Block Art

This program draws a randomly sized rectangle on screen using inverse-video block characters for the border and a fill character derived from the rectangle’s own height value. The dimensions are generated with RND: width T is scaled to 54 columns and height D to 8 rows, with a bias toward smaller heights (line 6 re-rolls D if it exceeds 4). The rectangle is rendered in three passes — a top border row, D middle rows each with side borders and an interior filled with CHR$ D, and a bottom border row — all positioned with AT and TAB expressions based on T and D. After drawing, the program waits for a keypress via CODE INKEY$ before looping back to draw a new rectangle.

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Program Analysis

Program Structure

The program is a tight loop spanning lines 4–110. Lines 4–6 generate random dimensions; lines 7–100 render the rectangle; line 105 provides a keypress gate; and line 110 restarts the loop. Lines 120–140 are bootstrap/save code that runs once on initial load.

1. Line 4–6: Randomise width T (0–54) and height D (0–8), re-rolling D if it exceeds 4 to bias toward shorter rectangles.
2. Lines 7–35: Position the cursor with AT and print the top border row of T inverse-space characters.
3. Lines 50–70: Print D middle rows, each starting and ending with an inverse-space border character, with T-2 interior cells filled by CHR$ D.
4. Lines 75–100: Print the bottom border row, mirroring the top.
5. Line 105: Waits until no key is pressed (NOT CODE INKEY$ is true when INKEY$ is empty), then returns — though there is no corresponding GOSUB, so RETURN here would cause an error; in practice the condition is reversed and the loop simply falls through to GOTO 4.

Dimension Generation

RND**54 uses the exponentiation operator applied twice (i.e. RND * *54 is parsed as RND ^ 54 — raising a 0–1 random number to the 54th power), which strongly biases T toward very small values near zero. Similarly RND**8 biases D toward small heights. The re-roll on line 6 (IF D>4 THEN LET D=RND**8) further skews the height distribution downward.

Rendering Technique

The border character is the inverse-video space (% in the source escaping), producing a solid block. The interior fill character is CHR$ D, meaning the actual character displayed depends on the rectangle’s own height — for small D values this will be a control or graphics character, giving the interior a somewhat unpredictable appearance.

Positioning uses the expression (1-T/2) and (1-D/2) inside AT and TAB. Since these are likely to produce values between 0 and negative numbers (given T can reach 54 and the screen is 32 columns wide on ZX81/TS1000), the coordinate arithmetic is approximate centering that may wrap or clip on smaller screens.

Key BASIC Idioms

• NOT CODE INKEY$ — tests whether INKEY$ is the empty string (CODE of “” = 0, NOT 0 = 1), used as a “no key pressed” sentinel.
• RND**N — raising RND to a power to shape the probability distribution toward smaller values.
• Repeated TAB (1-T/2) calls before each row to re-establish the horizontal position after each PRINT statement.

Bugs and Anomalies

Line	Issue
105	RETURN is used without a preceding GOSUB. When NOT CODE INKEY$ is true the program will attempt to return from a non-existent subroutine call, causing a RETURN without GOSUB error. The intent was likely IF CODE INKEY$ THEN GOTO 4 or similar.
4	RND**54 is parsed as RND ^ 54, not RND * 54. If the intent was a width scaled to 54, the correct expression would be RND*54. The power form produces values extremely close to zero nearly all the time.
7	The AT row/column expressions (1-D/2) and (1-T/2) will frequently yield zero or negative values, which may be clamped or cause errors depending on the interpreter’s AT bounds checking.