Mega Drive

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Fast Facts on the Sega Mega Drive

Made by: Sega
Variants: Mega Drive 2, Genesis 3, Sega Mega Jet, Sega Nomad, Sega Teradrive
{{ #if: |Add-Ons: Sega CD, 32X, Power Base Converter, Super Magic Drive
|}} Processor: Motorola 68000 (or equivalent), 7.67MHz NTSC/7.61MHz PAL; Zilog Z80 (or equivalent) for sound programming and Master System compatibility, 3.58MHz NTSC/3.55MHz PAL
Released in US: 9 January 1989
Released in EU: Winter 1990
Released in JP: 29 October 1988

The Sega Mega Drive, or Sega "Genesis" as it is called in the United States is a 4th generation home video game console produced by Sega. It was introduced to the market in 1989 as competition to rival Nintendo's Super Nintendo Console.

The Mega Drive is a video game console that is based mainly for home use. During its run, Sega had three models released (while the last model was not produced by Sega, but by another company under license) and each one becoming smaller than the original. The system outputted video and audio signals through an A/V out cable that connected to a television. The console itself boasted one of the fastest processors in its time, the Motorola 68000 series. Along with it, it also came equipped with a Zilog Z80 coprocessor that handled sound and music processing for the console. With all of this running inside, the Mega Drive was one of the first game console to boast of having 16-bit music and art, and to top it off, the Genesis had "Blast Processing," which was not much more than a marketing slogan.

Model 1 and Model 2 Mega Drives have an expansion port for the Sega CD peripheral. In addition to the Sega CD, in 1995 Sega released the 32X peripheral. This piece of equipment was to be placed directly into cartidge slot, and enhanced the console with two 32-bit CPUs and increased audio and video capabilities.

Hardware Revisions

Model 1

The Model 1 is the original release of the system in 1988. This version was released in all regions. The only differnces between the two versions is that the American Release had "Genesis" printed on it as opposed to "Mega Drive"

Basic Features

  • Two Controller ports
  • A/V out ports
  • The system had the switch that selected which channel the RF Signal would be picked up on
  • Capabilities to play up to 4 MB (more with special cartridges) games
  • Stereo sound output only through headphone port
  • EXT port on the back of the console, used mostly for the Mega Modem (removed in late Model 1 consoles)
  • Larger circuit board and less distorted sound output
noicon
A sample of the Mega Drive's sound capability.


The Model 1 Mega Drive can easily be identified by the large rectangular shape, and the non-distorted audio. The Model 1 Mega Drive also came bundled with the original Sonic the Hedgehog, and it very popular along console modders due to the large motherboard. A model 1 Genesis can also be identified by its FCC ID - if it is FJ846EUSASEGA or FJ846EUSASEGA one can usually be sure that you have a Model 1 at hand. Some Model 1's also used some sort of Motorola 68000 clone instead of the official CPU from Motorola.

Model 2

Sega eventually released a later version of the Mega Drive dubbed the "Mega Drive 2" or Model 2. It featured a smaller and more square-like appearance, removed the headphone jack, volume slider, as well as the 'EXT' port in the back of the console. Sega also changed the DIN plug at the back of the console to 9 pins (and shrunk it) to be able to output stereo sound. The backside of the Model 2 had a much simpler design, only having the standard AV out port and power adapter input.

