Sideways Billboard

Scrolls a message of your own across a blue band in the middle of the screen, in giant letters made from the ROM character set, and repeats it until BREAK is pressed. A menu offers two versions. 8X Sideways Billboard turns each font pixel into a whole character cell. 4X Sideways Billboard uses quarter-block graphics to make each font pixel 4×4 screen pixels, which gives smaller letters and a longer message. Both programs build the whole message before it moves, which the instructions say takes about six or seven seconds per character, and both ask for a message ending in a space.

Program Analysis

Program Structure

A small loader (added for this release) asks for 1 or 2 and loads BILBRD.B1 (8X) or BILBRD.B2 (4X) from tape. Each program shows its instructions, takes the message in A$ and builds a string array H$ that holds the enlarged message, one array row for each screen row. It then scrolls in three stages: lines 3000–3200 slide the message in from the right edge, lines 4000–4200 move a 32-column window along the array, and lines 5000–5200 scroll the tail off. Line 6000 starts again.

Reading the Font

Both versions read the ROM font directly with PEEK (15360+CODE A$(B)*8+C) for rows 1–6 of each character, leaving out the blank top and bottom rows. Each bit is tested by repeated subtraction of 128, 64, 32 and so on. In the 8X version, every set bit puts a full block :: into H$(6,LEN A$*7), so each letter is 7 cells wide and 6 rows tall. The 4X version first fills a 6×8 work array K$ with “1” and space characters. It then takes each 2×2 group of pixels as a four-character pattern and matches it against all 16 combinations (lines 2330–2345) to pick the matching quarter-block graphic, so each letter becomes 4 cells by 3 rows.

Notable Techniques

• Large text made by reading the ROM character bitmaps at 15360, with no UDGs or machine code.
• The 4X version maps 2×2 pixel groups onto the 16 Spectrum-style block graphics, so its letters have twice the resolution per cell.
• The lead-in loop uses negative AT columns (FOR J=-31 TO 0). The ROM takes the size of the value, so the message appears at the right edge and grows leftward.
• Each scroll step is one PRINT statement that reprints all six (or three) rows from string slices.

Potential Issues

• Messages shorter than the stated minimum (5 characters for 8X, 9 for 4X) make the slice arithmetic go out of range.
• The 4X version’s speed-control PAUSE lines (3135, 4135, 5135) are REMmed out, so its delay loops do nothing.

It comes from a Larken disk in the TS User Group’s collection (disk l-24). For this release it was converted to load and run from tape on a stock TS2068: the LKDOS RANDOMIZE USR 100 and OPEN #4,"dd" calls and the PRINT #4: prefixes on the colour statements were removed, and the menu loader was added so either version can be chosen. It was tested in ZEsarUX.