Spikes¶

Each of these settled a question about the BBC end that could not be settled by reading. They are kept as the record of how each answer was arrived at, which matters when an answer later looks surprising.

They are not part of the test suite. They need a local Beebium checkout, a Commstar ROM, and a couple of minutes each. Run one with:

BEEBIUM_ROM_DIR=~/Code/beebium/roms \
BEEBIUM_SERVER=~/Code/beebium/build-release/src/server/beebium-model-b \
uv run --no-project --python 3.12 \
    --with ~/Code/beebium/clients/beebium-python-client \
    --with . --with pytest \
    python -m pytest docs/spikes/spike_control_codes.py -s -v

Most drive Commstar through rpc-serial, which delivers bytes to the ACIA under the test’s control, and read the screen back as resolved SAA5050 cells. The transport is interchangeable for this purpose: the questions are about what Commstar does with bytes, not about how they arrived.

Spike	Question	Answer
`spike_control_codes.py`	How must attributes be encoded?	`ESC` + code + 0x40. The SAA5050’s own 0x80-0x9F codes vanish on a 7E1 line.
`spike_frame_geometry.py`	Where does a 24-row frame sit in a 25-row screen?	Rows 0-23. Column 40 wraps by itself; the bottom-right cell wraps to the top-left rather than scrolling.
`spike_page_number_buffer.py`	How long a page number will Commstar send?	At least twenty digits, untruncated. The nine-digit Prestel maximum was its database’s, not a terminal’s.
`spike_cursor_keys.py`	Do the BBC’s arrow keys reach us?	Yes, as 0x08-0x0B — the viewdata cursor-control codes, the eighth bit taken by 7E1. COPY is consumed locally.
`spike_editing_keys.py`	What does DELETE send?	0x7F, distinct from RETURN’s 0x5F. TAB sends 0x09; CTRL-H sends 0x19.
`spike_cursor_output.py`	Can we redraw part of a screen?	Yes. Moving the cursor erases nothing, and cursor up from row 0 wraps to row 23 — so the footer is two bytes away.
`spike_cursor_visibility.py`	Can the cursor be hidden?	0x11 and 0x14 are consumed as controls, taking no cell. Which shows and which hides was confirmed by eye, the cursor’s blink defeating a read-back.
`spike_trimmed_frames.py`	Is a trimmed frame identical on screen?	Yes, across six frames including the one that would break it — a row filled to column 40.
`spike_suggestion_block.py`	Can a block of rows be repainted as a reader types?	Yes, if each row is sent trimmed: a full-width row wraps by itself, so a cursor down after one moves two rows. Three suggestions cost 121 bytes, 1.0s at 1200 baud; the common keystroke costs 40.
`spike_search_page.py`	Does the real search page work on a real terminal?	Yes. Every byte from a real session over a real index; the furniture around the block survives, narrowing covers what it replaces, DELETE reaches the field. Keying TRONDHEIM cost 128 bytes at worst and 35 at best.

What they established is written up in Viewdata encoding, which keeps what was verified separate from what was inferred.

Writing another¶

Copy the fixture and helpers from any of them; they are deliberately repetitive rather than shared, so that a spike is a single file someone can read top to bottom.

Two things worth knowing, both learned the hard way:

The cursor flashes. One reading tells you which half of the blink you caught, not what the state is. spike_cursor_visibility.py samples across it.
Beebium’s teletext_screen().text maps codes to ASCII, not to glyphs. A cell holding 0x5F reads back as _ though the screen shows #. Assert on cell.character when identity matters.