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video: Add a text mode

Why?

  • A text mode would require 1/8:th of the VRAM compared to 1 bpp graphics.
  • A text mode is less CPU hungry than 1 bpp graphics.
  • A text mode requires less hazzle and code than 1 bpp graphics.

Changes

  • Add a new CMODE: TEXT1
  • Store the font in the palette memory.
    • Use SETPAL to load a font.
    • One 8x8 glyph is stored in two palette entries, so we get 128 glyph entries in total (e.g. 7-bit ASCII).
    • Do something fun with char codes 128-255 (e.g. invert the glyph pixels?).
  • Add two video registers for the background & foreground colors in text mode.
    • Names: TEXTFG and TEXTBG?
    • Pure 24-bit RGB, or something with an alpha channel (A8R5G6B5)?
  • Add a "sub row" video register (find a better name?).
    • The ADDR register marks the start of the text line.
    • The SUBROW register tells which row of the glyph to render (0-7).
  • The shift stage of the pixel pipeline needs some modification.
  • We need a modified palette lookup in text mode (different lookup address).
  • We need an extra pixel pipeline stage after the palette/glyph lookup to select the pixel color from the TEXTFG/TEXTBG registers.
Edited by mbitsnbites