diff --git a/index.html b/index.html
index bacdc380..73070393 100644
--- a/index.html
+++ b/index.html
@@ -154,6 +154,7 @@ if (window.location.host.endsWith('8bitworkshop.com')) {
                <li><a class="dropdown-item" href="?platform=coleco">ColecoVision</a></li>
                <li><a class="dropdown-item" href="?platform=sms-sg1000-libcv">Sega SG-1000</a></li>
                <li><a class="dropdown-item" href="?platform=sms-sms-libcv">Sega Master System</a></li>
+               <li><a class="dropdown-item" href="?platform=sms-gg-libcv">Sega Game Gear</a></li>
                <li><a class="dropdown-item" href="?platform=atari7800">Atari 7800</a></li>
                <!--
                <li><a class="dropdown-item" href="?platform=vectrex">Vectrex</a></li>
@@ -323,11 +324,17 @@ if (window.location.host.endsWith('8bitworkshop.com')) {
       <span class="control-def"><span class="control-key small">\</span> Select</span>
       <span class="control-def"><span class="control-key small">Enter</span> Start</span>
     </div>
-    <div class="emucontrols-msx emucontrols-coleco emucontrols-sms text-center small control-insns" style="display:none">
+    <div class="emucontrols-msx emucontrols-coleco text-center small control-insns" style="display:none">
       <span class="control-def"><span class="control-key">&#x2190;&#x2191;&#x2193;&#x2192;</span> Joypad</span>
       <span class="control-def"><span class="control-key small">Space</span> Button A</span>
       <span class="control-def"><span class="control-key small">Shift</span> Button B</span>
     </div>
+    <div class="emucontrols-sms text-center small control-insns" style="display:none">
+      <span class="control-def"><span class="control-key">&#x2190;&#x2191;&#x2193;&#x2192;</span> Joypad</span>
+      <span class="control-def"><span class="control-key small">Space</span> Button A</span>
+      <span class="control-def"><span class="control-key small">Shift</span> Button B</span>
+      <span class="control-def"><span class="control-key small">Enter</span> Start</span>
+    </div>
     <div class="emucontrols-vicdual emucontrols-galaxian emucontrols-vector text-center small control-insns" style="display:none">
       <span class="control-def"><span class="control-key">&#x2190;&#x2191;&#x2193;&#x2192;</span> Joystick</span>
       <span class="control-def"><span class="control-key small">Space</span> Button 1</span>
diff --git a/presets/sms-gg-libcv b/presets/sms-gg-libcv
new file mode 120000
index 00000000..3c624673
--- /dev/null
+++ b/presets/sms-gg-libcv
@@ -0,0 +1 @@
+sms-sms-libcv
\ No newline at end of file
diff --git a/presets/sms-sms-libcv/climber.c b/presets/sms-sms-libcv/climber.c
index cfa83cef..ccec9912 100644
--- a/presets/sms-sms-libcv/climber.c
+++ b/presets/sms-sms-libcv/climber.c
@@ -72,21 +72,23 @@ void music_play(byte index) {
 void music_stop() {
 }
 
-/*{pal:222,n:16}*/
-const char PALETTE0[16] = {
-  0x30, 0x38, 0x3E, 0x3F,
-  0x30, 0x38, 0x3E, 0x3F,
-  0x30, 0x38, 0x3E, 0x3F,
-  0x30, 0x38, 0x3E, 0x3F,
+#ifdef __PLATFORM_SMS_GG_LIBCV__
+/*{pal:444,n:32}*/
+const short PALETTE[32] = {
+  0xe00, 0xa60, 0xee3, 0xddd, 0xe00, 0xa60, 0xee3, 0xddd,
+  0xe00, 0xa60, 0xee3, 0xddd, 0xe00, 0xa60, 0xee3, 0xddd,
+  0xe00, 0x111, 0xddd, 0x43e, 0xaaaa, 0xcccc, 0xeeee, 0xffff,
+  0xe00, 0xa60, 0xee3, 0xddd, 0xe00, 0xa60, 0xee3, 0xddd,
 };
-
-/*{pal:222,n:16}*/
-const char PALETTE1[16] = {
-  0x30, 0x00, 0x3F, 0x03,
-  0x30, 0x38, 0x3E, 0x3F,
-  0x30, 0x38, 0x3E, 0x3F,
-  0x30, 0x38, 0x3E, 0x3F,
+#else
+/*{pal:222,n:32}*/
+const char PALETTE[32] = {
+  0x30, 0x38, 0x3E, 0x3F, 0x30, 0x38, 0x3E, 0x3F,
+  0x30, 0x38, 0x3E, 0x3F, 0x30, 0x38, 0x3E, 0x3F,
+  0x30, 0x00, 0x3F, 0x03, 0x30, 0x38, 0x3E, 0x3F,
+  0x30, 0x38, 0x3E, 0x3F, 0x30, 0x38, 0x3E, 0x3F
 };
+#endif
 
