diff --git a/include/reactos/libs/soft386/soft386.h b/include/reactos/libs/soft386/soft386.h
index c212e93acf3..b68550f6714 100644
--- a/include/reactos/libs/soft386/soft386.h
+++ b/include/reactos/libs/soft386/soft386.h
@@ -34,6 +34,13 @@
 #define SOFT386_IDT_INT_GATE_32 0xE
 #define SOFT386_IDT_TRAP_GATE_32 0xF
 
+#define SOFT386_PREFIX_SEG (1 << 0)
+#define SOFT386_PREFIX_OPSIZE (1 << 1)
+#define SOFT386_PREFIX_ADSIZE (1 << 2)
+#define SOFT386_PREFIX_LOCK (1 << 3)
+#define SOFT386_PREFIX_REPNZ (1 << 4)
+#define SOFT386_PREFIX_REP (1 << 5)
+
 struct _SOFT386_STATE;
 typedef struct _SOFT386_STATE SOFT386_STATE, *PSOFT386_STATE;
 
@@ -282,6 +289,8 @@ struct _SOFT386_STATE
     ULONG ControlRegisters[SOFT386_NUM_CTRL_REGS];
     ULONG DebugRegisters[SOFT386_NUM_DBG_REGS];
     ULONG ExceptionCount;
+    ULONG PrefixFlags;
+    INT SegmentOverride;
 };
 
 /* FUNCTIONS ******************************************************************/
diff --git a/lib/soft386/common.c b/lib/soft386/common.c
index 9f15c4e2ec0..71115d495bf 100644
--- a/lib/soft386/common.c
+++ b/lib/soft386/common.c
@@ -197,7 +197,8 @@ Soft386StackPush(PSOFT386_STATE State, ULONG Value)
 {
     BOOLEAN Size = State->SegmentRegs[SOFT386_REG_SS].Size;
 
-    // TODO: Handle OPSIZE prefix.
+    /* The OPSIZE prefix toggles the size */
+    if (State->PrefixFlags & SOFT386_PREFIX_OPSIZE) Size = !Size;
 
     if (Size)
     {
@@ -253,7 +254,8 @@ Soft386StackPop(PSOFT386_STATE State, PULONG Value)
     USHORT ShortValue;
     BOOLEAN Size = State->SegmentRegs[SOFT386_REG_SS].Size;
 
-    // TODO: Handle OPSIZE prefix.
+    /* The OPSIZE prefix toggles the size */
+    if (State->PrefixFlags & SOFT386_PREFIX_OPSIZE) Size = !Size;
 
     if (Size)
     {
@@ -706,7 +708,7 @@ Soft386GetIntVector(PSOFT386_STATE State,
 }
 
 VOID
-__fastcall
+FASTCALL
 Soft386Exception(PSOFT386_STATE State, INT ExceptionCode)
 {
     SOFT386_IDT_ENTRY IdtEntry;
diff --git a/lib/soft386/common.h b/lib/soft386/common.h
index aacf6448c6c..df9ba0a12e9 100644
--- a/lib/soft386/common.h
+++ b/lib/soft386/common.h
@@ -11,6 +11,10 @@
 
 /* DEFINES ********************************************************************/
 
+#ifndef FASTCALL
+#define FASTCALL __fastcall
+#endif
+
 #define GET_SEGMENT_RPL(s) ((s) & 3)
 #define GET_SEGMENT_INDEX(s) ((s) & 0xFFF8)
 
