Super Scroll

This program implements a scrolling word-display effect that renders user-input text one word at a time using pixel-level character plotting. It prompts the user for a message, then extracts words sequentially via a subroutine at line 1000, rendering each word by PEEKing the ZX81 character ROM at address 7680 to read 8×8 font bitmaps and plotting individual pixels. After displaying each word, the screen scrolls up by G lines (normally 4, but 21 at the end of the message to clear the screen), then loops back to the start of the message. The program handles inverse-video characters by subtracting 128 from their character code before the ROM lookup, storing the stripped character in M$ (though M$ is set but never used to adjust the PLOT call, which is a bug).

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Program Analysis

Program Structure

The program is divided into a main loop and two subroutines. Initialisation sets the word-start pointer S=1 and the scroll amount G=4. After prompting for input and appending two trailing spaces to B$ (line 80), the program enters a perpetual loop: call the word-extraction subroutine, scroll the screen, plot the word pixel by pixel, then repeat.

1. Lines 10–90 – Initialisation and user prompt.
2. Lines 130–360 – Main display loop: extract word, scroll, plot pixels, repeat.
3. Lines 1000–1070 – Word extraction subroutine: scans B$ from position S to the next space, returning the word in A$.
4. Lines 2000–2030 – End-of-message subroutine: resets S and adjusts scroll count when the last word is reached.
5. Line 9998–9999 – SAVE with auto-run flag, then RUN.

Character ROM Access

The pixel rendering (lines 200–350) reads font data directly from ROM. The ZX81 character set begins at address 7680, with each character occupying 8 consecutive bytes (one per pixel row). Line 260 computes the address as 7680 + C*8 + L, where C is the character code and L is the row (0–7). Each byte is then bit-shifted by successive halving of V (starting at 128) to isolate individual pixels, which are plotted at PLOT 8*(K-1)+J, 10-L.

Word Extraction (Subroutine 1000)

The subroutine scans B$ from the current word pointer S until it finds a space character, storing the word slice in A$ and advancing S past the delimiter. The two trailing spaces appended at line 80 ensure the final word is always followed by a space, preventing an out-of-bounds substring error.

End-of-Message Handling (Subroutine 2000)

Called once per pixel plot (line 305), this subroutine checks whether the current word index N has reached the end of B$. When it has, it resets S=1 (restarting from the first word on the next cycle) and sets G=21 to scroll the display clear before the loop begins again. For all other words, G is kept at 4.

Notable Techniques

• Bitmap font decoding: The successive-halving loop (V=128 → V=V/2) is a common ZX81 idiom for extracting bits without using AND or integer arithmetic beyond subtraction.
• Trailing-space sentinel: Appending " " to B$ at line 80 is a neat boundary-condition fix for the word-scanner loop.
• Dynamic scroll depth: Varying G between 4 and 21 allows normal inter-word scrolling while still clearing the screen between complete message cycles.
• Inverse-video stripping: Line 220 detects codes ≥128 (inverse characters) and subtracts 128 to get the normal code for the ROM lookup.

Bugs and Anomalies

Location	Issue
Lines 230–240	M$ is assigned the base character at line 230 and C is set to 0, but M$ is never used again. Setting C=0 means that any inverse character in the input is rendered as the @ character (code 0) rather than its correct inverse form — the intended logic to use M$‘s code is missing.
Line 305	GOSUB 2000 is called inside three nested FOR loops (over characters, rows, and columns), so the end-of-message check fires up to 64 times per character. This is harmless since the assignments are idempotent, but it is inefficient.
Line 1010	If the input message contains no spaces at all, the inner loop at line 1020 will run past the end of B$ (the two appended spaces provide a fallback, so this is only a concern if B$ overflows available string space).