Dog

This program renders an animated scene of a dog walking across the screen and interacting with a fire hydrant, using 21 user-defined graphics (UDGs “a” through “u”) loaded from DATA statements at startup. The UDG pixel data is POKEd directly into memory via USR addresses, assembling a multi-cell sprite that spans two character rows. Sound effects are generated using the SOUND keyword for both a startup chord and sound cues tied to the dog’s actions. After the dog animation completes, control jumps to a PLOT/DRAW routine that produces a looping graphical display with accompanying SOUND tones before cycling back to the dog scene.

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Program Analysis

Program Structure

The program begins at line 9500 with a brief loading message, then jumps via GO TO 9785 to the UDG setup subroutine call. The main logic is divided into three functional regions:

1. Lines 9500–9610: Startup banner and entry point
2. Lines 9620–9780: Abstract PLOT/DRAW/SOUND animation loop
3. Lines 9785–9870: Dog walking animation with hydrant interaction
4. Lines 9875–9885: UDG initialization subroutine
5. Lines 9890–9990: UDG pixel DATA (21 characters × 8 bytes = 168 bytes)

Execution flows: startup → UDG init (GO SUB 9875) → dog animation → PLOT/DRAW loop (GO TO 9620) → dog animation again, cycling indefinitely.

UDG Initialization

The subroutine at line 9875 uses RESTORE followed by a FOR loop from USR "a" to USR "u"+7, reading 168 DATA values and POKEing them sequentially into the UDG memory area. This covers all 21 UDGs (\a through \u, characters 144–164). The use of USR "a" as the base address is the standard Spectrum technique for locating UDG storage without hard-coding a memory address, making the code portable across different memory configurations.

Sprite Layout

The dog sprite is drawn across two character rows using UDGs arranged as follows:

Row offset	Columns (relative to x)	UDGs used
y (upper body)	x, x+1, x+2, x+3	\a, \b (walk), \c (tail/head)
y+1 (lower body)	x, x+1, x+2	\d, \e, \f / \g, \h, \i (alternating frames)

Two leg positions are cycled using the print sequence at lines 9815–9830: first \k\b\j / \g\h\i are printed, then replaced with \d\e\f / \a with the tail character \c. The sprite is erased by printing spaces behind the advancing x position. UDGs \l and \m handle the fire hydrant, while \n through \u cover additional hydrant and scene elements.

Animation Technique

The dog walks from column 0 to column 27 using a FOR x=0 TO 27 loop. Each iteration prints the “new” frame of the dog at position x, pauses briefly with PAUSE 10, then overwrites the trailing cell with spaces. A conditional at line 9835 checks x=12 to trigger a BEEP 2,-40 (a low-frequency, extended beep) along with a special UDG \n, marking a specific event mid-walk. The hydrant interaction at lines 9845–9845 fires when x+5 is between 17 and 30, playing a SOUND sequence and swapping hydrant UDGs.

PLOT/DRAW Animation (Lines 9620–9780)

This section produces an abstract oscillating graphic. A variable j starts at 100 and decrements each iteration of a 150-step loop, with an acceleration phase (j=j+2) when i>75. Two PLOT calls at lines 9665 and 9667 draw symmetric points above and below a center line. A subsequent double loop (lines 9700–9780) plays descending then ascending SOUND tones while drawing lines with DRAW. The outer counter a repeats this 6 times before displaying a “To Stop, C-Shift Break” message using FLASH 1.

SOUND Usage

SOUND is used extensively for both music and sound effects. Line 9620 sets up a multi-voice chord using registers 6–13. In the animation loops, register 0 carries the tone pitch, register 1 sets the fine pitch, register 8 sets the amplitude (15 = maximum), and register 7 (value 62 = %00111110) configures the mixer to enable tone on channel A only. Register 8 is set to 0 to silence output at the end of each sequence.

Notable Techniques

• RESTORE before reading UDG data ensures the DATA pointer is reset regardless of any prior reads.
• The screen row y is randomized with LET y=INT (RND*18)+2, keeping the dog at least 2 rows from the top and bottom, avoiding attribute clashes with the border.
• BORDER 0 sets a black border to frame the animation cleanly.
• The hydrant guard condition IF x+5<17 OR x+5>30 uses a skip-to-next idiom: when true it jumps to line 9850 (NEXT x), bypassing the hydrant sound and UDG swap.
• The SAVE "dog" LINE 0 at line 9995 saves the program with an auto-run directive targeting line 0; since line 0 does not exist, execution starts at the first available line (9500).

Potential Anomalies

• Line 9870 clears the sprite area then calls CLS and jumps to 9620 (the PLOT/DRAW section) rather than directly restarting the dog animation, meaning the abstract graphics sequence always plays between dog runs.
• The DRAW i+102,i+72 at line 9745 uses the SOUND pitch variable i as both the audio frequency and a draw displacement, coupling the visual and audio in an unconventional way that produces a diagonal spray of lines.
• Line 9780 loops GO TO 9730 instead of using a proper FOR/NEXT outer loop, which is an intentional structure but slightly unusual given the explicit a counter.