A Retro Dream

A series of what-if "fantasy" computer architectures that aim to be consistent with what was available at the time.

8bit Design

Built around a 65C02 CPU clocked at 4Mhz.

64K addressable memory, but with paging support:

• 32K main RAM, comprising:
• 12K shared memory
• 20K bank 1
• 32K main RAM, comprising:
• 20K bank 2 (shadow video RAM)
• 12K OS RAM
• 64k sideways RAM:
  • 16K sideways RAM bank 1
  • 16K sideways RAM bank 1
  • 16K sideways RAM bank 1
  • 16K sideways RAM bank 1

  Video Display Processor

  A custom 8bit Video Display Processor (VDP) clocked at 8Mhz that supports the following features:

  • Text/Character Modes
  • Bitmap Graphics Modes
  • Sprites
  • 4096 colour palette with 2 x 16 colour lookup tables (CLUTs)

  Text/Character Modes

  Supports both 80x32 and 40x32 character modes with the option of reducing this to 80x25 and 40x25 by inserting a blank line after each row (as per the BBC Microcomputer MODE3 and MODE6). Each of the characters can have a attribute byte which offers the following:

  • 4bit foreground colour and 4bit background colour (Thomson graphics mode), 256 characters
  • 3bit foreground colour, 3bit background colour, 1bit intensity/bright bit, 1bit foreground flash (Timex graphics mode), 256 characters
  • Tile attributes (see below)

  Tile Mode

  An special attribute used in the character modes that are optimised for tile-based games. Each tile attribute comprises:

  • 1bit priority
  • 1bit flip x
  • 1bit flip y
  • 1bit transparency set
  • 1bit character bank
  • 1bit palette select
  • 2bit sub-palette select (2bpp only)
  This allows for 512 different character tiles to be selected. The transparency bit sets whether a tile's pixel value of 00 or 0000 is treated as transparent.

  Bitmap Graphics Modes

  Supports a number of resolutions and colour depths:

  • 640x256x1bpp (BBC MODE0)
  • 320x256x2bpp (BBC MODE1)
  • 160x256x4bpp (BBC MODE2)
  • 320x256x1bpp (BBC MODE4)
  • 160x256x2bpp (BBC MODE5)
  • 320x256x1bpp + 8x1 pixel attributes (Thomson graphics mode)

  Sprites

  A Sprite Attribute Table (SAT) containing 64 entries. Capable of displaying 8 sprites on a scanline. Each sprite is 4bytes and can be either 8 pixels in 4bpp (16 colours), 8 pixels in 2bpp (4 colours) or 16 pixels in 2bpp.

  Each entry in the SAT takes 4 bytes. The entire table is 256 bytes. The attributes are:

  • Byte 1
  • 8bit vertical position
  • Byte 2
  • 1bit flip x
  • 1bit flip y
  • 1bit sprite bank select
  • 1bit palette select
  • 2bit sub-palette select (2bpp only)
  • 2bit horizontal position
  • Byte 3
  • 8bit horizontal position
  • Byte 4
  • 8bit sprite id

  Custom Sound Chip

  The custom sound chip is a basic wavetable synth, influenced but different from the sound chip in the PC Engine. There are six channels:

  • Channel 1: Wavetable
  • Channel 2: Wavetable or Modulation for Channel 1
  • Channel 3: Wavetable
  • Channel 4: Wavetable or Modulation for Channel 2
  • Channel 5: Noise
  • Channel 6: Noise or direct PCM playback
  This allows the computer to output many different combinations of sound.