair BASIC string quoting.
• The INPUT prompt at line 20 uses the inline string form, combining the prompt and the variable assignment in one statement.
• The program relies on the fact that OVER (and other print attributes like INK, PAPER) placed inline in a PRINT statement act as temporary modifiers scoped to the next printable item only.

Bugs and Anomalies

There are no functional bugs. One minor behavioral note: if the user enters exactly one character followed by pressing Enter, the LEN check at line 40 correctly rejects it. However, if INPUT is given an empty string (just Enter), q$ will be empty, the CHR$ 32 check will fail, and the LEN check will also catch it, so the loop still handles this gracefully.

   10 PRINT "EXPERIMENTS WITH "" OVER """''
   20 INPUT "Enter any 2 characters (or space to stop) ";q$
   30 IF q$=CHR$ 32 THEN STOP 
   40 IF LEN q$<>2 THEN GO TO 20
   50 PRINT q$(1);CHR$ 8; OVER 1;q$(2);" = ";q$(1);" OVER ";q$(2)
   60 GO TO 20
 9998 SAVE "OVERS 1" LINE 0