 /// METASPRITES
 
@@ -750,7 +752,7 @@ void pal_bright(byte level) {
   byte or = PALBR_OR[level];
   byte and = PALBR_AND[level];
   for (i=0; i<16; i++) {
-    pal[i] = PALETTE0[i] & and | or;
+    pal[i] = PALETTE[i] & and | or;
   }
   cvu_memtocmemcpy(0xc000, pal, 16);
 }
@@ -799,8 +801,7 @@ void play_scene() {
 
 void setup_graphics() {
   cvu_memtovmemcpy(PATTERN, CHR_GENERIC, sizeof(CHR_GENERIC));
-  cvu_memtocmemcpy(0xc000, PALETTE0, 16);
-  cvu_memtocmemcpy(0xc010, PALETTE1, 16);
+  cvu_memtocmemcpy(0xc000, PALETTE, sizeof(PALETTE));
   flip_sprite_patterns(PATTERN+0x80*32, CHR_GENERIC+0xd0*32, 0x30*32);
   cv_set_left_column_blank(true);
 }
diff --git a/presets/sms-sms-libcv/mode4test.c b/presets/sms-sms-libcv/mode4test.c
index 28ec4fa1..1aadc7ad 100644
--- a/presets/sms-sms-libcv/mode4test.c
+++ b/presets/sms-sms-libcv/mode4test.c
@@ -21,17 +21,23 @@ extern uintptr_t font_bitmap_0[];
 #define SPRITE_PATTERNS 0x3800	// 32*32 = 1024 bytes
 #define SPRITES		0x3c00	// 4*32 = 128 bytes
 
-/*{pal:222,n:16}*/
-const char PALETTE0[16] = {
-  0x10, 0x03, 0x0B, 0x0F, 0x13, 0x17, 0x1B, 0x1F,
-  0x22, 0x28, 0x2A, 0x2E, 0x30, 0x37, 0x3B, 0x3F,
-};
-
-/*{pal:222,n:16}*/
-const char PALETTE1[16] = {
+#ifdef __PLATFORM_SMS_GG_LIBCV__
+/*{pal:444,n:32}*/
+const short PALETTE[32] = {
+  0x2221, 0x4441, 0x6661, 0x8888, 0xaaaa, 0xcccc, 0xeeee, 0xffff,
+  0x2222, 0x4444, 0x6666, 0x8888, 0xaaaa, 0xcccc, 0xeeee, 0xffff,
+  0x2222, 0x4444, 0x6666, 0x8888, 0xaaaa, 0xcccc, 0xeeee, 0xffff,
+  0x2222, 0x4444, 0x6666, 0x8888, 0xaaaa, 0xcccc, 0xeeee, 0xffff,
+};
+#else
+/*{pal:222,n:32}*/
+const char PALETTE[32] = {
+  0x10, 0x03, 0x0B, 0x0F, 0x13, 0x17, 0x1B, 0x1F,
+  0x22, 0x28, 0x2A, 0x2E, 0x30, 0x37, 0x3B, 0x3F,
   0x10, 0x03, 0x0B, 0x0F, 0x13, 0x17, 0x1B, 0x1F,
   0x22, 0x28, 0x2A, 0x2E, 0x30, 0x37, 0x3B, 0x3F,
 };
+#endif
 
 /*{w:8,h:8,bpp:1,count:2,brev:1,np:4,pofs:1,sl:4}*/
 const char PATTERNS[64] = {
@@ -79,8 +85,7 @@ void setup_graphics() {
   // copy sprites to pattern table VRAM
   cvu_memtovmemcpy(PATTERN+32, PATTERNS, sizeof(PATTERNS));
   // copy palettes to VRAM
-  cvu_memtocmemcpy(0xc000, PALETTE0, 16);
-  cvu_memtocmemcpy(0xc010, PALETTE1, 16);
+  cvu_memtocmemcpy(0xc000, PALETTE, sizeof(PALETTE));
 }
 
 // image table has two bytes per cell (name + attribute)
diff --git a/src/common/baseplatform.ts b/src/common/baseplatform.ts
index 98ad3cb5..c0bc5a8e 100644
--- a/src/common/baseplatform.ts
+++ b/src/common/baseplatform.ts
@@ -671,7 +671,7 @@ export abstract class Base6809Platform extends BaseZ80Platform {
 
 
 //TODO: how to get stack_end?
-export function dumpStackToString(platform:Platform, mem:Uint8Array|number[], start:number, end:number, sp:number, jsrop:number) : string {
+export function dumpStackToString(platform:Platform, mem:Uint8Array|number[], start:number, end:number, sp:number, jsrop:number, bigendian?:boolean) : string {
   var s = "";
   var nraw = 0;
   //s = dumpRAM(mem.slice(start,start+end+1), start, end-start+1);
@@ -684,6 +684,7 @@ export function dumpStackToString(platform:Platform, mem:Uint8Array|number[], st
     // see if there's a JSR on the stack here
     // TODO: make work with roms and memory maps
     var addr = read(sp) + read(sp+1)*256;
+    if (bigendian) { addr = ((addr & 0xff) << 8) | ((addr & 0xff00) >> 8) }
     var jsrofs = jsrop==0x20 ? -2 : -3; // 6502 vs Z80
     var opcode = read(addr + jsrofs); // might be out of bounds
     if (opcode == jsrop) { // JSR
@@ -968,7 +969,6 @@ export abstract class BaseZ80MachinePlatform<T extends Machine> extends BaseMach
         var start = sp & 0xff00;
         var end = start + 0xff;
         if (sp == 0) sp = 0x10000;
-        console.log(sp,start,end);
         return dumpStackToString(<Platform><any>this, [], start, end, sp, 0xcd);
       }
       default: return isDebuggable(this.machine) && this.machine.getDebugInfo(category, state);
@@ -980,6 +980,35 @@ export abstract class BaseZ80MachinePlatform<T extends Machine> extends BaseMach
 
