Bulletin

This program displays an animated bulletin board splash screen and then scrolls two independent ticker-tape text messages across the screen. It uses PEEK of system variables at addresses 16396–16397 to find the base of the display file, then POKEs display-file bytes directly via a packed decimal offset string (M$) to selectively blank or set pixels for a graphical effect. The border is drawn using block-graphic characters (▛, ▜, ▙, ▟, etc.) and two framed windows scroll strings A$ and B$ with 28-character viewports. Playback controls allow the user to pause (P), scroll backward (B), or resume forward (F) independently for each ticker window.

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Program Analysis

Program Structure

The program is divided into several functional blocks:

1. Lines 1000–1200: Initialise the display — clear screen by printing S$ 24 times, draw a solid block-graphic border around the full 32×24 screen.
2. Lines 1210–1270: POKE selected bytes in the display file to 0 (clear) and then to 128 (set top pixel row) using offsets packed in string M$, then pause for effect.
3. Lines 1280–1350: Animate a “wipe” effect collapsing rows inward from top and bottom, then redraw the border with half-block graphics to create a 3-D-style frame.
4. Lines 1400–1430: Print a centred title panel (“THE BULLETIN BOARD FOR SEPTEMBER 1983”) and two subsidiary framed windows.
5. Lines 1500–1580: Main scroll loop — independently scroll strings A$ and B$ through 28-character viewports at rows 4 and 19 respectively, calling subroutines 2000 and 2500 for keypress handling. Loops back at line 1580.
6. Lines 2000–2160 / 2500–2660: Two parallel keypress-handler subroutines, one per ticker window, supporting pause (P), backward scroll (B), and resume-forward (F).
7. Lines 9998–9999: SAVE with auto-run flag, then restart.

Display File Manipulation

Lines 1000–1010 read the display file base address from system variables 16396 and 16397 (the standard ZX81/TS1000 D-FILE pointer) and store it in X. POKE 16418,0 disables the cursor. The string M$ encodes a sequence of three-digit decimal offsets; lines 1210–1260 step through it in strides of 3, extracting each offset with VAL M$(N TO N+2) and POKEing X + offset first to 0 then to 128. This directly manipulates pixel rows in the display file to produce a custom graphic pattern without any PLOT commands.

Scrolling Ticker Technique

The scrolling is achieved by sliding a 28-character window through a long string. For forward scroll, line 1510 prints A$(N TO N+27) at row 4, column 2, incrementing N each iteration. This is a classic Sinclair BASIC substring-window idiom. The loop bounds (LEN A$-28) ensure the window never overruns the string.

The backward-scroll subroutines (lines 2100–2160 and 2600–2660) reverse the window by decrementing from the current N back to 28, checking INKEY$ on each frame for an “F” (forward) keypress to break out, or “P” to pause. LET N=I on exit resynchronises the main loop counter to wherever the reverse scroll stopped.

Key BASIC Idioms

• VAL M$(N TO N+2) — converts a three-character decimal substring to a number, used as a memory-efficient way to store a table of offsets in a string variable rather than a DATA block.
• GOSUB 2000 / GOSUB 2500 called inside FOR–NEXT loops for per-frame event polling without blocking.
• Immediate RETURN when INKEY$="" (line 2010, 2510) keeps the scroll running at full speed when no key is held.
• PAUSE 40000 for a long pause on “P” — effectively indefinite on a 50 Hz machine (~800 seconds).

Block-Graphic Border Construction

The border is built in four passes (lines 1060–1200) using the \## escape, which represents the solid block character (█). A second, more elaborate border at lines 1310–1350 uses paired half-block characters (\:', \':, \:., \.:, etc.) to produce a shadowed or bevelled frame appearance using standard ZX character-set block graphics (chars 128–143).

Notable Techniques

• Two completely independent ticker channels (A$ top, B$ bottom) with separate scroll positions and separate backward-scroll subroutines — unusual for a single-tasking BASIC program.
• The main loop at lines 1500–1580 interleaves both scrollers: A$ advances one step and calls its handler, then B$ advances one step and calls its handler, giving approximate simultaneity.
• The auto-run SAVE at line 9998 uses an inverse-video character in the filename (“BULLETI%N”), causing the tape loader to jump to line 1000 automatically on load.

Anomalies and Observations

• Line 1060 is numbered 1060 after 1030/1040, skipping 1050 — and similarly 1100, 1140, 1180 skip intermediates. This is cosmetic line-number spacing with no functional effect.
• The backward-scroll loop (line 2100) runs FOR I=N TO 28 STEP -1. If N is already less than 28 (early in the string), the loop body never executes and the subroutine falls through to LET N=I with I holding whatever value it had from the outer loop — potentially a logic edge case if A$ is short.
• There is no INKEY$ debounce between the two subroutine calls within the main loop, so a held key will trigger both handlers on the same frame.