diff --git a/presets/coleco/common.c b/presets/coleco/common.c
index 5c82fb4e..41877c4c 100644
--- a/presets/coleco/common.c
+++ b/presets/coleco/common.c
@@ -1,4 +1,3 @@
-
 #include <stdlib.h>
 #include <string.h>
 #include <cv.h>
@@ -6,6 +5,11 @@
 
 #include "common.h"
 
+/* Unified common code for libcv platforms (coleco, msx-libcv, sms-sg1000-libcv,
+   sms-sms-libcv, sms-gg-libcv). Differences are selected by the CV_CV / CV_MSX /
+   CV_SMS defines (set by the platform) plus CV_MODE4 for SMS/GG mode-4 builds.
+   Keep this file identical across those preset directories. */
+
 volatile uint_fast8_t vint_counter;
 
 void vint_handler(void) {
@@ -22,16 +26,20 @@ byte reverse_bits(byte n) {
 void flip_sprite_patterns(word dest, const byte* patterns, word len) {
   word i;
   for (i=0; i<len; i++) {
+#if defined(CV_MODE4)
+    cvu_voutb(reverse_bits(*patterns++), dest++);
+#else
     cvu_voutb(reverse_bits(*patterns++), dest++ ^ 16); // swap left/right chars
+#endif
   }
 }
 
 void clrscr() {
-  cvu_vmemset(IMAGE, 0, COLS*ROWS);
+  cvu_vmemset(IMAGE, 0, ROWSTRIDE*ROWS);
 }
 
 word getimageaddr(byte x, byte y) {
-  return IMAGE + y*COLS + x;
+  return IMAGE + y*ROWSTRIDE + x;
 }
 
 byte getcharxy(byte x, byte y) {
@@ -48,6 +56,21 @@ void putstringxy(byte x, byte y, const char* string) {
   }
 }
 
+#if defined(CV_MODE4)
+// mode 4 aliases (same functions, shorter names)
+byte getchar(byte x, byte y) {
+  return getcharxy(x, y);
+}
+
+void putchar(byte x, byte y, byte attr) {
+  putcharxy(x, y, attr);
+}
+
+void putstring(byte x, byte y, const char* string) {
+  putstringxy(x, y, string);
+}
+#endif
+
 void delay(byte i) {
   while (i--) {
     wait_vsync();
@@ -103,11 +126,25 @@ __endasm;
 
 void vdp_setup() {
   cv_set_screen_active(false);
+#if defined(CV_MODE4)
+  cv_set_screen_mode(CV_SCREENMODE_4_224);
+  cv_set_character_pattern_t(PATTERN | 0x3000);
+  cv_set_image_table(IMAGE | 0x400);
+//  cv_set_color_table(COLOR | 0xfff);
+//  cv_set_sprite_pattern_table(SPRITE_PATTERNS | 0x1800);
+#else
   cv_set_screen_mode(CV_SCREENMODE_STANDARD);
   cv_set_image_table(IMAGE);
   cv_set_character_pattern_t(PATTERN);
   cv_set_color_table(COLOR);
   cv_set_sprite_pattern_table(SPRITE_PATTERNS);
+  // load the default font and set a visible default color (white on black).
+  // the MSX startup code does this for us, but the ColecoVision and
+  // SG-1000 leave VRAM zeroed -- without this, putstringxy() output is
+  // invisible (blank pattern table) and the color table is transparent.
+  copy_default_character_set();
+  cvu_vmemset(COLOR, 0xf1, 32);
+#endif
   cv_set_sprite_attribute_table(SPRITES);
   cv_set_sprite_big(true);
 }
@@ -124,6 +161,7 @@ void set_shifted_pattern(const byte* src, word dest, byte shift) {
   }
 }
 
+#if !defined(CV_MODE4)
 void copy_default_character_set() {
 #ifdef CV_MSX
   static byte __at(0xf91f) CGPNT;
@@ -133,3 +171,4 @@ void copy_default_character_set() {
   cvu_memtovmemcpy(PATTERN, (void *)(font_bitmap_0 - '0'*8), 256*8);
 #endif
 }
+#endif
diff --git a/presets/coleco/common.h b/presets/coleco/common.h
index 01ace8ba..12edfc74 100644
--- a/presets/coleco/common.h
+++ b/presets/coleco/common.h
@@ -2,27 +2,52 @@
 #ifndef _CV_COMMON_H
 #define _CV_COMMON_H
 