 }
 
+export abstract class Base6809MachinePlatform<T extends Machine> extends BaseMachinePlatform<T> {
+
+  getToolForFilename    = getToolForFilename_6809;
+
+  getDebugCategories() {
+    if (isDebuggable(this.machine))
+      return this.machine.getDebugCategories();
+    else
+      return ['CPU','Stack'];
+  }
+  getDebugInfo(category:string, state:EmuState) : string {
+    switch (category) {
+      case 'CPU':   return cpuStateToLongString_6809(state.c);
+      case 'Stack': {
+        var sp = (state.c.SP-1) & 0xffff;
+        var start = sp & 0xff00;
+        var end = start + 0xff;
+        if (sp == 0) sp = 0x10000;
+        return dumpStackToString(<Platform><any>this, [], start, end, sp, 0x17, true);
+      }
+      default: return super.getDebugInfo(category, state);
+    }
+  }
+  disassemble(pc:number, read:(addr:number)=>number) : DisasmLine {
+    // TODO: don't create new CPU
+    return Object.create(CPU6809()).disasm(read(pc), read(pc+1), read(pc+2), read(pc+3), read(pc+4), pc);
+  }
+}
+
 ///
 
 class SerialIOVisualizer {
diff --git a/src/common/cpu/6809.ts b/src/common/cpu/6809.ts
index e7ab7200..8bb123e1 100644
--- a/src/common/cpu/6809.ts
+++ b/src/common/cpu/6809.ts
@@ -2456,7 +2456,8 @@ return {
         }
         return f;
     },
-    disasm: disasm
+    disasm: disasm,
+    isStable: function() { return true; }
 };
 
 };
diff --git a/src/common/video/tms9918a.ts b/src/common/video/tms9918a.ts
index 9bee12c4..966d7ca7 100644
--- a/src/common/video/tms9918a.ts
+++ b/src/common/video/tms9918a.ts
@@ -1013,3 +1013,30 @@ export class SMSVDP extends TMS9918A {
     }
 };
 
+export class GameGearVDP extends SMSVDP {
+    cram = new Uint8Array(64); // color RAM
+    cram_latch = 0;
+
+    writeData(value: number) {
+        if (this.writeToCRAM) {
+            //console.log(hex(this.addressRegister), hex(value), hex(this.cram_latch));
+            if (this.addressRegister & 1) { // odd address?
+                let rgb4 = this.cram_latch + (value << 8);
+                let rgba = RGBA((rgb4&15)*17, ((rgb4>>4)&15)*17, ((rgb4>>8)&15)*17);
+                let palindex = this.addressRegister & (this.cram.length-1);
+                this.cram[palindex-1] = this.cram_latch;
+                this.cram[palindex] = value;
+                this.cpalette[palindex >> 1] = rgba;
+                this.prefetchByte = value;
+                this.addressRegister &= this.ramMask;
+                this.redrawRequired = true;
+            } else {
+                this.cram_latch = value; // even address
+            }
+            this.addressRegister++;
+        } else {
+            super.writeData(value);
+        }
+        this.latch = false;
+    }
+}
diff --git a/src/machine/sms.ts b/src/machine/sms.ts
index 9b119816..503a80c7 100644
--- a/src/machine/sms.ts
+++ b/src/machine/sms.ts
@@ -1,11 +1,9 @@
 
-import { Z80, Z80State } from "../common/cpu/ZilogZ80";
-import { BasicScanlineMachine } from "../common/devices";
 import { BaseZ80VDPBasedMachine } from "./vdp_z80";
-import { KeyFlags, newAddressDecoder, padBytes, Keys, makeKeycodeMap, newKeyboardHandler } from "../common/emu";
-import { hex, lzgmini, stringToByteArray } from "../common/util";
-import { TssChannelAdapter, MasterAudio, SN76489_Audio } from "../common/audio";
-import { TMS9918A, SMSVDP } from "../common/video/tms9918a";
+import { newAddressDecoder, padBytes, Keys, makeKeycodeMap } from "../common/emu";
+import { hex } from "../common/util";
+import { MasterAudio, SN76489_Audio } from "../common/audio";
+import { SMSVDP, GameGearVDP } from "../common/video/tms9918a";
 
 // http://www.smspower.org/Development/Index
 // http://www.smspower.org/uploads/Development/sg1000.txt
@@ -28,6 +26,7 @@ var SG1000_KEYCODE_MAP = makeKeycodeMap([
   [Keys.P2_A,     1, 0x4],
   [Keys.P2_B,     1, 0x8],
   [Keys.VK_BACK_SLASH,    1, 0x10], // reset
+  [Keys.VK_ENTER,    1, 0x80], // start/pause
 ]);
 
 export class SG1000 extends BaseZ80VDPBasedMachine {
@@ -61,33 +60,41 @@ export class SG1000 extends BaseZ80VDPBasedMachine {
   setMemoryControl(v:number) { }
   setIOPortControl(v:number) { }
 
