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==Technical specifications== {{stack| [[File:Mega Drive mboard.jpg|thumb|Mega Drive mainboard ([[PAL]])]] [[File:SMD2mobo.jpg|thumb|Mega Drive II mainboard (PAL)]] }} {| class="wikitable" |- | style=width:160px|Processor: | [[Motorola 68000]] 16/32-bit processor @ 7.67 [[MHz]] (MC68HC000, [[CMOS]] version) |- | Co-processor: | [[Zilog Z80]] 8-bit @ 3.58 [[MHz]] |- | Video display processor | Yamaha YM7101, derivative of the [[VDP]] from the [[Master System]] |- | Memory: | 64K work RAM (68000), 64 [[KiB]] video RAM, 8 [[KiB]] work RAM (Z80)<br/>Later hardware had an internal 1Kx16 ROM for the license display screen. |- | Display palette: | 512 colors (3:3:3 RGB) |- | Onscreen colors: | 64 (normal) or 183 (shadow/highlight mode) |- | Maximum onscreen sprites: | 80 (320-pixel wide display) or 64 (256-pixel wide display) |- | Resolutions: | 256Γ224, 256Γ448, 320Γ224, 320Γ448, (PAL and NTSC) 256Γ240, 256Γ480, 320Γ240, 320Γ480 (PAL only), 256Γ192 (SMS games only) |- | Sound: | [[Yamaha YM2612]] 5 channel [[Frequency modulation|FM]] and 1 channel FM/[[PCM]], [[Texas Instruments SN76489]] 4 channel PSG ([[Programmable Sound Generator]]) |} ===CPU and memory=== {{Main|Motorola 68000|Zilog Z80}} The Mega Drive's [[CPU]] is a 16/32-bit Motorola 68000.<ref>{{Cite web|url=http://www.ticalc.org/pub/text/68k/index.html|title=MC68000 Documentation|work=[http://www.ticalc.org/ TiCalc.org]|accessdate=2008-04-03}}</ref> The maximum addressable memory is 16 [[megabyte|MB]] from the ROM ($00000000-00400000 - 4 MB), to the RAM ($00FF0000-00FFFFFF - 64 [[Kilobyte|KB]]). The 68000 runs at 7.61 MHz in PAL consoles, 7.67 MHz in NTSC consoles.<ref name="pd">{{Cite web|url=http://picodrive.acornarcade.com/megadrive/background.html|title=Pico Drive Mega Drive Background|work=[http://picodrive.acornarcade.com/ PicoDrive]|author=Michael Drake, Adrian Lees, and Jeffrey Lee|accessdate=2010-07-16}}</ref> The Mega Drive also includes a [[Zilog Z80]], which serves as secondary processor along with allowing complete [[SEGA Master System|Master System]] compatibility with only a [[#Master System compatibility|passive adapter]]. The initial Mega Drive models used a Hitachi-made HD68HC000, while the Mega Drive 2 and later models used a Motorola MC68HC000, both fabricated in [[CMOS]]. There is 64 KB of Main [[Random Access Memory|RAM]], as part of the 68000 address space.<ref name="sreac">{{Cite web |url=http://skillreactor.org/tutortxt/megadrv/ |archiveurl=http://web.archive.org/web/20080522123832/http://skillreactor.org/ |archivedate=2008-05-22 |title=SEGA Mega Drive information |accessdate=2008-04-01 |work=Skill Reactor |author=Christoph Bolitz}}</ref> Also present is 64 KB of Video RAM,<ref name="ConsoleInfo"/> for exclusive use and access by the VDP (Video Display Processor). The Z80 has 8 [[kilobyte]]s of RAM for use as program RAM, which is also mapped into the 68000's address space. The Z80 can also access 32 [[kilobyte]]s of the 68000s memory using bank-switching, which can be used as a sound bank while in use as an audio controller. There is also 2 [[kilobytes|KB]] of Boot [[Read-only memory|ROM]], which is also known as the "[[Trademark Security System]]" (TMSS). When the 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.".<ref>{{Cite web|title=SEGA Enterprises Ltd. v. Accolade Inc.| work=[http://digital-law-online.info/ Digital Law Online]| date=1992-10-20| url=http://digital-law-online.info/cases/24PQ2D1561.htm| accessdate=2008-04-02}}</ref> Also, as a hardware-feature, with later versions of the Trademark Security System "SEGA" must be written into an area of I/O memory (A$14000) in order to turn on the VDP. The TMSS was the subject of the legal case [[SEGA v. Accolade]]. ===Audio and video=== There are two primary [[sound chip]]s which can both be controlled by the Z80 or the 68000; the [[Yamaha YM2612]] [[Frequency modulation synthesis|FM synth]] chip and the [[Texas Instruments SN76489]] [[Programmable Sound Generator|PSG]] chip.