+/* Unified common header for libcv platforms.
+   The build defines one of: CV_CV (ColecoVision), CV_MSX, CV_SMS (SG-1000/SMS/Game Gear).
+   CV_MODE4 is defined for SMS/GG mode-4 builds (2 bytes per tile, 32x28).
+   Keep this file identical across the coleco, msx-libcv, sms-sg1000-libcv,
+   sms-sms-libcv and sms-gg-libcv preset directories. */
+
+#if defined(CV_MODE4)
+/* VRAM map (mode 4)
+   0x0000 - 0x3fff character pattern table
+   0x3000 - 0x36ff image table
+   0x3c00 - 0x3fff sprite attribute table */
+#define PATTERN		((const cv_vmemp)0x0000)
+#define IMAGE		((const cv_vmemp)0x3000)
+#define SPRITES		((const cv_vmemp)0x3c00)
+#else
 /* VRAM map
    0x0000 - 0x17ff character pattern table
    0x1800 - 0x1aff image table
    0x2000 - 0x37ff color table
    0x3800 - 0x3bff sprite pattern table
-   0x3c00 - 0x3fff sprite attribute table
-*/
-
+   0x3c00 - 0x3fff sprite attribute table */
 #define PATTERN		((const cv_vmemp)0x0000)
 #define IMAGE		((const cv_vmemp)0x1800)
 #define COLOR		((const cv_vmemp)0x2000)
 #define SPRITE_PATTERNS ((const cv_vmemp)0x3800)
 #define SPRITES		((const cv_vmemp)0x3c00)
+#endif
 
 #ifndef COLS
 #define COLS 32
 #endif
 
 #ifndef ROWS
+#if defined(CV_MODE4)
+#define ROWS 28
+#else
 #define ROWS 24
 #endif
+#endif
+
+/* bytes per row in the name table (mode 4 has a color byte per tile) */
+#if defined(CV_MODE4)
+#define ROWSTRIDE (COLS*2)
+#else
+#define ROWSTRIDE COLS
+#endif
 
 typedef unsigned char byte;
 typedef signed char sbyte;
@@ -42,8 +67,12 @@ extern char font_bitmap_0[];
 #define COLOR_FG(fg) (((fg)<<4))
 
 #ifndef LOCHAR
+#if defined(CV_MODE4)
+#define LOCHAR 0x20
+#else
 #define LOCHAR 0x0
 #endif
+#endif
 
 #define CHAR(ch) (ch-LOCHAR)
 
@@ -64,6 +93,12 @@ extern word getimageaddr(byte x, byte y);
 extern byte getcharxy(byte x, byte y);
 extern void putcharxy(byte x, byte y, byte attr);
 extern void putstringxy(byte x, byte y, const char* string);
+#if defined(CV_MODE4)
+// mode 4 aliases (same functions, shorter names)
+extern byte getchar(byte x, byte y);
+extern void putchar(byte x, byte y, byte attr);
+extern void putstring(byte x, byte y, const char* string);
+#endif
 extern void delay(byte i);
 extern byte rndint(byte a, byte b);
 
@@ -77,6 +112,8 @@ extern word bcd_add(word a, word b);
 extern void vdp_setup();
 extern void set_shifted_pattern(const byte* src, word dest, byte shift);
 