+  readIO(addr:number) {
+    switch (addr & 0xc1) {
+      case 0x40: return this.getVCounter();
+      case 0x41: return this.getHCounter();
+      case 0x80: return this.vdp.readData();
+      case 0x81: return this.vdp.readStatus();
+      case 0xc0: return this.inputs[0] ^ 0xff;
+      case 0xc1: return this.inputs[1] ^ 0xff;
+    }
+    return 0;
+  }
+
+  writeIO(addr:number, val:number) {
+    switch (addr & 0xc1) {
+      case 0x00: return this.setMemoryControl(val);
+      case 0x01: return this.setIOPortControl(val);
+      case 0x40:
+      case 0x41: return this.psg.setData(val);
+      case 0x80: return this.vdp.writeData(val);
+      case 0x81: return this.vdp.writeAddress(val);
+    }
+  }
+
   newIOBus() {
     return {
       read: (addr:number) => {
         addr &= 0xff;
         //console.log('IO read', hex(addr,4));
-        switch (addr & 0xc1) {
-          case 0x40: return this.getVCounter();
-          case 0x41: return this.getHCounter();
-          case 0x80: return this.vdp.readData();
-          case 0x81: return this.vdp.readStatus();
-          case 0xc0: return this.inputs[0] ^ 0xff;
-          case 0xc1: return this.inputs[1] ^ 0xff;
-        }
-        return 0;
+        return this.readIO(addr);
       },
       write: (addr:number, val:number) => {
         addr &= 0xff;
         val &= 0xff;
         //console.log('IO write', hex(addr,4), hex(val,2));
-        switch (addr & 0xc1) {
-          case 0x00: return this.setMemoryControl(val);
-          case 0x01: return this.setIOPortControl(val);
-          case 0x40:
-          case 0x41: return this.psg.setData(val);
-          case 0x80: return this.vdp.writeData(val);
-          case 0x81: return this.vdp.writeAddress(val);
-        }
+        this.writeIO(addr, val);
       }
     };
   }
@@ -223,3 +230,17 @@ export class SMS extends SG1000 {
   }
 }
 
