Multiples

This program prints all multiples of a user-supplied number in the range 0–99, displaying them on screen with the non-multiples replaced by block graphic characters. The program prompts for any integer from 0 to 99, with a special case that substitutes 100 for 0 (since every number is a multiple of 0, which would cause a division-by-zero or trivial result). Numbers that are not multiples are rendered using ZX81 inverse-video block graphics via CHR$ values 156 and above, creating a visually distinctive display. The loop uses nested FOR loops over tens (J) and units (K) digits, reconstructing each two-digit number as 10*J+K. Line 600 saves the program with an auto-run flag, and line 610 re-runs it after saving.

---

Program Analysis

Program Structure

The program is divided into four logical sections:

1. Setup (lines 5–30): Displays the title in inverse video, prompts for input, and clears the screen.
2. Main loop (lines 100–300): Iterates through all numbers 0–99 using nested FOR loops and prints results.
3. Multiple handler (lines 500–520): Called via GOTO 500 when a multiple is found; prints the number using CHR$ block graphics.
4. Save/run stub (lines 600–610): Saves the program with an auto-run flag, then restarts.

Loop Mechanics

Rather than a single loop from 0 to 99, the program decomposes each number into tens digit J (0–9) and units digit K (0–9), reconstructing the value as M = 10*J+K at line 230. Line 210 contains the escape , which is the ZX81 block graphic for character code 66 — however, in context this appears inside a FOR statement and is likely a stray/erroneous character that is ignored or causes a syntax quirk; the effective upper bound for K is 9.

Line 220 adds a leading space when J=0 to align single-digit numbers visually.

Multiple Detection

Line 240 uses the classic integer-division test: IF INT(M/N)*N=M THEN GOTO 500. This checks whether M is exactly divisible by N without using the MOD operator (unavailable in ZX81 BASIC). When a multiple is found, execution jumps to line 500; otherwise the raw number is printed at line 250.

Block Graphic Output for Multiples

Lines 500–510 print the multiples using CHR$(J+156) and CHR$(K+156). On the ZX81, characters 156–165 are inverse-video digits (0–9), so adding 156 to each digit index produces the corresponding inverse digit character. This makes multiples visually stand out from the ordinary printed non-multiples — an effective use of the character set without any PRINT AT or attribute manipulation.

Zero Special Case

Line 110 replaces N=0 with N=100. Since every integer is a multiple of 0 only in a degenerate sense (or produces division-by-zero in the test), substituting 100 ensures no number in the 0–99 range satisfies the divisibility condition, so all numbers are printed in normal video — a pragmatic workaround.

Key BASIC Idioms and Techniques

• Integer-division modulo test: INT(M/N)*N=M (line 240)
• Inverse-digit rendering via CHR$(digit+156) (lines 500–510)
• GOTO 260 at line 520 re-enters the NEXT K line directly, continuing the inner loop after printing a multiple — this is a deliberate structured jump into the loop body.
• Title displayed in inverse video using %-escaped characters (line 5).
• After printing all results, GOTO 10 at line 300 loops back to prompt for a new number without restarting the program.

Bugs and Anomalies

Line	Issue	Impact
210	block graphic escape appears at end of FOR K=0 TO 9	Likely a spurious character; may cause a syntax error or be silently ignored depending on interpreter behaviour.
270–280	Two consecutive PRINT statements produce a blank line between each row of ten numbers	Cosmetic: produces double-spaced output rows; probably intentional for readability but wastes screen space.
110	Substituting 100 for 0 means no multiples are ever shown when 0 is entered	Arguably incorrect — all numbers are multiples of itself if N=0 is treated as “no filter”. The substitute value 100 is outside the display range, so the output appears to show no multiples at all.