Seasons Greetings

This program displays an animated Christmas and New Year greeting card with scrolling graphical decorations. It uses machine code via RAND USR 16514 and a POKE to address 16418 to initialise the display environment before the main loop. Three string variables (B$, C$, D$) each hold a 64-character pattern built from block graphics representing candles or decorative motifs, and are horizontally scrolled each frame by rotating the string one character left using the slice idiom s$(2 TO LEN s$)+s$(1). The PRINT statement alternates between inverse-video and normal-video versions of “MERRY CHRISTMAS” and “HAPPY NEW YEAR” text, creating a flashing effect on each loop iteration.

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Program Analysis

Program Structure

The program divides into four functional phases:

1. Initialisation (lines 2–6): Sets FAST mode, calls machine code at 16514, POKEs address 16418 to zero, prints a bottom border row, then restores SLOW mode.
2. String setup (lines 10–40): Builds the shared prefix A$ and constructs three display strings B$, C$, D$ each longer than 32 characters to support rotation.
3. Display loop (line 100): A single compound PRINT AT statement renders three rows of graphics and two text labels in both normal and inverse video.
4. Scroll update (lines 200–300): Each string is rotated left by one character and control returns to line 100.

Machine Code Usage

RAND USR 16514 at line 3 calls a machine-code routine at address 16514 in RAM. On a standard 16K or 64K TS1000/ZX81 this area sits within the system variables or early RAM; the exact function depends on what has been loaded or pre-POKEd into that location, but this pattern is a common technique to execute an initialisation stub before the BASIC program runs. The subsequent POKE 16418,0 writes to the system variable RAMTOP low byte or a related display variable to adjust memory layout or clear a flag.

String Construction and the Rotation Idiom

All three animated strings are constructed by concatenating A$ (32 spaces in inverse video acting as a left-pad buffer) with a pattern of block graphics. This makes each string 64+ characters long. The rotation at lines 200–220 uses the classic ZX81 cyclic-shift idiom:X$ = X$(2 TO LEN X$) + X$(1)

This drops the first character and appends it to the end, producing a one-character leftward scroll per frame. Printing only the first 32 characters with the slice B$( TO 32) then shows a moving window across the pattern, creating smooth horizontal animation without any array or loop overhead.

Block Graphics Usage

The three display rows use ZX81 block graphic characters to suggest a repeating decorative motif (likely stylised candles or Christmas trees):

String	Row	Graphics used	Visual role
B$	Top	▝▝▐, O, /	Flame / top of candle
C$	Middle	▖, dashes, %	Candle body
D$	Bottom	▌▌▌, ▐▐▐, \;;\;; sequences	Candle base / holder

The \;; sequences in D$ represent character code 59 (semicolon) used as a block-fill element for the base row.

Flashing Text Technique

Line 100 prints the two greeting strings twice within the same PRINT statement: first in inverse video (%H%A%P%P%Y%… and %M%E%R%R%Y%…) and then in normal video. Because SLOW mode re-enters the display interrupt on each frame, the two renderings appear in rapid alternation across loop iterations, producing a flashing effect without any explicit IF branch or flag variable.

Key BASIC Idioms

• Compound PRINT AT … ; AT … ; AT … in a single statement to minimise interpreter overhead per frame.
• String slicing with ( TO 32) to extract a fixed-width window from a longer rotating buffer.
• LET B$=A$+"…" pattern to share the common 32-character prefix across all three row strings, saving re-typing and memory.
• GOTO 100 infinite loop with no termination condition — typical for demo/greeting programs.