Hardware

  • Main CPU: Motorola 68000 running at 7.67MHz NTSC/7.61MHz PAL
    • The 68000 has a 24-bit address space, allowing access to up to 16MB of memory. Sega's memory map for the Mega Drive allowed games to be up to 4MB without the use of a memory mapper; games that tried to go up to 10MB would find their memory maps crushed by the Sega CD (which took the second 4MB block) and Sega 32X (which took 2MB of the third 4MB block). All devices are memory mapped.
      • Games using save memory also needed to have the memory in the cartridge map; larger games, such as Phantasy Star IV, used a mapper to swap out cart space for SRAM during a save.
  • Main RAM: 64KB of Pseudo-static RAM (repeated over the upper 2MB of address space)
    • Refreshed every 128 68k clocks - if accessing it during refresh, it will be delayed one 68k clock.
    • Refreshed by pulling /OE low and all other control pins high
  • Graphics: "Video Display Processor," or "VDP:" modified Texas Instruments TMS9918 based on the modifications made for the Master System; all TMS9918 modes were removed and replaced with several new modes.
    • Screen resolutions: 320x224, 256x224, 320x240 (PAL only), 256x240 (PAL only)
    • "Interlaced mode" doubles the height of all four; it was used by some games, such as Sonic 2 for two-player mode
    • Four graphics layers: two tile planes (just a grid of tiles), a "window" tile plane (cannot be transparent), and a sprite plane
    • 64KB internal VRAM — used to store graphics tiles, mappings for all layers, and horizontal scrolling
    • 64 9-bit words of internal CRAM — used to store the color palette
      • 64 colors split into four 16-color lines; each tile can be drawn with one of these four color lines
      • The first color in each line is transparent and any color of the entire palette can be used as a "background color" (when no pixels are drawn at a location); consequently the Mega Drive can display 61 colors on screen at once (unless raster effects or the Shadow/Highlight modes are used, in which case this number increases depending on the extent used)
      • Colors are 9-bit RGB with 3 bits per color component, allowing for 512 colors
        • When writing CRAM values to the VDP, however, they are word adjusted to 16 bits
      • Shadow/Highlight modes increase color gamut
    • 80 bytes internal VSRAM — used for vertical scrolling
  • Sound:
    • Sound CPU: Zilog Z80 running at 3.58MHz NTSC/3.55MHz PAL
      • Some games did not use the Z80, other games used it only for sample playback, but most used it for sound processing
      • 8KB program RAM which the 68000 and the Z80 can freely write to (though the 68000 must request the Z80 bus)
      • Can access 32KB of the 68000 memory map at once (while it should be used for accessing the cartridge, setting the bank register elsewhere can work on some hardware)
    • Yamaha YM2612 clocked at the 68000 clock speed
      • 6 channels of FM synthesis, Operator Type-N
      • The third channel can enter a Special Mode, or multifrequency mode, where each individual operator has a different frequency
      • The sixth channel can enter a DAC mode where the sound program constantly streams 8-bit unsigned PCM data to mix directly into the output waveform
      • Mapped to the Z80 address space — 68000 must request the Z80 bus to use
      • Some Mega Drive 2s actually use the core from the chip's CMOS equivalent, the YM3438
    • Texas Instruments SN76489 clocked at the Z80 clock speed and built into the VDP — same as with the Master System
      • Three channels of pure square tones and one noise channel
      • The noise channel can play either white noise or "periodic noise" either at one of three preset frequencies or using the frequency of the third tone channel (consequently, that channel will be mute)
      • Can be freely accessed by both the 68000 through the VDP and the Z80 through its memory map
    • The cartridge connector has two pins which allow stereo sound mixing directly from cart. No game used this, however the 32X uses it for its PWM audio.
    • The Mega Drive 1 has mono audio output from the TV output and stereo output from a built-in headphone jack, plus a built-in volume control. Future models drop the headphone jack and do stereo output from the TV output
  • Controller input: Two male DE-9 controller ports; one female DE-9 expansion port (early MD1s only)
    • Controller ports support two modes: parallel and serial
    • Parallel supports 7-bit bidirectional, with the console setting the direction of each bit.
    • Parallel also supports optional active-low interrupts on the TH line. (mapped to 68000 IRQ 2)
    • Serial mode supports up to 4800 bps. (used by the Mega Modem on port 3)
  • Expansion port: Used for Sega CD.
    • Provides access to /FDC ($A120xx) and /DISK to indicate Sega CD presence.
    • Maps Sega CD ROM to $000000 when no cartridge is present, $400000 otherwise.