Sketch

This program is a simple interactive pixel-drawing tool for the ZX81/TS1000’s 64×44 character-cell graphics display. The user enters a title string, then uses keys 5/6/7/8 to move a cursor left, down, up, and right respectively, with key 0 toggling the pen up and key 1 toggling the pen down to plot pixels. The variable A is initialised to 1 and used as the movement step size throughout, while P acts as a pen-state flag (P=A means draw, P=0 means move without drawing). A SAVE command triggered by key “S” stores the program under the user-supplied title string directly from within the drawing loop.

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Program Analysis

Program Structure

The program is divided into three logical phases:

1. Initialisation (lines 5–13): Sets A=1, X=A, Y=A, and P=A, placing the cursor at position (1,1) with the pen down.
2. Title entry (lines 15–27): Prompts for a title string A$, clears the screen, and prints the title.
3. Main drawing loop (lines 30–140): Continuously polls INKEY$ for movement and pen-state commands, plots or unplots the current pixel, then loops back to line 30.

Lines 150–170 are dead code — they are unreachable because the loop at line 140 always jumps back to line 30. Lines 180–200 appear to be a bootstrap/loader stub that would only execute if the program were run from line 180 directly.

Drawing Loop Mechanics

Each iteration of the main loop performs an UNPLOT at the current position (line 40) and then conditionally re-PLOTs it (line 45) if P=A (i.e., pen is down). This erase-then-redraw pattern is used rather than maintaining a separate cursor graphic, so the “cursor” and the drawn trail are the same pixel. Movement keys map as follows:

Key	Action
"5"	Move left: X = ABS(X - A)
"6"	Move down: Y = ABS(Y - A)
"7"	Move up: Y = Y + A
"8"	Move right: X = X + A
"0"	Pen up: P = 0
"1"	Pen down: P = A
"S"	Save program as A$

These key assignments (5=left, 6=down, 7=up, 8=right) mirror the standard ZX81 cursor-key convention used in many games of the era.

Boundary Clamping

Lines 120 and 130 implement simple boundary checks: if X exceeds 63 or Y exceeds 41, the coordinate is decremented by A (i.e., 1). This prevents the cursor from leaving the 64×44 pixel display area on the right and top edges. However, there is no explicit lower-bound check for X or Y going negative — instead, line 50 uses ABS(X - A) and line 60 uses ABS(Y - A), which means moving left or down past zero reflects the coordinate back to a positive value rather than clamping to zero. This produces a bounce rather than a hard stop at the left and bottom edges.

Use of Variable A as a Constant

Rather than using the literal 1 throughout the program, the step size is stored in A. This means that modifying line 5 alone would change the movement granularity across the entire program, making A an effective configuration constant. It also means the pen-state flag P uses the same value: P=A means pen down, P=0 means pen up, so the pen-down test on line 45 (IF P=A) would remain correct even if A were changed.

Notable Techniques and Anomalies

• The INKEY$ polling on lines 50–110 is not mutually exclusive — each line independently reads the keyboard. On a real machine this is fine since only one key is pressed at a time, but it does mean multiple checks are performed per loop iteration.
• The loop does not contain any PAUSE or delay, so cursor movement speed is governed purely by the speed of the BASIC interpreter executing lines 30–140.
• Lines 150–170 are entirely unreachable via normal flow due to the unconditional GOTO 30 at line 140. They appear to be vestigial code or an earlier draft of the loop exit logic.
• The SAVE A$ command on line 110 saves the program under the user’s chosen title, providing a convenient in-sketch save function, though it saves the entire program rather than just the drawing state.
• Line 190 contains a SAVE with an inverse digit in the filename, serving as the auto-run loader entry point.