+#if !defined(CV_MODE4)
 extern void copy_default_character_set();
+#endif
 
 #endif
diff --git a/presets/sms-sms-libcv/common.c b/presets/sms-sms-libcv/common.c
deleted file mode 100644
index 50e3ee01..00000000
--- a/presets/sms-sms-libcv/common.c
+++ /dev/null
@@ -1,125 +0,0 @@
-
-#include <stdlib.h>
-#include <string.h>
-#include <cv.h>
-#include <cvu.h>
-
-#include "common.h"
-
-volatile uint_fast8_t vint_counter;
-
-void vint_handler(void) {
-  vint_counter++;
-}
-
-const unsigned char reverse_lookup[16] = {
-0x0, 0x8, 0x4, 0xc, 0x2, 0xa, 0x6, 0xe, 0x1, 0x9, 0x5, 0xd, 0x3, 0xb, 0x7, 0xf, };
-
-byte reverse_bits(byte n) {
-  return (reverse_lookup[n&0b1111] << 4) | reverse_lookup[n>>4];
-}
-
-void flip_sprite_patterns(word dest, const byte* patterns, word len) {
-  word i;
-  for (i=0; i<len; i++) {
-    cvu_voutb(reverse_bits(*patterns++), dest++); // swap left/right chars
-  }
-}
-
-void clrscr() {
-  cvu_vmemset(IMAGE, 0, COLS*2*ROWS);
-}
-
-word getimageaddr(byte x, byte y) {
-  return IMAGE + y*COLS*2 + x;
-}
-
-byte getchar(byte x, byte y) {
-  return cvu_vinb(getimageaddr(x,y));
-}
-
-void putchar(byte x, byte y, byte attr) {
-  cvu_voutb(attr, getimageaddr(x,y));
-}
-
-void putstring(byte x, byte y, const char* string) {
-  while (*string) {
-    putchar(x++, y, CHAR(*string++));
-  }
-}
-
-void delay(byte i) {
-  while (i--) {
-    wait_vsync();
-  }
-}
-
-byte rndint(byte a, byte b) {
-  return ((byte)rand() % (b-a+1)) + a;
-}
-
-void memset_safe(void* _dest, char ch, word size) {
-  byte* dest = _dest;
-  while (size--) {
-    *dest++ = ch;
-  }
-}
-
-char in_rect(byte x, byte y, byte x0, byte y0, byte w, byte h) {
-  return ((byte)(x-x0) < w && (byte)(y-y0) < h); // unsigned
-}
-
-void draw_bcd_word(byte x, byte y, word bcd) {
-  byte j;
-  x += 3;
-  for (j=0; j<4; j++) {
-    putchar(x, y, CHAR('0'+(bcd&0xf)));
-    x--;
-    bcd >>= 4;
-  }
-}
-
-// add two 16-bit BCD values
-word bcd_add(word a, word b) __naked {
-  a; b; // to avoid warning
-__asm
- 	push	ix
- 	ld	ix,#0
-	add	ix,sp
- 	ld	a,4 (ix)
- 	add	a, 6 (ix)
-	daa
-	ld	c,a
- 	ld	a,5 (ix)
- 	adc	a, 7 (ix)
-	daa
- 	ld	b,a
- 	ld	l, c
- 	ld	h, b
-	pop	ix
- 	ret
-__endasm;
-}
-
-void vdp_setup() {
-  cv_set_screen_active(false);
-  cv_set_screen_mode(CV_SCREENMODE_4_224);
-  cv_set_character_pattern_t(PATTERN | 0x3000);
-  cv_set_image_table(IMAGE | 0x400);
-//  cv_set_color_table(COLOR | 0xfff);
-//  cv_set_sprite_pattern_table(SPRITE_PATTERNS | 0x1800);
-  cv_set_sprite_attribute_table(SPRITES);
-  cv_set_sprite_big(true);
-}
-
-void set_shifted_pattern(const byte* src, word dest, byte shift) {
-  byte y;
-  for (y=0; y<8; y++) {
-    byte a = src[y+8];
-    byte b = src[y];
-    cvu_voutb(a>>shift, dest);
-    cvu_voutb(b>>shift | a<<(8-shift), dest+8);
-    cvu_voutb(b<<(8-shift), dest+16);
-    dest++;
-  }
-}
diff --git a/presets/sms-sms-libcv/common.c b/presets/sms-sms-libcv/common.c
new file mode 120000
index 00000000..ce249516
--- /dev/null
+++ b/presets/sms-sms-libcv/common.c
@@ -0,0 +1 @@
+../coleco/common.c
\ No newline at end of file
diff --git a/presets/sms-sms-libcv/common.h b/presets/sms-sms-libcv/common.h
