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Text Mode

The text mode module [textmode.v] displays text using character data and colour from a dedicated memory: tram.

Text mode includes a 2 KiB character rom holding 128 characters for basic system functionality before additional storage is available; these characters cover Basic Latin and Block Elements. Text mode is currently limited by Isle's lack of storage; future versions will extend Unicode coverage and add support for full-width glyphs.

Text mode fetches the next glyph while the previous one is being rendered. With the default Isle setup, this means the minimum glyph width is 8 pixels; if you set GLYPH_WIDTH to less than 8 pixels you'll have rendering issues. If GLYPH_HEIGHT isn't a power of 2, the font_glyph module may infer a multiplier.

Text mode also has one cycle of latency (PIX_LAT) to register outputs pix and paint. The module compensates for PIX_LAT and CLUT_LAT by starting drawing early.

See the Text Mode blog post for more information on this module.

Hardware Scrolling

Vertically scrolling text mode is a very common operation, so we includes hardware scrolling support. For example, if you're appending a line to the bottom of the screen, hardware scrolling can move everything up one row and you only need change one row of text in tram.

Text rendering starts at scroll_row and continues down through tram, wrapping back to row 0 after the last row in memory. The valid range for scroll_row is 0 to TEXT_VRES-1.

Dependencies

The textmode module depends on three other modules:

  • tram - holds Unicode code points and colours
  • font_glyph - load font pixel data (embedded within textmode)
  • rom_sync - holds core system font (embedded within font_glyph)

Parameters

  • ADDRW - tram address width (bits)
  • CIDXW - colour index width (bits)
  • CLUT_LAT - CLUT latency (cycles)
  • CORDW - signed coordinate width (bits)
  • FILE_FONT - font glyph ROM file
  • FONT_COUNT - number of glyphs in font ROM
  • GLYPH_HEIGHT - glyph height (pixels)
  • GLYPH_WIDTH - half-width glyph width (pixels)
  • TEXT_HRES - text mode width (half-width chars)
  • TEXT_VRES - text mode height (chars)
  • TRAM_LAT - tram latency (cycles)
  • WORD - machine word size (bits)

For Isle, WORD must be set to 32, and CIDXW must be set to 4.

Text mode uses the first TEXT_HRES × TEXT_VRES words of tram and ADDRW must be wide enough to encompass this.

In tram, we pack Unicode code point and two 4-bit colours into a word. The Unicode standard limits the code point to 21 bits. See tram for how the bits are packed into each word.

See font glyph for details on font and glyph parameters.

Signals

Input

  • clk_pix - pixel clock
  • rst_pix - reset text mode state machine
  • start - start textmode rendering (before active painting area)
  • (dx, dy) - display position
  • win_start - text window start coords
  • win_end - text window end coords
  • scale - text scale
  • scroll_row - first text row to display
  • tram_data - character data from tram

The win_start and win_end inputs are a pair of signed 16-bit values, with the y-coordinate in the upper 16 bits. The scale input works in a similar way, with the vertical scale in the upper 16 bits and the horizontal scale in the lower 16 bits.

See Canvas Display Address Generation for further discussion of windows and scaling.

Output

  • tram_addr - tram address (word)
  • pix - pixel colour index
  • paint - text mode painting enable

pix and paint are delayed so they arrive at the same times as tram data.

The colour pix should be painted when paint is enabled. The paint area is the intersection of the text area and window coordinates (win_start and win_end).