Put CosmOS on the screen without changing a line of it

The console is now a display controller as well as a port: it owns a font, keeps a cursor,
handles newline, carriage return, backspace and wrapping, and scrolls. That is an ordinary
kind of chip - it is what a video terminal's character generator did - and it is the reason
this rung needed no changes to CosmOS at all. CosmOS already writes bytes to port 0x00.

It writes to BOTH the screen and standard output, which is deliberate. A machine with a
screen and a serial line is an ordinary machine, the emulator's standard output is that
serial line, and one console drives both. It is also what keeps all 165 recorded results
passing under Voyager, and what makes --screen work on the plain SplitBit: there is one
console and it drives everything it has.

Scrolling moves the video device's origin and no memory. The row arriving at the bottom is
cleared because the map is a ring and it holds what was there 128 rows ago; the rows going
off the top are not, and that is a hundred rows of scrollback nothing had to keep. The test
reads the register back rather than looking at the screen, because a console blitting rows
instead would look identical and cost twelve percent of a frame for every line printed.

The font is vendored from Hatchet-GPU with a note saying where it came from, since that
repository is not part of this one. 135 glyphs in ASCII order, which is the thing that makes
it worth keeping - PETSCII's whole inconvenience was that its order was not ASCII's, so a
machine using it needed a translation table in front of every string. Here the machine
subtracts 32. It is stored one bit a pixel and expanded into tile memory at reset: 1,088
bytes against 16 kilobytes.

Voyager gets a keyboard. A window has no standard input, and a machine blocking on it inside
a frame would stop drawing and stop answering, so a front end with a window installs a hook
that the console calls while it has nothing: it keeps the window alive and hands back a key.
The hook has to tell "nobody has typed yet", which happens sixty times a second, apart from
"the window has gone", which is the end of input - one value for both would have made the
first keystroke look like a closed machine. In line mode the console echoes what it is
given, because there is no terminal behind a window to do it and that was always the
terminal's job.

Tests/video.sh grew from 14 checks to 26, half of them about the console rather than the
device: those programs ask the video device for nothing and write bytes to port 0x00 like
every SplitBit program always has. Verified by breaking two things - removing the scroll
failed exactly the two checks about scrolling, and removing the cursor advance failed
exactly the three that depend on it.

Two video checks had quietly depended on palette entry 0 being black, which stopped being
true the moment a machine woke up able to show text. They now set what they are about to
look at, and a new check pins the waking state itself.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E2JrLzFvuFX9fgi1LDRjrW

Source/Emulator/video.c

@@ -3,6 +3,7 @@
 // Written by Anachronaut
 
 #include "video.h"
+#include "font.h"
 #include <stdio.h>
 #include <string.h>
 
@@ -29,12 +30,66 @@ static int renderedHeight = 0;
 static int columnsFor(uint8_t m) { return m == VIDEO_MODE_80x50 ? 80 : 40; }
 static int rowsFor(uint8_t m)    { return m == VIDEO_MODE_80x50 ? 50 : 25; }
 
+int videoColumns(void) { return columnsFor(mode); }
+int videoRows(void)    { return rowsFor(mode); }
+
+// ---- The two colours a machine wakes up with ----
+//
+// Only two, and the rest of the palette left at zero. A program that wants colour sets it,
+// and a machine that guessed sixteen entries on its behalf would be sixteen entries it had
+// to overwrite. What it must not do is wake up unable to show text at all.
+static const uint8_t defaultInk[3]   = { 0xDC, 0xE6, 0xDC };
+static const uint8_t defaultPaper[3] = { 0x10, 0x14, 0x12 };
+
+void videoLoadFont(void) {
+    // One bit a pixel becomes one byte a pixel: index 1 where the font has a dot and 0
+    // where it does not, which is what makes the two palette entries below mean ink and
+    // paper. Glyphs the font does not have are left blank rather than left as whatever was
+    // in tile memory.
+    memset(videoRAM + VIDEO_TILE_BASE, 0, (size_t)VIDEO_TILE_COUNT * VIDEO_TILE_BYTES);
+    for (int glyph = 0; glyph < CONSOLE_FONT_GLYPHS && glyph < VIDEO_TILE_COUNT; glyph++) {
+        uint8_t *tile = videoRAM + VIDEO_TILE_BASE + glyph * VIDEO_TILE_BYTES;
+        for (int y = 0; y < CONSOLE_FONT_BYTES; y++) {
+            const unsigned char row = consoleFont[glyph * CONSOLE_FONT_BYTES + y];
+            for (int x = 0; x < VIDEO_CELL_PIXELS; x++) {
+                tile[y * VIDEO_CELL_PIXELS + x] = (row & (0x80u >> x)) ? 1 : 0;
+            }
+        }
+    }
+    uint8_t *palette = videoRAM + VIDEO_PALETTE_BASE;
+    memcpy(palette + 0 * VIDEO_PALETTE_BYTES, defaultPaper, 3);
+    memcpy(palette + 1 * VIDEO_PALETTE_BYTES, defaultInk, 3);
+}
+
+void videoPutCell(int screenRow, int column, uint8_t tile, uint8_t attribute) {
+    if (screenRow < 0 || screenRow >= rowsFor(mode)) return;
+    if (column < 0 || column >= columnsFor(mode)) return;
+    const int mapRow = (scroll + screenRow) % VIDEO_MAP_ROWS;
+    uint8_t *cell = videoRAM + VIDEO_MAP_BASE + mapRow * VIDEO_MAP_STRIDE
+                  + column * VIDEO_CELL_BYTES;
+    cell[0] = tile;
+    cell[1] = attribute;
+}
+
+void videoScrollUp(void) {
+    scroll = (uint8_t)((scroll + 1) % VIDEO_MAP_ROWS);
+    // The row now at the bottom held whatever was there a ring ago, so it is cleared. The
+    // rows that went off the top are NOT cleared, which is the whole of the scrollback: a
+    // hundred rows of what has already been said, still sitting in the map.
+    const int bottom = rowsFor(mode) - 1;
+    const int mapRow = (scroll + bottom) % VIDEO_MAP_ROWS;
+    memset(videoRAM + VIDEO_MAP_BASE + mapRow * VIDEO_MAP_STRIDE, 0, VIDEO_MAP_STRIDE);
+}
+
 void videoReset(void) {
     memset(videoRAM, 0, sizeof(videoRAM));
     mode = VIDEO_MODE_40x25;
     scroll = 0;
     renderedWidth = 0;
     renderedHeight = 0;
+    // A machine wakes up able to show text. Everything here is ordinary video memory that a
+    // program may overwrite the moment it wants the screen for something else.
+    videoLoadFont();
 }
 
 uint8_t *videoMemory(uint32_t *capacity) {