+// https://segaretro.org/images/1/16/Sega_Game_Gear_Hardware_Reference_Manual.pdf
+export class GameGear extends SMS {
+  newVDP(frameData, cru, flicker) {
+    return new GameGearVDP(frameData, cru, flicker);
+  }
+  readIO(addr:number) {
+    switch (addr & 0xc1) {
+      case 0x00: return (~this.inputs[1] & 0x80) | 0x40;
+      case 0xdc: return this.inputs[0] ^ 0xff;
+      case 0xdd: return this.inputs[1] ^ 0xff;
+    }
+    return super.readIO(addr);
+  }
+}
diff --git a/src/machine/williams.ts b/src/machine/williams.ts
new file mode 100644
index 00000000..e53ea202
--- /dev/null
+++ b/src/machine/williams.ts
@@ -0,0 +1,398 @@
+import { hex } from "chroma-js";
+import { MasterAudio, WorkerSoundChannel } from "../common/audio";
+import { MemoryBus } from "../common/baseplatform";
+import { CPU6809 } from "../common/cpu/6809";
+import { BasicScanlineMachine } from "../common/devices";
+import { Keys, makeKeycodeMap, newAddressDecoder, newKeyboardHandler, padBytes } from "../common/emu";
+
+const INITIAL_WATCHDOG = 8;
+const SCREEN_HEIGHT = 304;
+
+export class WilliamsMachine extends BasicScanlineMachine {
+
+    xtal = 12000000;
+    cpuFrequency = this.xtal / 3 / 4;
+    //cpuCyclesPerLine = 64;
+    cpuCyclesPerLine = 54; // TODO: becuse we swapped width and height
+    canvasWidth = 256;
+    numTotalScanlines = 304;
+    numVisibleScanlines = 304;
+    defaultROMSize = 0xc000;
+    rotate = -90;
+    sampleRate = 1;
+
+    cpu;
+    membus: MemoryBus;
+    ram = new Uint8Array(0xc000);
+    nvram = new Uint8Array(0x400);
+    rom = new Uint8Array(0xc000);
+    portsel = 0;
+    banksel = 0;
+    watchdog_counter = 0;
+    watchdog_enabled = false;
+    pia6821 = new Uint8Array(8);
+    blitregs = new Uint8Array(8);
+
+    worker: Worker;
+    master;
+    audioadapter;
+
+    palette = new Uint32Array(16);
+    screenNeedsRefresh = false;
+    displayPCs;
+    cpuScale = 1;
+    waitCycles = 0;
+
+    constructor(public isDefender: boolean) {
+        super();
+        this.palette.fill(0xff000000);
+        this.initBus(isDefender);
+        this.initInputs(isDefender);
+        this.initAudio();
+        this.initCPU();
+    }
+
+    initInputs(isDefender: boolean) {
+        var DEFENDER_KEYCODE_MAP = makeKeycodeMap([
+            [Keys.A, 4, 0x1],
+            [Keys.RIGHT, 4, 0x2],
+            [Keys.B, 4, 0x4],
+            [Keys.VK_X, 4, 0x8],
+            [Keys.P2_START, 4, 0x10],
+            [Keys.START, 4, 0x20],
+            [Keys.LEFT, 4, 0x40],
+            [Keys.DOWN, 4, 0x80],
+            [Keys.UP, 6, 0x1],
+            [Keys.SELECT, 0, 0x4],
+            [Keys.VK_7, 0, 0x1],
+            [Keys.VK_8, 0, 0x2],
+            [Keys.VK_9, 0, 0x8],
+        ]);
+
+        var ROBOTRON_KEYCODE_MAP = makeKeycodeMap([
+            [Keys.P2_UP, 0, 0x1],
+            [Keys.P2_DOWN, 0, 0x2],
+            [Keys.P2_LEFT, 0, 0x4],
+            [Keys.P2_RIGHT, 0, 0x8],
+            [Keys.START, 0, 0x10],
+            [Keys.P2_START, 0, 0x20],
+            [Keys.UP, 0, 0x40],
+            [Keys.DOWN, 0, 0x80],
+            [Keys.LEFT, 2, 0x1],
+            [Keys.RIGHT, 2, 0x2],
+            [Keys.VK_7, 4, 0x1],
+            [Keys.VK_8, 4, 0x2],
+            [Keys.VK_6, 4, 0x4],
+            [Keys.VK_9, 4, 0x8],
+            [Keys.SELECT, 4, 0x10],
+        ]);
+
+        var KEYCODE_MAP = isDefender ? DEFENDER_KEYCODE_MAP : ROBOTRON_KEYCODE_MAP;
+        //this.inputs.set(this.pia6821);
+        this.handler = newKeyboardHandler(this.pia6821, KEYCODE_MAP);
+    }
+
+    initBus(isDefender: boolean) {
+        var ioread_defender = newAddressDecoder([
+            [0x400, 0x5ff, 0x1ff, (a) => { return this.nvram[a]; }],
+            [0x800, 0x800, 0, (a) => { return this.scanline; }],
+            [0xc00, 0xc07, 0x7, (a) => { return this.pia6821[a]; }],
+            [0x0, 0xfff, 0, (a) => { /*console.log('ioread',hex(a));*/ }],
+        ]);
+
+        var iowrite_defender = newAddressDecoder([
+            [0x0, 0xf, 0xf, this.setPalette.bind(this)],
+            [0x3fc, 0x3ff, 0, (a, v) => { if (v == 0x38) this.watchdog_counter = INITIAL_WATCHDOG; this.watchdog_enabled = true; }],
+            [0x400, 0x5ff, 0x1ff, (a, v) => { this.nvram[a] = v; }],
+            [0xc02, 0xc02, 0x1, (a, v) => { if (this.worker) this.worker.postMessage({ command: v & 0x3f }); }],
+            [0xc00, 0xc07, 0x7, (a, v) => { this.pia6821[a] = v; }],
+            [0x0, 0xfff, 0, (a, v) => { /* console.log('iowrite', hex(a), hex(v)); */ }],
+        ]);
+
+        var memread_defender = newAddressDecoder([
+            [0x0000, 0xbfff, 0xffff, (a) => { return this.ram[a]; }],
+            [0xc000, 0xcfff, 0x0fff, (a) => {
+                switch (this.banksel) {
+                    case 0: return ioread_defender(a);
+                    case 1: return this.rom[a + 0x3000];
+                    case 2: return this.rom[a + 0x4000];
+                    case 3: return this.rom[a + 0x5000];
+                    case 7: return this.rom[a + 0x6000];
+                    default: return 0; // TODO: error light
+                }
+            }],
+            [0xd000, 0xffff, 0xffff, (a) => { return this.rom ? this.rom[a - 0xd000] : 0; }],
+        ]);
+
+        var memwrite_defender = newAddressDecoder([
+            [0x0000, 0x97ff, 0, this.write_display_byte.bind(this)],
