Fire Works White on Black

This program renders an animated fireworks effect on screen using block graphics characters stored in a string variable. It first fills the entire display with a pattern of inverse-space and full-block characters to create a dark background, then draws five randomly positioned “burst” animations by expanding rings of characters outward in eight directions simultaneously. After the random bursts, a final centered explosion expands from position (10,15) with a radius of 8, before looping back to repeat. The program uses POKE 16418,0 to disable the system clock interrupt (or modify a system variable), and stores four animation-frame characters in A$ including block graphic characters for visual effect.

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Program Analysis

Program Structure

The program is divided into four logical phases:

1. Initialization (lines 1–6): Defines the animation character string A$, sets a system variable via POKE 16418,0, and fills the screen with a dense background pattern of inverse spaces and full-block graphics.
2. Random burst loop (lines 7–27): Iterates five times (C=1 TO 5), each time choosing a random screen position and animating an expanding ring of characters.
3. Centered explosion (lines 40–45): Performs a single large burst centered at row 10, column 15, expanding to radius 8.
4. Loop (line 46): Jumps to line 2 (not line 1), reusing the background without re-initialising A$, creating an infinite animation cycle.

Background Fill Technique

Lines 4–6 use a FOR X=0 TO 22 loop printing a long literal string at each row using PRINT AT X,0;. The string contains alternating inverse-space characters (% %) and a long run of full-block graphic characters (\''), creating a solid, patterned dark background that acts as the “night sky” for the fireworks.

Animation Frame Characters

The string A$ at line 1 contains four characters used as successive animation frames:

Index (Y)	Escape / Character	Visual
1	\.'	▞ (top-right + bottom-left block)
2	\:.	▙ (three-quarter block)
3	%"	inverse quote character
4	%.	inverse period character

Cycling through these four characters as the burst radius grows gives a crude animation effect simulating the changing density or shape of an explosion.

Eight-Direction Expansion

The core burst technique in lines 20–22 (and 42–44) prints the current frame character at eight compass positions relative to the burst centre simultaneously in a single PRINT statement:

• Left and right: AT A,B-Z and AT A,B+Z
• Up and down: AT A-Z,B and AT A+Z,B
• Four diagonals: AT A-Z,B-Z, AT A-Z,B+Z, AT A+Z,B-Z, AT A+Z,B+Z

Using a single PRINT statement with multiple AT items is an efficiency idiom that avoids repeated PRINT AT calls and their associated overhead.

POKE 16418,0

POKE 16418,0 writes to the ZX81/TS1000 system variable FRAMES (low byte), resetting the frame counter. This has no direct visual effect on the animation but may be used to synchronise timing or simply as a housekeeping step at program start.

Loop Architecture and GOTO 40

Line 28 uses GOTO 40 to skip the SAVE command at line 30 during normal execution. Line 30 is a SAVE "1009%5" which stores the program; the %5 represents an inverse digit 5 in the filename. Line 31 redirects to line 1 if the save path were ever reached. The main loop at line 46 uses GOTO 2 rather than GOTO 1, deliberately skipping the re-initialisation of A$ and the POKE, which is correct since neither needs repeating.

Potential Anomalies

• The expanding burst in lines 20–22 does not erase previous frames — each new character simply overwrites the previous one at the same position, meaning earlier ring positions remain drawn. This is likely intentional, building up a bloom effect.
• When Z or the random offsets place characters outside the valid screen area (rows 0–21, columns 0–31), a PRINT AT with out-of-range coordinates will produce an error. For the random bursts, the position is constrained (A=INT(RND*15)+3, B=INT(RND*25)+3) but Z runs to 3, so edge cases near borders could cause issues. The centered burst at radius 8 from (10,15) would reach column 23, still within bounds horizontally, but row 10+8=18 is safe; however row 10−8=2 and column 15−8=7 are also safe.
• Line numbers jump from 9 to 14, 15 to 20, 22 to 26, and 27 to 28, leaving gaps that serve no apparent purpose other than allowing future line insertions.