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Put the cursor home when the screen is cleared

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A screen with nothing on it and a cursor half way down it is not a cleared
screen: the next thing written lands where the last thing happened to leave
off, at a position whose meaning was just erased. The shell's clear did
exactly that, and left the next line one row below wherever it had been.

Both halves were missing. consoleClearScreen blanks the cells and does not
touch cursorRow or cursorColumn, and 2J on a terminal empties the screen
without moving anything - H is what puts the cursor at the top.

Life and Snake never showed this because they follow their clear with an
explicit 1;1H of their own. They were working around it, which is why the
bug survived until a command cleared the screen and then let somebody type.

The attribute is deliberately not reset. Clearing is about what is on the
screen rather than how the next thing will be drawn, and a program that
chose a colour and then cleared still wants that colour - which is what a
terminal does too.

Two checks in video.sh, and they took two goes to make independent. The
first pointed at the row where the letter lands when the cursor is NOT
homed, so removing the clear and removing the homing failed the same pair
and neither said which. The second now looks at a row nothing writes to
either way - and at a letter whose ink actually reaches the pixel it reads,
which "two" did not, so it passed on a screen that had never been cleared.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E2JrLzFvuFX9fgi1LDRjrW
This commit is contained in:
Anachronaut
2026-08-30 17:08:27 -04:00
co-authored by Claude Opus 5
parent 553882d28d
commit 0e0731e2b1
10 changed files with 51 additions and 9 deletions
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SplitBit Programming Manual.md
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@@ -694,7 +694,7 @@ Three more registers, because that is how this machine talks to everything else.
| --- | --- |
| 0x03 | Cursor row. Read and write. |
| 0x04 | Cursor column. Read and write. |
| 0x05 | Command. Write 1 to clear the screen. |
| 0x05 | Command. Write 1 to clear the screen, which also puts the cursor at the top left. |
| 0x06 | Attribute. Read and write. |
**A cursor is shown only when it is asked for**, with bit 2 of the Control port, and status bit 4 says whether one is being shown. Off is the right default for a machine: a program painting its own screen does not want something blinking in the middle of it, and a system that reads lines from a person turns it on.