+            [0x9800, 0xbfff, 0, (a, v) => { this.ram[a] = v; }],
+            [0xc000, 0xcfff, 0x0fff, iowrite_defender.bind(this)],
+            [0xd000, 0xdfff, 0, (a, v) => { this.banksel = v & 0x7; }],
+            [0, 0xffff, 0, (a, v) => { /* console.log(hex(a), hex(v)); */ }],
+        ]);
+
+        // Robotron, Joust, Bubbles, Stargate
+
+        var ioread_robotron = newAddressDecoder([
+            [0x804, 0x807, 0x3, (a) => { return this.pia6821[a]; }],
+            [0x80c, 0x80f, 0x3, (a) => { return this.pia6821[a + 4]; }],
+            [0xb00, 0xbff, 0, (a) => { return this.scanline; }],
+            [0xc00, 0xfff, 0x3ff, (a) => { return this.nvram[a]; }],
+            [0x0, 0xfff, 0, (a) => { /* console.log('ioread',hex(a)); */ }],
+        ]);
+
+        var iowrite_robotron = newAddressDecoder([
+            [0x0, 0xf, 0xf, this.setPalette.bind(this)],
+            [0x80c, 0x80c, 0xf, (a, v) => { if (this.worker) this.worker.postMessage({ command: v }); }],
+            //[0x804, 0x807, 0x3,   function(a,v) { console.log('iowrite',a); }], // TODO: sound
+            //[0x80c, 0x80f, 0x3,   function(a,v) { console.log('iowrite',a+4); }], // TODO: sound
+            [0x900, 0x9ff, 0, (a, v) => { this.banksel = v & 0x1; }],
+            [0xa00, 0xa07, 0x7, this.setBlitter.bind(this)],
+            [0xbff, 0xbff, 0, (a, v) => { if (v == 0x39) { this.watchdog_counter = INITIAL_WATCHDOG; this.watchdog_enabled = true; } }],
+            [0xc00, 0xfff, 0x3ff, (a, v) => { this.nvram[a] = v; }],
+            //[0x0,   0xfff, 0,     function(a,v) { console.log('iowrite',hex(a),hex(v)); }],
+        ]);
+
+        var memread_robotron = newAddressDecoder([
+            [0x0000, 0x8fff, 0xffff, (a) => { return this.banksel ? this.rom[a] : this.ram[a]; }],
+            [0x9000, 0xbfff, 0xffff, (a) => { return this.ram[a]; }],
+            [0xc000, 0xcfff, 0x0fff, ioread_robotron],
+            [0xd000, 0xffff, 0xffff, (a) => { return this.rom ? this.rom[a - 0x4000] : 0; }],
+        ]);
+
+        var memwrite_robotron = newAddressDecoder([
+            [0x0000, 0x97ff, 0, this.write_display_byte.bind(this)],
+            [0x9800, 0xbfff, 0, (a, v) => { this.ram[a] = v; }],
+            [0xc000, 0xcfff, 0x0fff, iowrite_robotron.bind(this)],
+            //[0x0000, 0xffff, 0,      function(a,v) { console.log(hex(a), hex(v)); }],
+        ]);
+
+        var memread_williams = isDefender ? memread_defender : memread_robotron;
+        var memwrite_williams = isDefender ? memwrite_defender : memwrite_robotron;
+        this.membus = {
+            read: memread_williams,
+            write: memwrite_williams,
+        };
+        this.membus = this.probeMemoryBus(this.membus);
+        this.readAddress = this.membus.read;
+    }
+
+    initAudio() {
+        this.master = new MasterAudio();
+        this.worker = new Worker("./src/common/audio/z80worker.js");
+        let workerchannel = new WorkerSoundChannel(this.worker);
+        this.master.master.addChannel(workerchannel);
+    }
+
+    initCPU() {
+        this.rom = new Uint8Array(this.defaultROMSize);
+        this.cpu = this.newCPU(this.membus);
+        //this.connectCPUMemoryBus(this);
+    }
+
+    newCPU(membus: MemoryBus) {
+        var cpu = Object.create(CPU6809());
+        cpu.init(membus.write, membus.read, 0);
+        return cpu;
+    }
+
+    readAddress;
+
+    // d1d6 ldu $11 / beq $d1ed
+
+    setPalette(a, v) {
+        // RRRGGGBB
+        var color = 0xff000000 | ((v & 7) << 5) | (((v >> 3) & 7) << 13) | (((v >> 6) << 22));
+        if (color != this.palette[a]) {
+            this.palette[a] = color;
+            this.screenNeedsRefresh = true;
+        }
+    }
+
+    write_display_byte(a: number, v: number) {
+        this.ram[a] = v;
+        this.drawDisplayByte(a, v);
+        if (this.displayPCs) this.displayPCs[a] = this.cpu.getPC(); // save program counter
+    }
+
+    drawDisplayByte(a, v) {
+        var ofs = ((a & 0xff00) << 1) | ((a & 0xff) ^ 0xff);
+        this.pixels[ofs] = this.palette[v >> 4];
+        this.pixels[ofs + 256] = this.palette[v & 0xf];
+    }
+
+    setBlitter(a, v) {
+        if (a) {
+            this.blitregs[a] = v;
+        } else {
+            var cycles = this.doBlit(v);
+            this.waitCycles -= cycles * this.cpuScale; // wait CPU cycles
+        }
+    }
+
+    doBlit(flags) {
+        //console.log(hex(flags), blitregs);
+        flags &= 0xff;
+        var offs = SCREEN_HEIGHT - this.blitregs[7];
+        var sstart = (this.blitregs[2] << 8) + this.blitregs[3];
+        var dstart = (this.blitregs[4] << 8) + this.blitregs[5];
+        var w = this.blitregs[6] ^ 4; // blitter bug fix
+        var h = this.blitregs[7] ^ 4;
+        if (w == 0) w++;
+        if (h == 0) h++;
+        if (h == 255) h++;
+        var sxinc = (flags & 0x1) ? 256 : 1;
+        var syinc = (flags & 0x1) ? 1 : w;
+        var dxinc = (flags & 0x2) ? 256 : 1;
+        var dyinc = (flags & 0x2) ? 1 : w;
+        var pixdata = 0;
+        for (var y = 0; y < h; y++) {
+            var source = sstart & 0xffff;
+            var dest = dstart & 0xffff;
+            for (var x = 0; x < w; x++) {
+                var data = this.membus.read(source);
+                if (flags & 0x20) {
