Rockets

This program draws an ASCII-art rocket on screen using a sequence of BORDER and BEEP calls to create a countdown effect before the rocket appears. After rendering the rocket with character-graphics lines (nose cone, body, and exhaust flames using backslashes and exclamation marks), it scatters 40 randomly positioned stars across the screen using PRINT AT with RND-generated row and column values. The program ends with a “HURRAY!” message and loops back via GO SUB to repeat the border-color and beep sequence. Line 15’s STOP is bypassed on auto-run because execution begins at line 20, leaving line 15 as a manual-run guard.

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Program Analysis

Program Structure

The program is divided into three logical phases. Lines 10–90 implement a countdown using alternating BORDER color changes and BEEP tones with increasing PAUSE durations (60, 120, 180 frames). Lines 100–270 draw the rocket entirely with ASCII characters via successive PRINT statements. Lines 280–370 scatter random stars, print a victory message, then loop back to line 10 via GO SUB 10 to repeat the light-and-sound sequence indefinitely.

Rocket Drawing Technique

The rocket is drawn starting at AT 21,16 — near the bottom-right of the screen — and uses the double-comma (,,) idiom at the end of each PRINT statement. In Sinclair BASIC, a trailing comma after a PRINT item moves to the next print zone; two commas together effectively advance to the start of the next line at the same horizontal tab position, stacking the rocket body vertically in a column. The rocket consists of 17 lines: a pointed nose cone, rectangular body, and flame exhaust using /, \, and ! characters.

Star Field Generation

Lines 320–340 use a FOR loop to place 40 asterisks at random screen positions. INT(RND*32) generates a column in the range 0–31 and INT(RND*22) generates a row in 0–21, covering the full 32×22 character display. These are passed directly to PRINT AT Z,A;"*", which can overwrite parts of the previously drawn rocket — a minor cosmetic anomaly.

Control Flow and the GO SUB Idiom

GO SUB 10 at line 360 jumps back into the countdown sequence but never formally returns via RETURN, since the countdown falls through into the rocket-drawing and star-scattering code again. This creates an infinite animation loop rather than a true subroutine call, and the return address pushed onto the GOSUB stack is never consumed, which will eventually exhaust stack space after many iterations.

Notable Techniques and Anomalies

• Line 15 contains a STOP statement that is skipped during normal auto-run execution (which begins at line 20), acting as a safeguard if the program is manually RUN from line 1.
• PAPER 0 and INK 7 at lines 315–317 set black background and white ink before the star field, providing contrast for the asterisks but potentially altering the appearance of the already-drawn rocket text.
• The increasing PAUSE durations (60, 120, 180) create an accelerating countdown feel despite being evenly spaced musically.
• The BEEP pitches rise by one semitone each step (3, 4, 5, then 6), giving an ascending scale effect synchronized with the border color changes.
• The trailing STOP at line 370 is unreachable in normal execution flow due to the unconditional GO SUB 10 at line 360.

Line Reference Table

Lines	Function
10–90	Countdown: cycling BORDER colors, BEEP tones, PAUSE delays
100–270	ASCII rocket drawing via stacked PRINT statements
280–310	Post-draw border/beep flash
315–317	Set PAPER/INK attributes for star field
320–340	Random star field loop (40 stars)
350–370	Victory message, loop via GO SUB, unreachable STOP