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Rework the terminal from a static one-tile-per-screen-position model to
LINE-BOUND tiles with an assign[] indirection, so scrolling is cheap:
- Each line-slot L owns the 20 VRAM tiles at positions [L*20..L*20+19].
- assign[screen_row] -> line-slot; the tilemap points each screen row at
its slot's tiles.
- Scroll = rotate assign[], blank+rebuild only the new bottom line (20
tiles), and rewrite the tilemap. O(1 line) instead of rebuilding all
360 tiles.
- term_putc: printable + \n \r \b, line wrap at col 40, cursor advance
with scroll-on-overflow.
- Cursor: an underline OR'd into the current cell's tile via wCurMask
(no sprites/OAM); moving it just re-renders the old and new tiles.
- term_demo drives 24 lines through term_putc to exercise scroll+cursor.
Also fixes a vicious bug this shook out: build_tile advanced its 16-bit
glyph pointers with `ld hl, wGlyphL+1 / inc [hl]`, which CLOBBERS HL --
and HL is the live VRAM destination pointer. Whenever a glyph's 8 bytes
straddled a page boundary (common with long/varied lines) the next tile
bytes were written into WRAM at $D580+, corrupting wGlyphL/wVram/wCurRow/
wCurCol and freezing the display. Now the pointers are bumped via A so HL
is preserved. (Note: the emulator build had also been silently failing on
an unrelated debug edit, masking this for a while.)
Verified by screenshot: 24 lines scroll to show the last 18 + a visible
cursor at the "ready$" prompt; serial shell and filesystem still work.
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Replace the small-caps placeholder font with a real baseline-aligned
4x8 set covering all 96 printable ASCII: distinct lowercase with true
ascenders (b d f h k l t) and descenders (g j p q y), digits, and
punctuation. Metrics: caps/ascenders rows 1-5, x-height rows 2-5,
baseline row 5, descenders into rows 6-7. Glyphs are ~3px wide in the
4px cell -- cramped but legible; the tile mechanism itself imposes no
character-set limit.
genfont.py now takes per-glyph (top, rows) so glyphs sit on the baseline.
Demo strings in term_test updated to show mixed case + punctuation.
Verified by screenshot: descenders visibly drop below the baseline.
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Add a background-tile text terminal that packs TWO 4px-wide characters
into each 8x8 tile, giving a 40x18 grid instead of the 20x18 you'd get
from an 8x8 font. The tilemap is static (one dedicated VRAM tile per
screen position); we rebuild a tile's 16 bytes from two glyphs whenever
a character changes. 360 tiles exceed the 256 a single tilemap can
address, so positions 256-359 live in VRAM bank 1 via the CGB tilemap
attribute bank-bit -- hence CGB-only.
- Switch the ROM to CGB (rgbfix -C). Safe for the FS: every process has
PROC_WRAMB=1, so SVBK stays on bank 1 and the WRAMX FS caches don't move.
- CGB BG palette 0 = white bg / black text via BCPS/BCPD.
- src/term.asm: term_init (palette + static tilemap + clear buffer),
build_tile (combine two 4px glyphs -> one 8x8 tile, correct VRAM bank),
term_redraw (rebuild all tiles), term_puts, term_show, term_test.
- src/font.asm: 96-glyph 3x5-in-4x8 font, generated by tools/genfont.py.
- 40x18 text buffer at $D600 (WRAMX, above the FS caches).
- tools/ppmview.py: render a --shot PPM as ASCII so the LCD is inspectable
from the shell during development.
Verified via emulator screenshot: "GBOS TERMINAL", the alphabet, and a
bank-1 row all render legibly at 40 columns. Serial shell + filesystem
still work unchanged.
Note: a full term_redraw builds 360 tiles and takes ~8 frames; fine for
incremental single-char updates, but scrolling needs a smarter path
(next milestone). Input (no keyboard) is also still TODO.
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