Knights Move

This program implements the Knight’s Tour puzzle, where the player must move a chess knight to every square on an 8×8 board without revisiting any square. The board is drawn using alternating inverse-video and normal characters to simulate a chequered pattern, and move numbers are printed directly onto the board at calculated screen positions using AT coordinates. Input is taken as a two-character string (e.g. “34” for row 3, column 4), with the subroutine at line 410 handling validation of range and already-visited squares. The program runs in FAST mode during board setup and switches to SLOW for interactive play, a common ZX81 technique to speed up the initial screen draw.

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Program Analysis

Program Structure

The program divides cleanly into several phases:

1. Initialisation (lines 10–160): Dimensions the 8×8 board array, draws the chequered grid, and zeroes all cells.
2. Starting position (lines 170–270): Prompts the player for a starting square, validates it via the subroutine, marks it, and records the position.
3. Main game loop (lines 280–400): Repeatedly asks for the next move, validates it, checks knight-move legality, and updates the board until all 64 squares are visited or the player quits.
4. Input/validation subroutine (lines 410–480): Parses the two-character input string, range-checks both coordinates, and rejects already-visited squares.
5. End/replay (lines 490–520): Offers replay on NEWLINE or halts on any other input.

Board Display

The chequered board is constructed from two alternating string patterns. A$ (line 20) contains inverse-video characters forming one row style, and B$ (line 30) contains the complementary row. These are printed in a loop (lines 80–100) using TAB and the ;; double-semicolon idiom to suppress spacing. Move numbers are placed directly on the board using PRINT AT L*2, C*3+3, mapping logical board coordinates to screen positions. A trailing space (line 240) is printed when the move count is less than 10 to keep single-digit numbers tidy.

Input Parsing

Player input is a two-character string such as "35", parsed at lines 430–440 using VAL C$(1) and VAL C$(2) to extract the row and column digits individually. This avoids any need for numeric input or comma-separated values, keeping entry fast. Entering "0" at line 340 allows the player to quit mid-game.

Knight-Move Validation

Lines 370–380 check whether the proposed square is a legal knight’s move from the current position (L1, C1). The logic attempts to replicate the two valid L-shaped move families:

• Line 370: L=L1±2 paired with C=C1±1
• Line 380: L=L1±1 paired with C=C1±2

However, these conditions contain a well-known ZX81 BASIC operator-precedence bug. The expression L=L1-2 OR L=L1+2 AND C=C1-1 OR C=C1+1 is evaluated as L=L1-2 OR (L=L1+2 AND C=C1-1) OR C=C1+1 because AND binds more tightly than OR. This means a move is accepted whenever either the column offset is ±1 (regardless of the row) or the row offset is ±2 with column offset −1. The correct check would require parentheses grouping both row and column conditions together. The same flaw appears on line 380. As a result, several illegal moves will be accepted and some valid ones may behave unexpectedly.

Array Usage and State Tracking

The 2D array B(8,8) acts as a visited-squares bitmap: cells are set to 1 at line 270 when visited, and the subroutine at line 452 rejects any square where B(L,C)=1. The move counter M serves double duty as both a display label and a win condition (M=64 at line 280).

FAST/SLOW Usage

FAST is engaged at line 50 before the board is drawn, suppressing the display during the computationally intensive screen-building loop. SLOW is restored at line 170 before interactive input begins, restoring the live display for the player.

Notable Idioms and Anomalies

Line	Idiom / Issue
90	;; (double semicolon) suppresses the automatic space between PRINT items
210	Sentinel value L=9 is used to signal invalid input from the subroutine — a classic ZX81 out-of-band return technique
310, 410	Fixed-width space strings are used to blank previous messages rather than CLS, preserving the board
370–380	Operator precedence bug makes knight-move validation incorrect (see above)
510	An empty INPUT response (bare NEWLINE) returns C$="", used as the replay trigger