Editing Sega Mega Drive / Genesis (Model 1)

{{Infobox computer
| name         = Sega Mega Drive – Model 1
| image        = [[File:Sega-Mega-Drive-JP-Mk1-Console-Set.jpg|250px]]
| caption      = First-generation (“High Definition Graphics – Stereo Sound”) Mega Drive released in Japan & Europe
| manufacturer = Sega Enterprises, Ltd.
| type         = 16-bit home video-game console
| release date = 29 October 1988 (JP)<br />14 August 1989 (NA – as '''Genesis''')<br />30 November 1990 (EU)
| discontinued = April 1997 (worldwide)
| price        = JP ¥21 000 • US US$189 (1989) • UK £189.99 (1990)
| cpu          = Motorola 68000 @ 7.67 MHz<ref>NTSC master clock 53.693175 MHz ÷7.</ref><br />Zilog Z80 @ 3.58 MHz (audio & Master-System compatibility)
| memory       = 64 KB 16-bit Work RAM • 64 KB VRAM • 8 KB Z80 RAM
| storage      = ROM cartridges (up to 4 MB officially; larger with bank-switch)<br />CD-ROM via [[Sega CD (Model 1)]] add-on
| display      = 320 × 224 px (256 × 224, 320 × 240 & 256 × 240 modes) • 64 colours onscreen from 512-colour palette • 80 sprites
| sound        = Yamaha '''YM2612''' 6-ch FM + SN76489 PSG 4-ch + 8-bit PCM (via 68000 DMA)
| dimensions   = 280 mm W × 210 mm D × 70 mm H
| weight       = 1.2 kg
| os           = None (game cartridge provides code)
| predecessor  = [[Master System II]]
| successor    = [[Sega Mega Drive / Genesis (Model 2)]]
| codename     = '''MK-1601''' (HAA-2510 PCB)
| model        = “VA0–VA6” motherboard revisions
}}
The '''Sega Mega Drive'''—released as the '''Sega Genesis''' in North America—is a 16-bit fourth-generation console that propelled Sega into direct competition with Nintendo’s Super Famicom/SNES.  Its Motorola 68000 CPU and sophisticated Yamaha FM audio gave arcade-like performance at home, while aggressive “Genesis does what Nintendon’t” marketing helped drive '''≈ 30 million units''' sold worldwide.

== Architecture & Dual-CPU Design ==
{| class="wikitable styled-table" style="width:70%; text-align:center;"
! Sub-system !! Specification (Model 1 VA4, NTSC)
|-
| '''Main CPU''' || Motorola 68000 @ 7.67 MHz (16/24-bit CISC, 68010-class core)
|-
| '''Sound CPU / 8-bit legacy''' || Zilog Z80 @ 3.579 MHz – controls PSG & streams PCM to YM2612 DAC  
Also hosts complete Master-System I/O for backward compatibility (cartridge mode pin M3)
|-
| '''VDP''' || Sega Custom '''YM7101 (315-5313)''' – derive from Yamaha V9938: 2 scrolling BG planes + window + 80 sprites, 64 colours onscreen
|-
| '''Audio''' || Yamaha '''YM2612''' (OPN2) 6-ch FM + integrated 8-bit DAC (channel 6)  
Texas Instruments '''SN76489''' PSG (3 tone + noise) clocked from 3.58 MHz ÷ 16
|-
| '''RAM''' || 2 × 32 KB 16-bit DRAM (Work RAM)  
2 × 32 KB VRAM (dual-ported)  
8 KB static RAM for Z80 
|-
| '''ROM space''' || Up to 4 MB linear; larger games (''Virtua Racing'', 6 MB) use mapper ASIC
|-
| '''DMA''' || 68000→VDP HSYNC DMA (13.4 MB/s burst) and VRAM copy modes
|}

== Memory Map (NTSC, no add-ons) ==
{| class="wikitable styled-table" style="width:70%;"
|+'''Mega Drive Model 1 Address Space (68000)'''
! Range !! Size !! Purpose
|-
| <code>000000–3FFFFF</code> || 4 MB || Cartridge ROM / SRAM
|-
| <code>400000–7FFFFF</code> || Mirrors / future mapper
|-
| <code>A00000–A0FFFF</code> || 64 KB || '''Z80 address space''' (via bus request)
|-
| <code>A10000–A1001F</code> || 32 B || I/O ports: controller, version, TMSS
|-
| <code>A11100</code> || 1 B || Bus-grant register (68k⇄Z80)
|-
| <code>A11200</code> || 1 B || Z80 reset
|-
| <code>C00000–C0001F</code> || 32 B || VDP registers
|-
| <code>C00000–C0FFFF</code> || 64 KB || '''VRAM''' (through VDP)
|-
| <code>C00020–C0003F</code> || 32 B || PSG, YM2612, CRTC HV counter
|-
| <code>FF0000–FFFFFF</code> || 64 KB || '''System RAM''' (work RAM)
|}

Key ROM vectors: <code>000004</code> initial SSP, <code>000006</code> initial PC (game entry), TMSS “SEGA” string check at <code>000100</code> on later VA6(+).

== Video Timing ==
{| class="wikitable styled-table" style="width:70%; text-align:center;"
! Mode !! Pixel clock !! Pixels/line !! Lines/frame !! Refresh
|-
| NTSC 320-wide || 10.738 MHz || 342 active / 427 total || 262 || 59.922 Hz
|-
| PAL 320-wide || 10.054 MHz || 342 / 427 || 312 || 49.701 Hz
|}

''The VDP fetches one 16-bit VRAM word every 4 cycles; HSYNC width ≈ 84 master clocks.''

