Sega Genesis

From Nintendo

The Sega Mega Drive released in Japan and Europe.

The Sega Genesis, which was better known in Japan and Europe as the Sega Mega Drive (メガドライブ, Mega Doraibu), was a 16-bit video game console released by Sega in Japan (October 29, 1988), Europe (November 30, 1990) and most of the rest of the world. It debuted under the name Sega Genesis in North America (August 14, 1989), as Sega was unable to secure legal rights to the Mega Drive name in that region. It was a competitor to the Super Nintendo and the last Sega game system regarded in hindsight as successful.

[edit] History

Although the Sega Master System had proved a success in Brazil and Europe, it failed to ignite much interest in the North American or Japanese markets, which, by the mid-to-late 1980s, were both dominated by Nintendo's large market shares. Meanwhile in the arcades, the Sega System 16 became a success. Hayao Nakayama, Sega's CEO at the time, decided to make its new home system utilize a similar 16-bit architecture. The final design was eventually also used in the Mega-Tech, Mega-Play and System-C arcade machines. Any game made for the Mega Drive hardware could easily be ported to these systems.

The first name Sega considered for its console was the MK-1601, but it ultimately decided to call it the "Sega Mega Drive". Sega used the name Mega Drive for the Japanese, European, Asian, Australian and Brazilian versions of the console. The North American version went by the name "Genesis" due to a trademark dispute, while the South Korean versions were called Super Gam*Boy (수퍼겜보이) and Super Aladdin Boy (transliterated from 수퍼알라딘보이; this was the Korean version of the Mega Drive 2). The Korean consoles were licensed and distributed by Samsung Electronics.

[edit] Technical specifications

[edit] CPU

The CPU is a 16/32-bit Motorola 68000. It is a 32 bit microprocessor sitting on a 16-bit-wide data bus. Maximum addressable memory is 16 megabytes. The 68000 Runs at 7.61 MHz in PAL consoles, 7.67 MHz in NTSC consoles. The Mega Drive also includes a Zilog Z80 as the sound CPU. The Z80 Runs at 3.55 MHz in PAL consoles, 3.58 MHz in NTSC consoles.

The 68000 uses a technique called Direct Memory Access (DMA) which is a high speed technique for accessing the VRAM, CRAM and VSRAM. DMA has 3 modes of access which may be applied to VRAM, CRAM or VSRAM. During memory to VRAM/CRAM/VSRAM DMA the 68000 is stopped, but the Z80 will continue to run as long as it does not attempt to access the 68000 memory space.

Processor:

16-bit/32-bit Motorola 68000

Runs at 7.61 MHz in PAL consoles, 7.67 MHz in NTSC consoles.
Some systems contained 68000s manufactured by Motorola licensees such as Hitachi and Signetics. Signetics 68K only found in early revisions as it is known to be inefficient.

Secondary Processor:

Zilog Z80

Runs at 3.55 MHz in PAL consoles, 3.58 MHz in NTSC consoles
Used as sound CPU or the main CPU in Master System compatibility mode.

[edit] Video

The Mega Drive has a dedicated VDP (Video Display Processor) for playfield and sprite control. This is an improved version of the Sega Master System VDP, which in turn is derived from the Texas Instruments TMS9918. It contains both mode 4 (for Master System compatibility) and mode 5 (for native 16-bit games). However, Master System programs can switch the VDP into mode 5 and make use of advanced VDP features. This page only discusses mode 5 capabilities.

Images could be output at 256 pixels (32 tiles) or 320 pixels (40 tiles) across and 224 scanlines (28 tiles) or 240 scanlines (30 tiles) down. The 240-line resolutions are only used on 50 Hz (i.e. PAL) displays, as the extra lines end up in the overscan of a 60 Hz (NTSC) signal. Instead, NTSC games use the 224-line resolution to free up more vertical blanking time to send more updates to the VDP. Colors are chosen from a total of 512 possible colors; some games used a small amount of flicker to simulate more colors. Graphics consist of up to 64 sprites on 3 planes. Palettes are stored in color RAM (CRAM) and consist of 16 colors each for a total of 64 colors.[citation needed]

Sprites can be any combination of 8, 16, 24 and 32 by 8, 16, 24 and 32. Up to 20 sprites may be used on a single line and each of these sprites will be on one of the 3 planes. The priority of each sprite can be changed allowing sprites on different planes to appear in a different order than the planes themselves.

Playfields, 2 of the available planes, can both be scrolled horizontally and vertically as a whole. Horizontal scrolling allows for individual character line or scan line scrolling while vertical scrolling is limited to 2 character wide columns.

Planes: 4 (2 scrolling playfields, 1 sprite plane, 1 'window' plane), per-tile priority

Sprites: Up to 64 (32H)/80 (40H) on-screen, 16/20 per line, 256/320 pixels per line, per-sprite priority

Palette: 512 colors (1536 using shadow/highlight mode)

On-screen colors: 64 × 9-bit words of color RAM, 4 lines of 15 colors plus transparent, allowing 61 on-screen colors (up to 1536 via raster effects and shadow/highlight)

Screen resolution:

256x224 (32Hx28V), 320x224 (40Hx28V), 256x240 (32Hx30V, PAL only), 320x240 (40Hx30V, PAL only)
Interlace mode 1 provides no increase in resolution, but still generates a true interlaced signal
Interlace mode 2 can provide double the vertical resolution (i.e. 320×448 for NTSC, 320x480 for PAL). Used in Sonic 2 for two-player split screen

Scroll size: Width and height independently set to 32, 64, or 128 cells as VRAM allows

[edit] Audio

There are two primary sound chips which are both controlled by the Z80; the Yamaha YM2612 Frequency Modulation (FM) chip and the Texas Instruments SN76489 Programmable Sound Generator (PSG) chip. The FM sound synthesis IC resembles the Yamaha YM2151 (used in Sega's coin-op machines) and the chips used in Yamaha's synthesizers. There is 8 KBytes of dedicated sound RAM available to them.