<ref name="ConsoleInfo">{{Cite web|url=http://www.consoledatabase.com/consoleinfo/segamegadrive/|title=SEGA Mega Drive/Genesis Console Information|accessdate=2008-04-03|work=Console Database/Dale Hansen}}</ref> The YM2612 is a stripped-down version of the [[Yamaha YM2608|YM2608]], which is an upgraded version of the prolific [[Yamaha YM2203]], used in many gaming machines throughout the 1980s and early 1990s. The Yamaha uses six FM channels with four operators each, and its clock speed is the same as that of the main CPU. [[Stereophonic sound|Stereo sound]] is output only through the headphone [[Jack (connector)|jack]] on model 1 systems and through AV out on model 2 systems along with mono signal.<ref name="sreac"/> Changes in the mixing circuit of late model 1 revisions, and all model 2's resulted in the hardware producing more distorted sound output than earlier models that bore the "[[Variations of the SEGA Mega Drive#North America (as SEGA Genesis)|High Definition Graphics]]" logo.<ref name="Audio Mixing in model #2 hardware revisions">{{Cite web|url=http://arcade.ym2149.com/megadrive/|title="Megadrive sound" at arcade.ym2149.com|work=[http://arcade.ym2149.com/ Arcade PCB]|accessdate=2008-10-05}}</ref> The Mega Drive uses the Yamaha YM7101 for graphics generation and [[Sprite (computer graphics)|sprite]] control.<ref>{{Cite web|url=http://gendev.spritesmind.net/forum/viewtopic.php?p=7758#7758|title=Gen cart pinout (Circuit diagram images)|work=[http://gendev.spritesmind.net/ SpritesMind]|accessdate=2010-02-16}}</ref> The YM7101 is based on the [[SEGA Master System#Video|Master System's VDP]] (or [[Graphics processing unit|Video Display Processor]]), which in turn is derived from the [[Texas Instruments TMS9918]]. The YM7101 includes additional display modes and capabilities along with the Master System VDP's original display modes (minus the original TMS9918 modes). Images can 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. 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, 3 bits per color channel; some games used a small amount of flicker to simulate more colors. Graphics consist of up to 80 sprites on screen and three background [[Plane (mathematics)|planes]] (Window, ScrollA, ScrollB), two of which (ScrollA and Window) share the same screen space. Palettes are stored in color RAM (CRAM) and consist of 16 colors each for a total of 64 colors.<ref name="cw">{{Cite web|url=http://www.captainwilliams.co.uk/sega/megadrive/megadrive.php|title=The SEGA Mega Drive/Genesis | The best 16-BIT Console|accessdate=2008-04-03|work=[http://www.captainwilliams.co.uk/ Captain Williams]}}</ref> ===Inputs and outputs=== On the front of the console are two controller input ports, which use 9-pin male [[D-subminiature]] connectors.<ref name="cw"/> On the rear of all first-model Japanese Mega Drive units and on early American Genesis and PAL (European, [[Australasia]]n and Asian) Mega Drive units is the EXT input port; a DE-9F (9-pin female D-connector) that was used with the [[Meganet]] modem peripheral, released only in Japan.<ref name="cw"/> The power input varies depending on the model - a model 1 uses a 2.1 mm [[DC connector|barrel connector]] with a negative tip, and requires 9-10 [[volt]]s [[Direct current|DC]] at 1.2 [[ampere|A]]. The model 2 uses a [[EIAJ connector|EIAJ-03 connector]] with a positive tip, and requires 9-10 volts DC at 0.85 A . There is also an Expansion input port which is an [[Edge connector]] on the bottom right hand side of the console. It is used almost exclusively for connection for the Mega-CD/SEGA CD, though it was also used for the SEGA Genesis 6 Cart Demo Unit (DS-16) in stores. This port is not present on the Genesis 3 model.<ref name="sreac"/> The console's A/V output consists of a [[DIN connector]] with [[composite video]], [[RGB|RGB video]] and audio outputs.<ref name="sreac"/> The Mega Drive and the first model Genesis have an 8-pin DIN socket (same as [[SEGA Master System]]) which supports mono audio only, while the Mega Drive 2, Multi-Mega/CDX and other models have a 9-pin [[mini-DIN connector]] with both mono and stereo audio.<ref name="sreac"/> Stereo audio for the Mega Drive and the first model Genesis were supplied by the headphone jack, which is not present on later models.<ref name="sreac"/> Original model European and Asian Mega Drives and the North American Genesis model also include a built in [[RF modulator]], which outputs via an [[RCA jack]] on the rear of the console; other models must use an external RF modulator for RF video/audio.