Draw

Draw is a joystick-driven freehand drawing program that lets users sketch on the screen and overlay geometric shapes. Movement is handled by reading STICK values that map eight compass directions (1, 2, 4, 5, 6, 8, 9, 10) to pixel increments, with boundary wrapping at the 256×176 pixel canvas edges. The program supports drawing circles centered on the cursor, tangent circles in any of eight directions, XOR-mode erasing/duplicating of the last circle via CIRCLE OVER 1, and a COPY command to print the screen. A flag variable tracks which circle type and position was last drawn so that the erase subroutine can reproduce its exact geometry. ON ERR is used for error trapping on the COPY command path, and PAUSE with STICK polling is used in the tangent-circle subroutine to read a directional joystick input before placing the new circle.

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Program Structure

The program is organized into a main loop and two subroutines:

• Lines 10–180: Initialization and main input/draw loop.
• Lines 500–515: Tangent-circle subroutine — prompts for diameter, polls joystick, draws a tangent circle in the chosen direction.
• Lines 1000–1028: Erase/duplicate subroutine — redraws the last circle using OVER 1 (XOR mode) to toggle it.
• Line 9999: Clean exit, restoring paper and ink attributes before STOP.

Main Loop Logic

The loop begins at line 40 with PLOT x,y to mark the cursor, then reads both fire (f) and direction (c) from STICK. Lines 80–110 compute new x and y positions using Boolean arithmetic: each direction code evaluates to 0 or 1, and combined additions and subtractions move the cursor diagonally or cardinally. Lines 100–110 implement wrapping so the cursor re-enters from the opposite edge when it goes out of the 0–255 / 0–175 range.

Lines 130–170 handle drawing vs. erasing. If fire is not pressed (f=0), the loop simply repositions. If fire is held, line 140 detects a lit pixel and erases it with PLOT OVER 1; line 150 detects a dark pixel, plots it, then immediately re-plots with OVER 1 to restore the cursor dot on top. This gives a freehand drawing effect while maintaining a visible cursor.

Direction Code Mapping

STICK returns a numeric code encoding eight directions. The program maps these as follows:

STICK code	Direction	Δx	Δy
1	Up	0	+1
2	Down	0	−1
4	Left	−1	0
8	Right	+1	0
5	Up-Left	−1	+1
9	Up-Right	+1	+1
6	Down-Left	−1	−1
10	Down-Right	+1	−1

Flag System for Circle Tracking

The variable flag records which circle was most recently drawn and its positional relationship to the cursor. A value of 11 means a simple circle centered at (x,y); values 1, 2, 4, 5, 6, 8, 9, 10 correspond to tangent circles offset in each of the eight joystick directions by the diameter d. The erase subroutine at line 1000 uses this flag together with the current x, y, and d to reconstruct the exact circle command with OVER 1, toggling it off or duplicating it.

Notable Techniques

• Boolean arithmetic for movement: Expressions like (c=9)+(c=8) exploit the fact that a true condition returns 1 and false returns 0, avoiding IF branches for cursor movement (lines 80–90).
• XOR drawing with OVER 1: Both freehand erasing (line 140) and circle toggling (subroutine 1000) use PLOT OVER 1 / CIRCLE OVER 1 to invert pixels rather than unconditionally set or clear them.
• ON ERR for COPY robustness: Line 72 sets ON ERR GO TO 75 before the COPY command path (line 73), so a printer error does not crash the program. Line 71 uses ON ERR RESET before jumping to 9999 to clear the handler on exit.
• Joystick-timed input in subroutine: Lines 501–512 use PAUSE 90 followed by STICK polling with individual IF tests for each direction, then PAUSE 20 for debounce before returning.
• Edge wrapping: Lines 100–110 use the same Boolean arithmetic trick: x=x-(x>255)+(x<0) wraps the coordinate in a single expression.

Bugs and Anomalies

• The variable d is shared between the simple circle (line 75) and the tangent circle subroutine (line 500). If the user draws a circle via “c” and then presses “e”, the erase subroutine correctly uses the stored d; however, if the user subsequently draws a tangent circle with a different diameter, d is overwritten and erase of the original circle will use the wrong radius.
• Lines 501–509 check each STICK direction independently with sequential IF statements rather than ELSE-IF logic, so if two conditions are somehow true simultaneously (unlikely with real hardware), multiple circles could be drawn and flag would be set to the last matched value.
• Line 79 clears the screen with CLS but does not reset flag to 0, meaning the erase subroutine could attempt to redraw a circle from a now-cleared screen.
• The REM at line 7 references a “joystik” (typo for joystick) and instructs all input in lower case, which is consistent with the INKEY$ comparisons in the main loop.