deleted file mode 100644
index c87ad62f..00000000
--- a/presets/sms-sms-libcv/common.h
+++ /dev/null
@@ -1,68 +0,0 @@
-
-#ifndef _CV_COMMON_H
-#define _CV_COMMON_H
-
-/* VRAM map
-   0x0000 - 0x3fff character pattern table
-   0x3000 - 0x36ff image table
-   0x3c00 - 0x3fff sprite attribute table
-*/
-
-#define PATTERN		((const cv_vmemp)0x0000)
-#define IMAGE		((const cv_vmemp)0x3000)
-#define SPRITES		((const cv_vmemp)0x3c00)
-
-#define COLS 32
-#define ROWS 28
-
-typedef unsigned char byte;
-typedef signed char sbyte;
-typedef unsigned short word;
-
-#ifndef CV_SMS
-uintptr_t __at(0x6a) font_bitmap_a;
-uintptr_t __at(0x6c) font_bitmap_0;
-#else
-extern char font_bitmap_a[];
-extern char font_bitmap_0[];
-#endif
-
-#define COLOR_FGBG(fg,bg) (((fg)<<4)|(bg))
-#define COLOR_FG(fg) (((fg)<<4))
-
-#define LOCHAR 0x20
-#define HICHAR 0xff
-
-#define CHAR(ch) (ch-LOCHAR)
-
-#define wait_vsync() __asm__("halt")
-
-extern volatile uint_fast8_t vint_counter;
-
-extern void vint_handler(void);
-extern byte reverse_bits(byte n);
-extern void flip_sprite_patterns(word dest, const byte* patterns, word len);
-
-extern char cursor_x;
-extern char cursor_y;
-
-extern void clrscr();
-
-extern word getimageaddr(byte x, byte y);
-extern byte getchar(byte x, byte y);
-extern void putchar(byte x, byte y, byte attr);
-extern void putstring(byte x, byte y, const char* string);
-extern void delay(byte i);
-extern byte rndint(byte a, byte b);
-
-extern void memset_safe(void* _dest, char ch, word size);
-extern char in_rect(byte x, byte y, byte x0, byte y0, byte w, byte h);
-// print 4-digit BCD value
-extern void draw_bcd_word(byte x, byte y, word bcd);
-// add two 16-bit BCD values
-extern word bcd_add(word a, word b);
-
-extern void vdp_setup();
-extern void set_shifted_pattern(const byte* src, word dest, byte shift);
-
-#endif
diff --git a/presets/sms-sms-libcv/common.h b/presets/sms-sms-libcv/common.h
new file mode 120000
index 00000000..d69f50fa
--- /dev/null
+++ b/presets/sms-sms-libcv/common.h
@@ -0,0 +1 @@
+../coleco/common.h
\ No newline at end of file
diff --git a/src/tools/nodemock.ts b/src/tools/nodemock.ts
index 9a158cab..54c62ca2 100644
--- a/src/tools/nodemock.ts
+++ b/src/tools/nodemock.ts
@@ -29,6 +29,7 @@ export function mockAudio() {
         writeRegister() { }
         writeRegisterSN() { }
         writeRegisterAY() { }
+        readRegister() { return 0; }
     }
     class NullMasterChannel {
         addChannel() { }
diff --git a/src/worker/platforms.ts b/src/worker/platforms.ts
index 08f4d96d..a337ec49 100644
--- a/src/worker/platforms.ts
+++ b/src/worker/platforms.ts
@@ -410,6 +410,8 @@ export var PLATFORM_PARAMS = {
     },
 };
   
-  PLATFORM_PARAMS['sms-sms-libcv'] = PLATFORM_PARAMS['sms-sg1000-libcv'];
+  // SMS/GG mode-4 variants of the shared libcv common.[ch] (2 bytes per tile, 32x28)
+  PLATFORM_PARAMS['sms-sms-libcv'] = { ...PLATFORM_PARAMS['sms-sg1000-libcv'],
+    extra_preproc_args: ['-I', '.', '-D', 'CV_SMS', '-D', 'CV_MODE4'] };
   PLATFORM_PARAMS['sms-gg-libcv'] = PLATFORM_PARAMS['sms-sms-libcv'];
   
\ No newline at end of file