<ref name="sreac"/> ===Master System compatibility=== One of the key design features of the console is its [[backwards compatibility]] with SEGA's previous console, the [[Master System]]. The 16-bit design is based upon the 8-bit design, albeit enhanced and extended in many areas. In order to achieve backwards compatibility, the Master System's central processor and sound chip (the [[Zilog Z80]]<ref name="sreac"/> and SN76489 respectively) are included as coprocessors in the Mega Drive, and the Mega Drive's Video Display Processor (VDP) is capable of the Master System's VDP mode 4, though it cannot run in modes 0, 1, 2, or 3 (so the Mega Drive is not compatible with [[SG-1000]] software or Master System software which uses these modes).{{citation needed|date=October 2011}} As the cartridge slot is of a different shape, SEGA released the Power Base Converter, a separate device that sits between a Master System cartridge and the Mega Drive's cartridge slot. The Power Base Converter does not contain any Master System components, instead functioning as a pass-through device, and consisting almost entirely of passive circuitry. The converter contains a top slot for cartridge-based games along with a front slot for card-based games, as well as the 3D glasses adapter. When a Master System game is inserted, the system puts the Z80 in control, leaving the Mega Drive's main 68000 processor idle. The Power Base Converter had inferior capacitors however, meaning that after a few years use, the system may suffer from glitchy play; to rectify this the user must remove the capacitors from the board or replace them.<ref name="sreac"/> In Japan the device is known as the "[[Mega Adapter]]" (γ‘γ¬γ’γγγΏ). The ''Mega Adapter'' is built for the Japanese Mega Drive cartridge slot, so it does not fit into the European Mega Drive and North American Genesis cartridge slots, like Japanese Mega Drive cartridges. It also has the Master System cartridge slot changed to the SEGA Mark III/Japanese Master System pinout. Because of the Genesis VDP limitations listed before, it does not run SG-1000/SC-3000 games or Master System games that use the SG-1000 video modes. The Mega Adapter does not have the Yamaha [[YM2413]] FM chip that enhances the sound of certain games. The PAL variant is called the "Master System Converter" in Europe.<ref name="MegaAdapter">{{Cite web|url=http://www.old-computers.com/museum/computer.asp?st=2&c=843|title=Mega Drive II|accessdate=2007-10-18|author=Old-Computers.com Staff|work=[http://www.old-computers.com/ OldComputers.com]}}</ref> The [[Power Base Converter]] is not fully compatible with the redesigned Mega Drive 2. A second version, the "[[Master System Converter II]]", was released to address this problem. This second version adapter was produced in a far smaller quantity, lacks the slot for card-based games, and was only released in Europe.<ref name="sreac"/> The only Master System game which does not work with this device is ''[[F-16 Fighting Falcon]]''.<ref>{{Cite book |title=Master System Converter Instruction Manual |publisher=[[SEGA]] |page=7}}</ref> It was originally thought that the game card had more pins than the adapter could interface with, but it is actually the compatibility mode of the Mega Drive/Genesis that is responsible for the game not working, not the Power Base Converter itself. This is because, as previously said, the Genesis can only run in VDP mode 4 and the game ran in a different VDP mode.{{Citation needed|date=October 2011}} Some Master System games (such as ''[[Shanghai]]'' and ''[[Alien Syndrome]]'') are incompatible with the Genesis control pad. However, it is possible to correct this by modifying the control pad, or by using a Master System control pad instead.<ref>{{Cite web|url=http://www.sega-16.com/forum/archive/index.php/t-9755.html|title=SMS games with the Genesis Controller [archive]|accessdate=2012-01-11|work=SEGA-16.com forum archive}}</ref> As it has the same connection port, the Master System pad can be plugged directly into Mega Drive controller ports without any kind of adapter.<ref>{{Cite web|url=http://www.vidgame.net/SEGA/MASTERS.html|archiveurl=http://web.archive.org/web/20080108225400/www.vidgame.net/SEGA/MASTERS.html|archivedate=2008-01-08|title=SEGA Master System|accessdate=2011-01-22|author=Barr, Adrienne|work=vidgame.net}}</ref>
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