Editing Sega Master System (Model 1)

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{{Infobox computer
| name          = Sega Master System&nbsp;(Model 1)
| image         = [[File:Sega-Master-System-Set.jpg|260px|frameless]]
| caption       = First-generation «Master System» console (MK-2000, 1986) with cartridge &amp; Sega Card slots
| manufacturer  = Sega Enterprises, Ltd.
| type          = Home video-game console
| release date  = {{Plainlist|
* JP (as '''Sega Mark III''') – '''20 Oct 1985'''
* NA – '''Jun 1986'''
* EU / Oceania – '''Sep 1987'''
}}
| discontinued  = {{Plainlist|
* JP – 1989 (replaced by Master System [JP])
* NA – 1991
* EU – 1996 (budget rerelease by Majesco)
}}
| price         = US$199 (NA launch bundle, 1986)
| cpu           = Zilog Z80A @ '''3 579 545 Hz''' (NTSC) / '''3 546 895 Hz''' (PAL)
| memory        = 8 KB work-RAM • 16 KB video-RAM
| storage       = ROM cartridges (max 4 Mbit) • Sega Card (256 Kbit) • optional FM sound BIOS (Japan)
| display       = VDP 315-5124/5246 → 256 × 192 (nom.) / 32-colour on-screen from 64-colour palette
| sound         = SN76489-compatible PSG (3 square + 1 noise) • optional '''YM2413 FM''' add-on (JP)
| dimensions    = 365 mm W × 170 mm D × 66 mm H
| weight        = ≈ 2.1 kg
| os            = None (8 KB BIOS v1.3 – menu + hidden Snail Maze)
| predecessor   = [[Sega SG-1000 II]]
| successor     = [[Sega Master System II]]
| codename      = ''Mark III''
| model         = MK-2000 / MK-3000
}}

The '''Sega Master System''' (first-generation “Model 1”, product code '''MK-2000''') is an 8-bit home console released by Sega to compete directly with Nintendo’s Famicom/NES.  Re-branded from the Japan-only '''Sega Mark III''', the Master System introduced the distinctive black-and-red industrial design, a combo cartridge + “[[Sega Card]]” slot, and an edge expansion port supporting FM audio, 3-D glasses and future accessories.  Although outsold by Nintendo in Japan & North America, it enjoyed major success across Europe, Brazil and Australasia, shipping an estimated '''≈ 13 million units''' worldwide.

== System Architecture ==
{| class="wikitable styled-table" style="width:70%; text-align:center;"
! Sub-system !! Specification (Master System Model 1)
|-
| '''CPU''' || Z80A 8-bit @ 3.58 MHz (NTSC) / 3.55 MHz (PAL) • 16-bit address – 64 KB space
|-
| '''VDP''' || Sega 315-5124 (NTSC) / 315-5246 (PAL)  
TMS9918-derived: 2× pattern tables, 32 sprites (8 per scan-line)  
VRAM 16 KB (dual 8 KB SRAM, 5 ns page registers)
|-
| '''Palette''' || 64 colours (6-bit RGB) – 32 on-screen (background & sprite)
|-
| '''Audio''' || TI SN76489 PSG (integrated into VDP) – 3 square + 1 noise  
Expansion pin-out for YM2413 FM (used in JP “FM Unit” & Mark III)
|-
| '''Main RAM''' || 2 × 8 KB HM6264 (work-RAM) @ ~150 ns
|-
| '''Cartridge ROM''' || Up to 4 Mbit/512 KB (LoROM/HiROM mapper chips: Sega 315-5235, 315-5208, etc.)
|-
| '''BIOS''' || 8 KB mask ROM ver 1.3 (NA/EU) — intro menu + hidden Snail Maze (hold ↑+1+2)
|-
| '''I/O''' || 2× DE-9 game-pad, RF-UHF & 8-pin AV, Card slot (36-pin), 50-pin cart, 35-pin EXT
|}

=== Exact 64 KB CPU Memory-Map (Model 1) ===
{| class="wikitable styled-table" style="width:70%; text-align:center;"
|+''Z80 Address Space''
! Range !! Size !! Purpose / Notes
|-
| $0000 – $3FFF || 16 KB || '''ROM–0''' (Cartridge / Card / BIOS)
|-
| $4000 – $7FFF || 16 KB || '''ROM–1''' (bank-selectable via mapper)
|-
| $8000 – $BFFF || 16 KB || '''ROM–2''' (bank-selectable)
|-
| $C000 – $DFFF || 8 KB || '''Work-RAM''' (on-board)
|-
| $E000 – $FFFF || 8 KB || Mirror of $C000 – $DFFF
|}

''Mapper registers'' are written through bogus addresses $FFFC–$FFFF (upper bits decoded by on-cart ASICs such as 315-5196).

=== BIOS Entry Points (v1.3) ===
{| class="wikitable styled-table" style="width:70%; text-align:center;"
! Address !! Routine
|-
| $0000 || Soft reset  →  Sega splash &amp; checksum
|-
| $00A8 || PSG mute / init
|-
| $0150 || Card detect loop
|-
| $0200 || Hidden game “Snail Maze” (activate ↑+1+2 at logo)
|}

== Video Timing ==
{| class="wikitable styled-table" style="width:70%; text-align:center;"
! Parameter !! NTSC !! PAL
|-
| Master clock || 10.738635 MHz || 10.644606 MHz
|-
| Pixel clock || 5.369 MHz || 5.322 MHz
|-
| Lines/frame || 262 || 313
|-
| Refresh || 59.92 Hz || 49.70 Hz
|}