The Yamaha uses six FM channels, four operators each and runs at 7.67 MHz. The sixth channel can be used as a 8-bit digitized audio channel or as a regular FM channel. The use of the digitized audio allows for stereo sound and is output only through the headphone jack.

The SN76489 device is built into the VDP and has a four-channel PSG (Programmable Sound Generator). It has three square wave channels, one white noise channel and programmable tone/noise and attenuation. It is mainly used for Master System compatibility mode but can be used to supplement FM channels.

Main sound chip: Yamaha YM2612

Six FM channels, four operators each; channel 6 can be used for PCM data or as a regular channel
PCM channel has no DMA, and requires data to be written to the chip from the Z80
Programmable low-frequency oscillator and stereo panning
Runs at 7.67 MHz

Secondary sound chip: Texas Instruments SN76489 compatible device built into VDP.

Four-channel PSG (Programmable Sound Generator)
Three square wave channels, one white noise channel
Programmable tone/noise and attenuation
Used for Master System compatibility mode as well as to supplement FM
Different random noise generation compared to a real SN76489/SN76489A chip

[edit] Memory

Boot ROM:

2 KB (KiB)
Known as the "Trademark Security System" (TMSS)
When console is started, it checks the game for certain code given to licensed developers
Unlicensed games without the code are thus locked out, but if a game is properly licensed, the ROM will display "Produced by or under license from Sega Enterprises Ltd."
Boot ROM is not present on earlier versions of the Mega Drive and Genesis and some earlier games not designed for the TMSS may not work in later consoles

Main RAM:

64 KB
Part of M68000 address space

Video RAM:

64 KB
Cannot be accessed directly by CPU, must be read and written via VDP (Video Display Processor - see below)

Secondary RAM:

8 KB
Part of Z80 address space
Used as main RAM in Master System compatibility mode

Audio RAM

8 KB

Cartridge memory area (ROM space):

Up to 4 MB (MiB) (32 megabits)
Part of M68000 address space
Game cartridges larger than 4 MB must use bank switching

[edit] Inputs and outputs

RF output: RCA jack connects to TV antenna input

Exists on original model European and Asian Mega Drive and North American Genesis only
Other models must use external RF modulator which plugs into A/V output

A/V output: DIN connector with composite video, RGB video, and audio outputs

Mega Drive and the first model Genesis have an 8-pin DIN socket (same as Sega Master System) which supports mono audio only
Mega Drive 2, Multimega, and other models have a 9-pin mini DIN socket with both mono and stereo audio

Power input:

Positive tip barrel connector
Requires 9-10 volts DC, 0.85-1.2 A depending on model

Headphone output: Amplified 3.5-mm stereo jack on front of console with volume control

Exists only on original model Mega Drive and Genesis units
Provides stereo audio on models which have the mono 8-pin DIN A/V output
Also suitable for passive speakers
Can be used for mixing audio from the SegaCD

"EXT" port: DE-9F (9-pin female D-connector) on back of console

Used with the Meganet modem peripheral, released only in Japan
Exists on all first-model Japanese Mega Drive units, and on early American Genesis and PAL (European, Australasian and Asian) Mega Drive units
May have been used for game selection on arcade adaptations of the Mega Drive/Genesis console

Control pad inputs: Two DE-9M (9-pin male D-connectors) on front of console

Expansion port: Edge connector on bottom right hand side of console

used almost exclusively for Sega Mega-CD connection
not present on Genesis 3 model
also used for the Sega Genesis 6 Cart Demo Unit (DS-16) in stores

[edit] Nomad

In 1995, Sega released their second handheld system, known as the Nomad. The idea of the Nomad was to have a handheld system compatible with Genesis cartridges. However, it had issues with the Genesis' add-ons such as Sega CD or Sega 32X.

Like the Game Gear before it, the Nomad also suffered from high power usage due to its color screen and backlight. It also required a full discharge before recharging was possible. A battery pack was available for a costly $80. Rechargable batteries were incompatible due to voltage problems.

Like the Game Gear before it, the Nomad was criticized for its bulkiness. While the Game Boy had been just as thick as the Game Gear, its vertical design made it more comfortable compared to the Game Gear's horizontal design.

Finally, the Nomad suffered from a late release. By 1995, the Genesis was being phased out for the new Sega Saturn, and Genesis games were slowly being discontinued.

[edit] Legacy on Nintendo Consoles

After Sega had stopped producing consoles, they started releasing games on other systems, including the Game Boy Advance. Many of the games that was released included games that had previously released on the Genesis.

[edit] Virtual Console

Sega Genesis games are currently being released via the Wii Virtual Console on the Wii. On the launch of the Wii, two games from the Genesis were immedately available: Altered Beast and Sonic the Hedgehog. Several more have been released in the weeks following with more to follow.

[edit] Genesis titles currently available

[edit] Planned titles for future release

Currently, of all Sega consoles, only titles from the Sega Genesis are confirmed for release on the Wii Virtual Console.

[edit] External Links