Genesis Technical Overview

GENESIS:
  • 68000 @ 8 MHz
    • Main CPU
    • 1 MByte ROM area
    • 64 KByte RAM area
  • VDP (Video Display Processor)
    • Dedicated video display processor
      • Controls playfield & sprites
      • Capable of DMA
      • Horizontal & Vertical interrupts
    • 64 KByte of dedicated VRAM (Video RAM)
    • 64 x 9-bits of CRAM (Color RAM)
  • Z80 @ 4 MHz
    • Controls PSG (Programmable Sound Generator)
    • 8 KByte of dedicated sound RAM
VIDEO:
  • NOTE: Playfield and Sprites are character-based
  • Display Area (visual)
    • 40 chars wide x 28 chars high
      • Each char is 8 x 8 pixels
      • Pixel resolution is 320 x 224 (NTSC)
    • 3 planes
      • 2 scrolling playfields
      • 1 sprite plane
      • Definable priorities between planes
    • Playfields
      • Each char position takes 2 bytes, that includes:
        • Char name (10 bits); points to char definition
        • Horizontal flip
        • Verical flip
        • Color palette (2 bits); index into CRAM
        • Priority
      • Scrolling
        • 1 pixel scrolling resolution
        • Horizontal
          • Whole playfield as unit
          • Each character line
          • Each scan line
        • Vertical:
          • Whole playfield as unit
          • 2 char wide columns
    • Sprites
      • 1 x 1 char up to 4 x 4 chars
      • Up to 80 sprites can be defined
      • Up to 20 sprites displayed on a scan line
      • Sprite priorities
    • Character definitions
      • 4 bits/pixel; points to color register
      • 4 bytes/scanline of char
      • 32 bytes for complete char definition
      • Playfield & sprite chars are the same!
COLOR:
  • Uses CRAM (part of the VDP)
    • 64 9-bit wide color registers
      • 3 bits of Red
      • 3 bits of Green
      • 3 bits of Blue
    • 4 palettes of 16 colors
      • 0th color (of each palette) is always transparent
OTHER:
  • DMA
    • Removes the 68000 from the BUS
    • Can move 205 bytes/scanline during VBLANK
      • There are 36 scanlines during VBLANK
      • DMA can move 7380 bytes during VBLANK
  • Horizontal & Vertical interrupts
SOUND:
  • Z80 controls
    • PSG chip (TI 76489)
    • FM chip (Yamaha YM 2612)
      • 6-channel stereo
    • Z80 can access ROM data
    • 8 KByte RAM