Christmas Special

This program renders an animated starfield scene with custom UDG characters and scrolling text banners, accompanied by a musical fanfare. Seven UDGs (labeled “a” through “f” and “e”) are defined using BIN literals to create pixel-art sprites — likely figures and decorative elements — by poking 8-byte bitmaps into the UDG area via USR. The drawing section uses PLOT and DRAW commands to construct geometric shapes (possibly a building or trophy outline) and fills regions using nested FOR loops. Two string variables, a$ and b$, each 65 characters wide, implement horizontal marquee scrolling at screen rows 3 and 6 by rotating characters through string slicing. The BEEP-based music routines at lines 4000–5110 play a short repeating fanfare with recognizable rhythmic patterns using pitch values and durations.

---

Program Analysis

Program Structure

The program is organized into several clearly separated phases:

1. Initialization (lines 5–80, 100–570): Variables are set and seven UDG characters are defined via POKE.
2. Scene setup (lines 1000–1100): Scrolling string buffers are initialized and the screen is cleared with black paper.
3. Background drawing (lines 2000–2010): Stars (dots) and UDG \f characters are scattered randomly.
4. PLOT/DRAW artwork (lines 3000–3270): Geometric shapes are drawn and filled using nested loops.
5. Scrolling loops (lines 3300–3999): Two marquee text rows scroll indefinitely, with music subroutine calls between passes.
6. Music subroutines (lines 4000–5110): BEEP-based fanfares organized into modular subroutines.

UDG Character Definitions

Lines 10–570 define six UDGs using POKE USR "x"+n, BIN xxxxxxxx to write 8-byte bitmaps. The UDGs defined are:

UDG	Escape	Notes
"e"	\e	Asymmetric figure, likely a person or alien sprite
"a"	\a	Mirror/variant of \e; row 6 missing (line 170 absent)
"b"	\b	Dense rectangular shape, possibly a trophy or object
"c"	\c	Horizontal line in center row only (row 3 = BIN 11111111); acts as a spacer/divider
"d"	\d	Mirror of \b
"f"	\f	Diamond/star shape; used in the starfield foreground

Note that UDG "a" is missing its row 6 definition (line between 160 and 180 is absent), so USR "a"+6 retains its previous value — a minor anomaly that may affect the sprite’s appearance.

Background and Scene Drawing

Lines 2000–2010 scatter 250 dot characters and 52 \f UDG instances at random screen positions to create a starfield. The \f UDG (a diamond pattern) is placed in INK 4 (green) to differentiate foreground stars.

Lines 3000–3270 use PLOT and DRAW to render what appears to be a building or monument silhouette, complete with a doorway cutout (the conditional skip at lines 3100–3110) and a curved roof drawn with incrementing/decrementing loop variable a. The nested fill loops in 3090–3190 use floating-point increments (a+.7, a-.7) to approximate diagonal edges.

Scrolling Text Technique

Two string variables implement horizontal marquee scrolling:

• a$ (65 characters): scrolls right-to-left at row 3 by rotating the last character to the front: LET a$=a$(64)+a$(TO 63).
• b$ (65 characters): scrolls left-to-right at row 6 by rotating the first character to the end: LET b$=b$(2 TO LEN b$)+b$(1).

Each loop prints only the first 32 characters of the buffer (a$(TO 32)) to fill one screen row. PAUSE 20 and PAUSE 10 control scroll speed. The strings embed UDG characters (\b, \c, \e, \a, \d) to display sprite-based content within the scrolling banner.

Music Subroutines

The music is organized as a small call tree:

• GO SUB 4000 — master fanfare: calls 4005, then 4050, then 4005, then falls through to 5100.
• GO SUB 4005 — plays three beeps via GO SUB 5050, then a melodic phrase (lines 4010–4040).
• GO SUB 4050 — plays a short five-note motif.
• GO SUB 5050 — plays three identical beeps at pitch 10.
• GO SUB 5100 — plays a descending four-note phrase with a long final note, then pauses.

Line 4000 uses GO TO 5100 rather than GO SUB 5100: RETURN, relying on 5110 RETURN to unwind back to the original caller — a stack-efficient tail-call pattern.

Notable Anomalies

• Line 3075 contains REM go to 3200, indicating the PLOT/DRAW fill section (3090–3190) was intentionally bypassed during development but later re-enabled by removing or not executing a GO TO 3200. The REM is left as a comment artifact.
• Line 3300 uses variable i as the loop start (FOR n=i TO 64), but i is last set during the PLOT/DRAW section (line 3230, loop variable). This couples the drawing phase to the scroll loop timing and means the first scroll pass may be shorter than subsequent ones.
• Variables sk and s initialized at line 5 and the DIM c$(32) array are never used in the visible program — likely remnants of a removed gameplay or scoring feature.
• Line 4999 (STOP) is unreachable dead code between the music subroutines.