+                    pixdata = (pixdata << 8) | data;
+                    this.blit_pixel(dest, (pixdata >> 4) & 0xff, flags);
+                } else {
+                    this.blit_pixel(dest, data, flags);
+                }
+                source += sxinc;
+                source &= 0xffff;
+                dest += dxinc;
+                dest &= 0xffff;
+            }
+            if (flags & 0x2)
+                dstart = (dstart & 0xff00) | ((dstart + dyinc) & 0xff);
+            else
+                dstart += dyinc;
+            if (flags & 0x1)
+                sstart = (sstart & 0xff00) | ((sstart + syinc) & 0xff);
+            else
+                sstart += syinc;
+        }
+        return w * h * (2 + ((flags & 0x4) >> 2)); // # of memory accesses
+    }
+
+    blit_pixel(dstaddr, srcdata, flags) {
+        var curpix = dstaddr < 0xc000 ? this.ram[dstaddr] : this.membus.read(dstaddr);
+        var solid = this.blitregs[1];
+        var keepmask = 0xff;          //what part of original dst byte should be kept, based on NO_EVEN and NO_ODD flags
+        //even pixel (D7-D4)
+        if ((flags & 0x8) && !(srcdata & 0xf0)) {   //FG only and src even pixel=0
+            if (flags & 0x80) keepmask &= 0x0f; // no even
+        } else {
+            if (!(flags & 0x80)) keepmask &= 0x0f; // not no even
+        }
+        //odd pixel (D3-D0)
+        if ((flags & 0x8) && !(srcdata & 0x0f)) {   //FG only and src odd pixel=0
+            if (flags & 0x40) keepmask &= 0xf0; // no odd
+        } else {
+            if (!(flags & 0x40)) keepmask &= 0xf0; // not no odd
+        }
+        curpix &= keepmask;
+        if (flags & 0x10) // solid bit
+            curpix |= (solid & ~keepmask);
+        else
+            curpix |= (srcdata & ~keepmask);
+        if (dstaddr < 0x9800) // can cause recursion otherwise
+            this.membus.write(dstaddr, curpix);
+    }
+
+    startScanline(): void {
+        this.audio && this.audioadapter && this.audioadapter.generate(this.audio);
+        // TODO: line-by-line
+        if (this.screenNeedsRefresh && this.scanline == 0) {
+            for (var i = 0; i < 0x9800; i++)
+                this.drawDisplayByte(i, this.ram[i]);
+            this.screenNeedsRefresh = false;
+        }
+        if (this.scanline == 0 && this.watchdog_enabled && this.watchdog_counter-- <= 0) {
+            console.log("WATCHDOG FIRED, PC =", this.cpu.getPC().toString(16)); // TODO: alert on video
+            // TODO: this.breakpointHit(cpu.T());
+            this.reset();
+        }
+    }
+    drawScanline(): void {
+        // interrupts happen every 1/4 of the screen
+        let sl = this.scanline;
+        if (sl == 0 || sl == 0x3c || sl == 0xbc || sl == 0xfc) {
+            if (!this.isDefender || this.pia6821[7] == 0x3c) { // TODO?
+                if (this.cpu.interrupt)
+                    this.cpu.interrupt();
+                if (this.cpu.requestInterrupt)
+                    this.cpu.requestInterrupt();
+            }
+        }
+    }
+    read(a: number): number {
+        return this.membus.read(a);
+    }
+    write(a: number, v: number): void {
+        this.membus.write(a, v);
+    }
+    readConst(a: number): number {
+        if (a >= 0xc000 && a <= 0xcbff) return 0xff;
+        else return this.membus.read(a); // TODO
+    }
+    reset() {
+        super.reset();
+        this.watchdog_counter = INITIAL_WATCHDOG;
+        this.watchdog_enabled = false;
+        this.banksel = 1;
+    }
+
+    loadSoundROM(data) {
+        console.log("loading sound ROM " + data.length + " bytes");
+        var soundrom = padBytes(data, 0x4000);
+        this.worker.postMessage({ rom: soundrom });
+    }
+
+    loadROM(data) {
+        if (data.length > 2) {
+            if (this.isDefender) {
+                this.loadSoundROM(data.slice(0x6800));
+                data = this.rom.slice(0, 0x6800);
+            } else if (data.length > 0xc000) {
+                this.loadSoundROM(data.slice(0xc000));
+                data = this.rom.slice(0, 0xc000);
+            } else if (data.length > 0x9000 && data[0x9000]) {
+                this.loadSoundROM(data.slice(0x9000));
+            }
+            data = padBytes(data, 0xc000);
+        }
+        super.loadROM(data);
+    }
+
+    loadState(state) {
+        this.cpu.loadState(state.c);
+        this.ram.set(state.ram);
+        this.nvram.set(state.nvram);
+        this.pia6821.set(state.inputs);
+        this.blitregs.set(state.blt);
+        this.watchdog_counter = state.wdc;
+        this.banksel = state.bs;
+        this.portsel = state.ps;
+    }
+    saveState() {
+        return {
+            c: this.cpu.saveState(),
+            ram: this.ram.slice(0),
+            nvram: this.nvram.slice(0),
+            inputs: this.pia6821.slice(0),
+            blt: this.blitregs.slice(0),
+            wdc: this.watchdog_counter,
+            bs: this.banksel,
+            ps: this.portsel,
+        };
+    }
+    loadControlsState(state) {
+        this.pia6821.set(state.inputs);
+    }
+    saveControlsState() {
+        return {
+            inputs: this.pia6821.slice(0),
+        };
+    }
+}
diff --git a/src/platform/sms.ts b/src/platform/sms.ts
index 5248346c..5dcf5b27 100644
--- a/src/platform/sms.ts
+++ b/src/platform/sms.ts
@@ -1,5 +1,5 @@
 