== Edge Connector (64-pin Cartridge Slot) ==
{| class="wikitable styled-table" style="width:90%; text-align:center;"
! Pin !! Signal !! Description !! Pin !! Signal !! Description
|-
| A1 || GND || Ground || B1 || +5 V
|-
| A2 || A23 || Address bus 23 || B2 || A22
|-
| A3 || A21 || … || B3 || A20
|-
| ⋯ || ⋯ || ⋯ || ⋯ || ⋯ || ⋯
|-
| A30 || '''M3''' || Low = Master-System mode || B30 || '''/CART_INH''' || Pull low to disable ROM (CD/32X)
|-
| A32 || /OE || 16-bit ROM output enable || B32 || /WE (SRAM)
|}

''Full 64-pin table located on the [[Mega Drive Cartridge Pin-out]] page.''

== Model 1 Motherboard Revisions ==
{| class="wikitable styled-table" style="width:80%; text-align:center;"
|+'''Key Mega Drive VA-board Changes'''
! Rev (JP) !! ASIC !! Notable characteristics
|-
| '''VA0''' (launch, 1988) || 315-5211 VDP || Rare; discrete RGB amp, noisier audio
|-
| '''VA2–VA4''' (1989) || 315-5313 VDP || Adds low-pass in audio, fixes VDP register 1 bug
|-
| '''VA5''' (1991) || 315-5476 combined I/O || ˜1 dB louder YM2612 DAC, TMSS “LOCK-ON”
|-
| '''VA6''' (1992) || same || TMSS mandatory; “Licensed by Sega” BIOS check
|}

VA4 & later drop the “EXT PORT” text on the front but keep the 9-pin SUB-D connector.  Revision affects RGB quality and capacitor layout—see the dedicated [[Sega Mega Drive / Genesis (Model 1) Maintenance Guide|maintenance guide]] for board-specific recap lists.

== Audio Path ==
68000 DMA → YM2612 FM → mixed with PSG → pre-emphasis RC filter → Sony CXA1034 op-amp → headphone amp & rear AV-out.  
*Early VA0/VA1 units suffer the well-known “Mega-Drive whistle.”*  Installing 470 µF / 100 µF low-ESR caps and moving the audio return after the regulators mitigates it (documented in the [[Mega Drive AV Output Guide]]).

== Historical Context & Sales ==
* Conceived under codename '''“Mark V”''' to supersede the 8-bit Master System using Sega’s System-16 arcade silicon.  
* Japanese uptake was modest (≈ 400 k units by 1990) until ''Sonic the Hedgehog'' (1991) exploded sales.  
* '''North America (Genesis):''' > 18 million; '''Europe/Middle East:''' > 8 million; '''Brazil (Tectoy):''' 3 million+.  
* Sixth-year production cost with VA6 board < US$50, leading to sub-£100 “Mega Drive II” bundles in 1995.

== Common Faults & Preventive Maintenance ==
* '''Electrolytic capacitor leakage''' around CXA1034 (audio) and power rails (5 V, -12 V op-amp).  See [[Sega Mega Drive / Genesis (Model 1) Capacitor Replacement Guide]].  
* '''No video / jailbars''' – cracked solder on AV DIN, VDP or RAM ICs.  
* '''RESET line stuck low''' – faulty MC14069 (Hex Schmitt inverter) on VA0-VA3.  
* '''Z80 bus contention''' causes garbled FM – reseat / clean edge cartridge & check 315-5410 ASIC.  
* Full troubleshooting flow-chart on [[Sega Mega Drive / Genesis (Model 1) Troubleshooting Guide]].

== Expansion & I/O ==
* '''Cartridge slot''' 64-pin edge (mapped @ 0–4 MB)  
* '''EXT PORT''' 9-pin mini-DIN (unused by most games; utilised by Sega CD side-bus)  
* '''AV Out''' 8-pin mini-DIN (RGB 0.7 Vpp, composite, mono audio)  
* '''Controller ports''' 2× DE-9 (compatible with Atari/VCS & MS pads)  
* '''Add-ons:''' [[Sega CD (Model 1)]] (CD-ROM storage, additional 12.5 MHz 68000 & PCM) and [[Sega 32X]] (2× Hitachi SH-2 @ 23 MHz, 32-colour cell-based renderer)

== Trivia & Pop-culture ==
* Early Japanese units proudly display '''“High Definition Graphics • Stereo Sound”''' silk-screen; later cost-reduced versions removed the legend though hardware remained identical.  
* The console appears (model-accurate) in the 2020 film ''Sonic the Hedgehog''—complete with a VA3 style headphone volume wheel.  
* An undocumented VDP test pattern can be triggered by holding <code>A+B+C+START</code> on power-up with no cartridge inserted (development boards only).  

== Further reading / guides ==
* [[Sega Mega Drive / Genesis (Model 1) Maintenance Guide]] – disassembly, PSU voltages, cleaning  
* [[Sega Mega Drive / Genesis (Model 1) Troubleshooting Guide]] – no-power, black-screen, audio issues  
* [[Sega Mega Drive / Genesis (Model 1) Capacitor Replacement Guide]] – board-specific recap BOM  
* [[Sega Mega Drive AV Output Guide]] – RGB, composite, stereo mod & jailbar fixes
<references />
[[Category:Sega Systems]]