13 Colonies

This program displays the original 13 colonies in the order they ratified the Constitution, animating a shooting-star or firework effect for each colony before drawing a representation of the 13-Colonies flag. Each colony name is printed at the bottom of the screen while a character animates upward from a random horizontal position, followed by a burst pattern built from block graphic characters (CHR$ 133, 134, and 6). The flag is rendered using string arrays pre-filled with CHR$ 128 (a solid block graphic) for red and white stripes, CHR$ 8 for green stripes, and CHR$ 155 for a circular star arrangement in the canton area. The program loops continuously after displaying “HAPPY FOURTH OF JULY” until a key is pressed.

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Program Analysis

Program Structure

The program has three distinct phases:

1. Initialization (lines 5–260): A REM comment describes the program’s purpose; string arrays are dimensioned and pre-filled; the machine switches to SLOW mode for display.
2. Colony animation loop (lines 270–350): For each of the 13 colonies, the colony name is printed, a rising character is animated from a random column, a burst is drawn at the top, and the screen is cleared.
3. Flag display (lines 360–500): The 13-Colonies flag is drawn using block graphics, a “HAPPY FOURTH OF JULY” message is shown, and the loop restarts after a keypress.

Array Pre-filling Strategy

Lines 190–250 pre-fill three string arrays before the display phase begins:

• B$(F) (30 chars): each character set to CHR$ 128 — a solid block, used for red stripes.
• G$(F) (30 chars): each character set to CHR$ 8 — used for white/green stripe rows.
• S$(F) (12 chars): each character set to CHR$ 128, used for the solid canton background.

By filling these arrays once and then PRINTing them as whole strings, the program avoids repeated character-by-character output during the flag drawing phase.

Colony Animation Technique

For each colony (line 270–350), the program picks a random column R = RND*20+5 (lines 5–25). A FOR loop (lines 300–320) counts from row 20 up to row 5; at each step it prints CHR$ 133 (a block graphic resembling a rising spark), then immediately overwrites it with CHR$ 0 (a space/null character) to erase it, creating the illusion of upward motion. At the apex (line 330), four additional block graphics (CHR$ 134 and CHR$ 6) are printed in a cross/burst pattern around the final position, simulating an explosion or firework burst. The screen is then cleared with CLS before the next colony.

Flag Rendering

The flag is built in two passes. Lines 360–410 print alternating rows of B$ (solid blocks) and G$ (lighter fill) six times each, plus one final B$, producing 13 stripe rows. Lines 420–450 overprint the upper-left canton area with S$ (12-character solid block strings) to create the blue canton background.

Line 460 then places CHR$ 155 characters (a star UDG or block graphic) in a circular arrangement across several rows and columns to represent the ring of 13 stars in the canton. The positioning uses a combination of AT row,col and TAB within a single long PRINT statement.

Key BASIC Idioms

• FAST / SLOW (lines 10, 260): Initialization is done in FAST mode to suppress display; animation runs in SLOW mode for visible output.
• IF INKEY$="" THEN GOTO 480 (line 480): Standard busy-wait keypress loop — waits until any key is pressed before restarting the colony animation.
• GOTO 270 (line 500): Returns to the top of the colony loop, creating an infinite cycle.
• DIM C$(13,14) (line 50): A two-dimensional string array with 13 rows of 14 characters stores the colony names, padded to the fixed length.

Notable Observations and Anomalies

• Line 330 contains a syntax irregularity: AT 4,R;CHR$ 128;AT 3,R-1;CHR$ 134;5,R-1;CHR$ 6 — the third AT keyword appears to be missing before 5,R-1. On the ZX81/TS1000, omitting AT while a row/column pair follows a semicolon may cause unpredictable positioning.
• Line 610 is numbered 610, well past line 500, making it unreachable during normal execution — it serves only as a SAVE command embedded in the listing.
• CHR$ 8 used for G$ is the ZX81 cursor-left control character, not a block graphic; its use as a “stripe” filler likely produces blank or unexpected characters rather than a visible green stripe.
• The RND*20+5 expression (line 290) can produce column values up to 25, which may push burst characters off the right edge of the 32-column screen depending on the value of R.