-import { SG1000, SMS } from "../machine/sms";
+import { GameGear, SG1000, SMS } from "../machine/sms";
 import { Platform, BaseZ80MachinePlatform } from "../common/baseplatform";
 import { PLATFORMS } from "../common/emu";
 
@@ -46,7 +46,17 @@ class SMSPlatform extends BaseZ80MachinePlatform<SMS> implements Platform {
   readVRAMAddress(a)    { return this.machine.readVRAMAddress(a); }
 }
 
+class GameGearPlatform extends BaseZ80MachinePlatform<GameGear> implements Platform {
+
+  newMachine()          { return new GameGear(); }
+  getPresets()          { return SMS_PRESETS; }
+  getDefaultExtension() { return ".c"; };
+  readAddress(a)        { return this.machine.read(a); }
+  readVRAMAddress(a)    { return this.machine.readVRAMAddress(a); }
+}
+
 ///
 
 PLATFORMS['sms-sg1000-libcv'] = SG1000Platform;
 PLATFORMS['sms-sms-libcv'] = SMSPlatform;
+PLATFORMS['sms-gg-libcv'] = GameGearPlatform;
diff --git a/src/platform/williams.ts b/src/platform/williams.ts
index a760d6e4..0d478eef 100644
--- a/src/platform/williams.ts
+++ b/src/platform/williams.ts
@@ -1,8 +1,11 @@
 
-import { Platform, BaseZ80Platform, Base6809Platform } from "../common/baseplatform";
+import { Platform, BaseZ80Platform, Base6809Platform, Base6809MachinePlatform } from "../common/baseplatform";
 import { PLATFORMS, RAM, newAddressDecoder, padBytes, noise, setKeyboardFromMap, AnimationTimer, RasterVideo, Keys, makeKeycodeMap } from "../common/emu";
 import { hex } from "../common/util";
 import { MasterAudio, WorkerSoundChannel } from "../common/audio";
+import { WilliamsMachine } from "../machine/williams";
+
+// https://www.arcade-museum.com/manuals-videogames/D/Defender.pdf
 
 var WILLIAMS_PRESETS = [
   { id: 'gfxtest.c', name: 'Graphics Test' },
@@ -469,7 +472,23 @@ var WilliamsDefenderPlatform = function(mainElement, options) {
 ] } };
 }
 
-PLATFORMS['williams'] = Williams6809Platform;
+class NewWilliamsPlatform extends Base6809MachinePlatform<WilliamsMachine> implements Platform {
+
+  newMachine()          { return new WilliamsMachine(false); }
+  getPresets()          { return WILLIAMS_PRESETS; }
+  getDefaultExtension() { return ".c"; };
+  readAddress(a)        { return this.machine.readConst(a); }
+  getMemoryMap() { 
+    return { main:[
+      {name:'Video RAM',start:0x0000,size:0xc000,type:'ram'},
+      {name:'I/O Registers',start:0xc000,size:0xc00,type:'io'},
+      {name:'NVRAM',start:0xcc00,size:0x400,type:'ram'},
+      {name:'ROM',start:0xd000,size:0x3000,type:'rom'},
+    ] } };
+}
+
+PLATFORMS['williams'] = NewWilliamsPlatform;
+PLATFORMS['williams.old'] = Williams6809Platform;
 PLATFORMS['williams-defender'] = WilliamsDefenderPlatform;
 PLATFORMS['williams-z80'] = WilliamsZ80Platform;
 
diff --git a/src/worker/workermain.ts b/src/worker/workermain.ts
index 632c094d..1706d618 100644
--- a/src/worker/workermain.ts
+++ b/src/worker/workermain.ts
@@ -388,6 +388,7 @@ var PLATFORM_PARAMS = {
 };
 
 PLATFORM_PARAMS['sms-sms-libcv'] = PLATFORM_PARAMS['sms-sg1000-libcv'];
+PLATFORM_PARAMS['sms-gg-libcv'] = PLATFORM_PARAMS['sms-sms-libcv'];
 
 var _t1;
 export function starttime() { _t1 = new Date(); }
@@ -1060,6 +1061,8 @@ export function preprocessMCPP(step:BuildStep, filesys:string) {
   if (step.mainfile) {
     args.unshift.apply(args, ["-D", "__MAIN__"]);
   }
+  let platform_def = (platform.toUpperCase() as any).replaceAll(/[^a-zA-Z0-9]/g,'_');
+  args.unshift.apply(args, ["-D", `__PLATFORM_${platform_def}__`]);
   if (params.extra_preproc_args) {
     args.push.apply(args, params.extra_preproc_args);
   }