Draw

This is an interactive drawing program that lets the user move a cursor around the ZX81/TS1000 screen using keys 5, 6, 7, and 8 (left, up, down, right) and toggle between draw and erase modes with keys R and D. The program uses PLOT and UNPLOT to set and clear individual pixels, with the cursor constrained to the plotting area (X: 0–50, Y: 10–43). Mode switching is handled by storing the current mode in the string variable F$, where “R” means erase (UNPLOT before each PLOT) and “D” means draw. The SAVE command on line 500 includes an inverse-video character in the filename, which acts as an auto-run flag.

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Program Analysis

Program Structure

The program is divided into logical blocks by line number ranges:

1. Lines 5–30: Initialisation — title print, cursor position (X=0, Y=10), and default mode (F$="R").
2. Lines 100–110: Mode-switching input — checks for R (erase) or D (draw).
3. Lines 190–230: Erase step and movement input — conditionally UNPLOTs, then reads directional keys.
4. Lines 300–330: Boundary clamping — keeps cursor within the drawable area.
5. Line 340: Plots the new cursor position.
6. Line 400: Main loop jump back to line 100.
7. Lines 500–510: SAVE and LIST utilities, not part of the running program.

Drawing Modes

The variable F$ controls whether the program is in draw or erase mode. When F$="R", line 190 executes UNPLOT X,Y before the new position is plotted at line 340, effectively erasing the previous pixel and producing a moving cursor with no trail. When F$="D", line 190 is skipped, leaving every visited pixel set — this is the actual drawing mode. The naming is slightly counter-intuitive: “R” (for “Rubber”?) erases, while “D” draws.

Input Handling

All input is polled via INKEY$ inside the main loop with no debouncing or waiting. Movement uses the classic ZX81 cursor key layout: 5=left, 6=up, 7=down, 8=right. Mode switching on lines 100 and 110 re-reads INKEY$ independently on each line, meaning the key must still be held when each line executes — a potential race condition where a quick keypress might be missed or partially registered.

Boundary Clamping

Lines 300–330 clamp X and Y to the valid plotting range before the PLOT on line 340. The Y range of 10–43 keeps the cursor out of the text area used by the title on line 5. The X range of 0–50 is well within the ZX81’s 64-pixel-wide display. Clamping is done with simple IF/LET pairs rather than MIN/MAX functions (which are not available in this BASIC).

Notable Techniques and Anomalies

• The program uses separate INKEY$ reads for mode switching (lines 100, 110) and movement (lines 200–230), meaning up to five independent key polls happen per loop iteration — potentially causing inconsistent behaviour at borderline key timings.
• In erase mode (F$="R"), UNPLOT is applied to the current position before the move is processed, so the old pixel is cleared and then the new position is plotted, giving a single-pixel travelling cursor effect.
• The default mode at startup is F$="R" (erase/cursor), so the user must press D to begin drawing.
• There is no mechanism to detect or handle INKEY$ returning an empty string differently from no input — movement simply does not occur if no key is held, which is the expected ZX81 idiom for real-time control.

Variable Summary

Variable	Purpose
X	Horizontal cursor position (0–50)
Y	Vertical cursor position (10–43)
F$	Mode flag: “R” = erase/cursor, “D” = draw