Emulator genesis plus gx Game Gear

genesis_plus_gx — Sega Master System + Game Gear

One core, two Sega machines. EmulatorJS + genesis_plus_gx (the accuracy-focused Sega libretro core, also used for the Mega Drive) drives both the handheld Game Gear and the console Master System from a single shared .data file. The Game Gear came first as a handheld; the Master System landed later (04/07/2026, replacing the retired Vectrex) and reuses the same already-shipped core. The Game Gear story is below; the Master System section follows it.

Game Gear

Sega's colour, backlit answer to the Game Boy. It's the third and final platform in the Handheld Consoles category, completing the set alongside the Emulator-gambatte-Game-Boy and Emulator-handy-Atari-Lynx.

Same bezel style — held up in class

The Game Gear reuses the handheld bezel style: the device held up in the foreground over the classroom photo, bottom-anchored and sized to 75%. gamegear.png (1523×818, wide landscape, screen keyed transparent), EJS in the screen hole. See Emulator-gambatte-Game-Boy for the full layer recipe — only the device image and hole coordinates differ.

The core: system segaGG

genxBootEJS({
  title: 'Sega Game Gear',
  coreName: 'segaGG',          // EJS resolves 'segaGG' → genesis_plus_gx
  defaultControls: { 0: { 0:{value:'z'}, 8:{value:'x'}, 3:{value:'enter'},
                          4:{value:'up arrow'},5:{value:'down arrow'},6:{value:'left arrow'},7:{value:'right arrow'} } }
});

genesis_plus_gx stable works — no nightly swap. No BIOS needed.

Volume — EJS doesn't clamp above 1.0

The Game Gear's PSG is quiet, and EJS defaults its master volume to 0.5. Useful finding: EJS reads EJS_volume and passes it straight to a Web Audio gain node with no clamp (emulator.js: gain.gain.value = volume), so values above 1.0 genuinely amplify. This bundle runs window.EJS_volume = 1.5; (150%) in its play.html — Game-Gear-only, not the shared boot. (At >1.0 the loudest passages can clip a touch; 1.25 is a safer ceiling if it sounds harsh.) The same trick works for any bundle that's too quiet.

Controls

Arrows = d-pad, Z = Button 1, X = Button 2, Enter = Start. Most games use the two buttons as jump + attack (either way round); each gamedoc spells out the per-game meaning.

Library

A Sega showcase, year-sorted: Columns (the pack-in), Sonic the Hedgehog (the 8-bit GG game, distinct from the Mega Drive one), Shinobi, Aerial Assault, Streets of Rage, Defenders of Oasis (a full RPG), Land of Illusion (Mickey), Mortal Kombat (Sega kept the gore), Dr. Robotnik's Mean Bean Machine, Tails Adventure. Gamedocs at gamedocs/gamegear/<key>.html. ROMs are .gg.

Bundle layout

systems/gamegear/
├── play.html            ← genxBootEJS wrapper (coreName: 'segaGG', EJS_volume 1.5)
├── controls.html
├── games.json
└── roms/
    └── *.gg

The core lives once in systems/_shared-ejs/ejs/data/cores/genesis_plus_gx-legacy-wasm.data — shared with the Master System bundle below (and with any future genesis_plus_gx machine).

Master System

The 8-bit console the Game Gear was built from. It shipped 04/07/2026 as systems/sms/, replacing the retired Vectrex, and rides the same genesis_plus_gx core already on disk for the Game Gear — no extra .data.

The core: system segaMS, but name the core explicitly

genxBootEJS({
  title: 'Sega Master System',
  coreName: 'genesis_plus_gx',   // NOT 'segaMS'
  controlScheme: 'segaMS'        // forwarded so EJS applies SMS button labels
});

The gotcha: EJS's segaMS system alias resolves to smsplus, a core we don't bundle. So coreName is set to the concrete genesis_plus_gx (which runs the SMS natively), while controlScheme: 'segaMS' is passed through genx-ejs-boot.js so the pad/keyboard still gets the Master System's control layout. stable core, no BIOS.

CRT bezel — it's a console, not a handheld

Unlike the Game Gear's held-up-device bezel, the Master System wears the shared 80s.png wood-cabinet CRT television (on the 70s-bg.png wallpaper, screen keyed transparent) — the same TV bezel the Atari 400/800 bundles use. It sits under the CONSOLE parent menu.

Library

Year-sorted, ten titles: Alex Kidd in Miracle World (1986, the de-facto pack-in), Fantasy Zone, Wonder Boy, Out Run (1987), Phantasy Star (1988, with a battery save that persists to IndexedDB), R-Type, Shinobi, Psycho Fox (1989), Castle of Illusion (1990), and the 8-bit Sonic the Hedgehog (1991). Gamedocs at gamedocs/sms/<key>.html; ROMs are .sms. Five save-state slots via the shared _shared/genx-savestate.js, plus native USB gamepad support.

Related

• Emulator-gambatte-Game-Boy · Emulator-handy-Atari-Lynx — sibling handhelds, same bezel style
• Emulators — index