== Edge-Connector Pin-out ==
=== 50-pin Cartridge Bus (looking into console) ===
{| class="wikitable styled-table" style="width:95%; text-align:center; font-size:90%;"
! Pin !! Signal !! Description !! Pin !! Signal !! Description
|-
| 1 || GND || Ground || 26 || A11 || Address 11
|-
| 2 || +5 V || +5 V DC || 27 || A9 || Address 9
|-
| 3 || CLOCK || M-clock (5.37 MHz) || 28 || A8
|-
| 4 || A15 || Address 15 || 29 || A13
|-
| 5 || A14 || Address 14 || 30 || A7
|-
| 6 || A12 || Address 12 || 31 || A6
|-
| 7 || /MREQ || Z80 Memory Request || 32 || A5
|-
| 8 || /IORQ || I/O Request || 33 || A4
|-
| 9 || /RD || Read strobe || 34 || A3
|-
| 10 || /WR || Write strobe || 35 || A2
|-
| 11 || D7 || Data 7 || 36 || A1
|-
| 12 || D6 || Data 6 || 37 || A0
|-
| 13 || D5 || Data 5 || 38 || CE ''(low)''
|-
| 14 || D4 || Data 4 || 39 || +5 V
|-
| 15 || D3 || Data 3 || 40 || /BUSREQ (rarely used)
|-
| 16 || D2 || Data 2 || 41 || PAUSE (pulled-up: cartridge can assert)
|-
| 17 || D1 || Data 1 || 42 || /RESET
|-
| 18 || D0 || Data 0 || 43 || AUDIO IN (FM/stereo mods)
|-
| 19 || M1 || Op-code fetch || 44 || /NMI (3-D glasses)
|-
| 20 || RFSH || Refresh || 45 || GND
|-
| 21 || /HALT || CPU halt || 46 || GND
|-
| 22 || A10 || Address 10 || 47 || +5 V
|-
| 23 || '''-''' || (n/c) || 48 || +5 V (cart power)
|-
| 24 || '''-''' || (n/c) || 49 || '''-'''
|-
| 25 || A16 || Address 16 (mapper) || 50 || '''-'''
|}

== Historical Context &amp; Sales ==
* '''1985''' – Sega launches the ''Mark III'' in Japan; hardware overhaul of the SG-1000 line.  
* '''1986''' – Re-packaged for North America as the '''“Sega System”''', quickly re-branded '''Master System''' under Tonka; bundled with ''Hang-On/Safari Hunt'' combo cartridge.  
* '''1987–1992''' – Becomes the dominant 8-bit console across Europe, distributed by Virgin Mastertronic; library balloons to 300+ PAL titles.  
* '''Brazil''' – Tec Toy begins local production (1989) and continues updated revisions into the 2020s; > 8 million units sold in Brazil alone.  
Global lifetime sales are estimated at '''≈ 13 million''' (vs ~62 M NES).

== Maintenance &amp; Common Faults ==
* Cracked solder on AV DIN → intermittent video.  
* 7805 regulator runs hot (VA2 boards) – recap & replace thermal pad.  
* Dying 315-5216 I/O ASIC causes random controller dropouts.  
[[File:Sega Master System (Model 1) PCB.png|none|thumb|601x601px|Sega Master System (Model 1) PCB]]
Detailed procedures: see '''[[Sega Master System General Maintenance]]''', '''[[Sega Master System Troubleshooting Guide]]''', '''[[Sega Master System Capacitor Replacement Guide]]''' and '''[[Sega Master System AV Output Modifications]]'''.

== Board Revisions &amp; Known Errata ==
{| class="wikitable styled-table" style="width:90%; text-align:center;"
! Rev !! Region (year) !! Notes / quirks
|-
| VA0 || JP (1985) || Mark III PCB – discrete VDP/PSG, no BIOS, external FM Unit
|-
| VA2 || NA/EU (1986) || First “Master System” – large RF can, 8 KB BIOS v1.3, 5 × 5 V regulators; early units prone to jail-bar video (unscreened VRAM lines)
|-
| VA3 || EU (1987) || Card slot EMI shield deleted, improved RF modulator
|-
| VA5 || EU (1989) || 315-5246 PAL VDP, YM2413 traces removed; RGB DIN 8 output added
|-
| VA6 || BR (1991) || Tec Toy – onboard TMSS lockout stub, composite only (no RF)
|}

== Game Media ==
* '''ROM Cartridges''' up to 512 KB; larger titles employ paging ASICS (315-5208, 315-5235).  
* '''Sega Card''' thin 35 g card, limited to 32 KB/256 Kbit – discontinued 1989.  
* FM soundtrack detected via mapper bit 6; Western carts often contain unused FM data later unlocked by community patches.

== Trivia &amp; Pop-Culture ==
* Hidden '''“Snail Maze”''' game embedded in BIOS v1.3 – launch by holding ↑ + 1 + 2 at the Sega logo.  
* Original Japanese unit’s 3-D glasses used active shutter synced via the /NMI pin; later re-used on Mega Drive add-ons.  
* The console appears in '''Charlie Brooker’s _Black Mirror_''' episode “Bandersnatch” (retro game-dev setting).  
* Tec Toy released Brazilian-exclusive ports: ''Street Fighter II'', ''Mônica no Castelo do Dragão'' (Wonder Boy II reskin), sustaining the platform into the late 1990s.

== Further Reading ==
* ''[[Sega Master System Service Manual (1986)]]'' – Part #420-5718-01  
* Chris Covell, “SMS VDP documentation & palette research” (covell.ski)  
* Charles MacDonald, “Master System Hardware Notes” (github.com/CharlesMacDonald)

[[Category:Sega Systems]]