Mystery

This program implements a classic six-card binary number-guessing trick, where the computer deduces a chosen number between 1 and 63. Each of the six cards displays a subset of numbers corresponding to a specific bit position in binary representation; the user presses Y or N for each card, and the program accumulates the answer using powers of 2. The title screen uses inverse-video characters for the “PRESS ANY KEY” prompts at lines 80 and 320. The subroutine structure is unusual: GOSUB 500 handles the first card’s Y/N input via a FOR loop, while GOSUB 550 re-enters that same loop mid-execution by jumping to the NEXT E statement, a deliberate shared-loop technique. The final calculation at line 260 sums B(X)*2^X across all six cards, then divides by 2 at line 290 to correct for the off-by-one in the exponent, correctly recovering the chosen number.

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Program Analysis

Program Structure

The program is divided into distinct phases: title/instructions (lines 5–90), card display and input collection (lines 100–230), result calculation and display (lines 240–350), and two subroutines (lines 500–570). A RUN at line 350 restarts the program for replay. Lines 580–590 appear to be a saved-program header artifact and are never reached during normal execution.

1. Lines 5–90: Initialisation, title box, instructions, and wait for keypress
2. Lines 100–230: Six card screens shown in sequence, each calling a subroutine
3. Lines 240–290: Binary accumulation and result reveal
4. Lines 300–350: Pause, replay prompt, restart via RUN
5. Lines 500–540: Subroutine — FOR loop body collecting Y/N for each card
6. Lines 550–570: Subroutine entry point for cards 2–6, jumping into the middle of the FOR loop

The Shared FOR-Loop Subroutine Technique

The most notable structural feature is the split subroutine. GOSUB 500 (line 130) initiates a FOR E=1 TO 6 loop and reads the first card’s response. Subsequent cards (lines 150–230) each call GOSUB 550, which enters at the NEXT E statement, advancing the same loop variable and reading subsequent cards before returning. This deliberately reuses the FOR loop stack frame across multiple GOSUB/RETURN pairs — an unusual but functional technique that saves repeating the input-reading logic.

Input Handling

Y/N input at line 510–530 uses a busy-wait on INKEY$, accepting only "Y" or "N". Line 530 stores VAL A$ into the array B(E). Since VAL "Y" and VAL "N" both evaluate to 0 in Sinclair BASIC, this is a bug — both answers will store 0, making the trick non-functional. The programmer likely intended to store 1 for “Y” and 0 for “N”, perhaps using a conditional assignment. Line 535 provides a key-debounce loop to drain any held key before continuing.

Binary Calculation and Off-by-One Correction

Lines 250–270 accumulate the result as F = F + B(X) * 2^X for X = 1 to 6. Because the exponent starts at 1 rather than 0, the accumulated value is exactly double the intended result. Line 290 corrects this by printing F/2. The cards themselves correspond to bit positions 0–5 of a 6-bit number (covering 1–63), which is consistent with dividing by 2 at output time.

Key BASIC Idioms

• IF INKEY$="" THEN GOTO 90 — standard busy-wait for any keypress
• PRINT AT row,col; — precise cursor positioning for card layout
• Double-comma ,, in PRINT statements skips two lines efficiently
• RUN at line 350 resets all variables and restarts cleanly, avoiding the need for explicit re-initialisation
• Inverse-video text (e.g., %P%R%E%S%S%) used for prominent prompts at lines 80 and 320

Card Display and Graphics

Each card is framed by block-graphic border rows printed at lines 111’s AT 6 and AT 14 positions, using \:′ and \:. corner/edge block characters to form a horizontal rule above and below the number grid. The numbers within each card correspond precisely to the correct binary bit-mask subsets for a 6-bit guessing trick (e.g., card 1 shows all odd numbers; card 2 shows numbers where bit 1 is set, etc.).

Bugs and Anomalies

Line	Issue	Effect
530	LET B(E)=VAL A$ — VAL "Y" and VAL "N" both yield 0	All six cards record 0; computed result is always 0, breaking the trick
300–310	FOR C=1 TO 20: NEXT C is an empty delay loop	Negligible pause on fast hardware; an inadequate substitute for PAUSE
560–570	STOP and CLEAR follow the subroutine and are unreachable in normal flow	No effect; likely leftover debugging lines

Variable Summary

Variable	Purpose
F	Accumulator for the binary sum
B(6)	Array storing Y/N response (0 or 1) for each of the 6 cards
Y, N	Declared at lines 20–30 but never used (likely vestigial)
E	FOR loop variable shared across the split subroutine
A$	Temporary storage for the keypress character
C	Loop counter for the delay at line 300
X	Loop variable for the binary accumulation at line 250