diff --git a/lib/soft386/opcodes.c b/lib/soft386/opcodes.c
index 8ff4ed217f9..9e642946ee6 100644
--- a/lib/soft386/opcodes.c
+++ b/lib/soft386/opcodes.c
@@ -24,5 +24,423 @@
 SOFT386_OPCODE_HANDLER_PROC
 Soft386OpcodeHandlers[SOFT386_NUM_OPCODE_HANDLERS] =
 {
-    NULL
+    NULL, // TODO: OPCODE 0x00 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x01 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x02 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x03 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x04 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x05 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x06 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x07 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x08 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x09 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x0A NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x0B NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x0C NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x0D NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x0E NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x0F NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x10 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x11 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x12 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x13 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x14 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x15 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x16 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x17 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x18 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x19 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x1A NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x1B NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x1C NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x1D NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x1E NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x1F NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x20 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x21 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x22 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x23 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x24 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x25 NOT SUPPORTED
+    Soft386OpcodePrefix,
+    NULL, // TODO: OPCODE 0x27 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x28 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x29 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x2A NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x2B NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x2C NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x2D NOT SUPPORTED
+    Soft386OpcodePrefix,
+    NULL, // TODO: OPCODE 0x2F NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x30 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x31 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x32 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x33 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x34 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x35 NOT SUPPORTED
+    Soft386OpcodePrefix,
+    NULL, // TODO: OPCODE 0x37 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x38 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x39 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x3A NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x3B NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x3C NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x3D NOT SUPPORTED
+    Soft386OpcodePrefix,
+    NULL, // TODO: OPCODE 0x3F NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x40 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x41 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x42 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x43 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x44 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x45 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x46 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x47 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x48 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x49 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x4A NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x4B NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x4C NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x4D NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x4E NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x4F NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x50 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x51 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x52 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x53 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x54 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x55 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x56 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x57 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x58 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x59 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x5A NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x5B NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x5C NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x5D NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x5E NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x5F NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x60 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x61 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x62 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x63 NOT SUPPORTED
+    Soft386OpcodePrefix,
+    Soft386OpcodePrefix,
+    Soft386OpcodePrefix,
+    Soft386OpcodePrefix,
+    NULL, // TODO: OPCODE 0x68 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x69 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x6A NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x6B NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x6C NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x6D NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x6E NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x6F NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x70 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x71 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x72 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x73 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x74 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x75 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x76 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x77 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x78 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x79 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x7A NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x7B NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x7C NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x7D NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x7E NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x7F NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x80 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x81 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x82 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x83 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x84 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x85 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x86 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x87 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x88 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x89 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x8A NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x8B NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x8C NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x8D NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x8E NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x8F NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x90 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x91 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x92 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x93 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x94 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x95 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x96 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x97 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x98 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x99 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x9A NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x9B NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x9C NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x9D NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x9E NOT SUPPORTED
+    NULL, // TODO: OPCODE 0x9F NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xA0 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xA1 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xA2 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xA3 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xA4 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xA5 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xA6 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xA7 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xA8 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xA9 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xAA NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xAB NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xAC NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xAD NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xAE NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xAF NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xB0 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xB1 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xB2 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xB3 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xB4 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xB5 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xB6 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xB7 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xB8 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xB9 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xBA NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xBB NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xBC NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xBD NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xBE NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xBF NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xC0 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xC1 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xC2 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xC3 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xC4 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xC5 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xC6 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xC7 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xC8 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xC9 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xCA NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xCB NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xCC NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xCD NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xCE NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xCF NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xD0 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xD1 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xD2 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xD3 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xD4 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xD5 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xD6 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xD7 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xD8 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xD9 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xDA NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xDB NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xDC NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xDD NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xDE NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xDF NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xE0 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xE1 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xE2 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xE3 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xE4 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xE5 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xE6 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xE7 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xE8 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xE9 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xEA NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xEB NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xEC NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xED NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xEE NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xEF NOT SUPPORTED
+    Soft386OpcodePrefix,
+    NULL, // Invalid
+    Soft386OpcodePrefix,
+    Soft386OpcodePrefix,
+    NULL, // TODO: OPCODE 0xF4 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xF5 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xF6 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xF7 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xF8 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xF9 NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xFA NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xFB NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xFC NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xFD NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xFE NOT SUPPORTED
+    NULL, // TODO: OPCODE 0xFF NOT SUPPORTED
 };
+
+BOOLEAN
+FASTCALL
+Soft386OpcodePrefix(PSOFT386_STATE State, UCHAR Opcode)
+{
+    BOOLEAN Valid = FALSE;
+
+    switch (Opcode)
+    {
+        /* ES: */
+        case 0x26:
+        {
+            if (!(State->PrefixFlags & SOFT386_PREFIX_SEG))
+            {
+                State->PrefixFlags |= SOFT386_PREFIX_SEG;
+                State->SegmentOverride = SOFT386_REG_ES;
+                Valid = TRUE;
+            }
+
+            break;
+        }
+
+        /* CS: */
+        case 0x2E:
+        {
+            if (!(State->PrefixFlags & SOFT386_PREFIX_SEG))
+            {
+                State->PrefixFlags |= SOFT386_PREFIX_SEG;
+                State->SegmentOverride = SOFT386_REG_CS;
+                Valid = TRUE;
+            }
+
+            break;
+        }
+
+        /* SS: */
+        case 0x36:
+        {
+            if (!(State->PrefixFlags & SOFT386_PREFIX_SEG))
+            {
+                State->PrefixFlags |= SOFT386_PREFIX_SEG;
+                State->SegmentOverride = SOFT386_REG_SS;
+                Valid = TRUE;
+            }
+
+            break;
+        }
+
+        /* DS: */
+        case 0x3E:
+        {
+            if (!(State->PrefixFlags & SOFT386_PREFIX_SEG))
+            {
+                State->PrefixFlags |= SOFT386_PREFIX_SEG;
+                State->SegmentOverride = SOFT386_REG_DS;
+                Valid = TRUE;
+            }
+
+            break;
+        }
+
+        /* FS: */
+        case 0x64:
+        {
+            if (!(State->PrefixFlags & SOFT386_PREFIX_SEG))
+            {
+                State->PrefixFlags |= SOFT386_PREFIX_SEG;
+                State->SegmentOverride = SOFT386_REG_FS;
+                Valid = TRUE;
+            }
+
+            break;
+        }
+
+        /* GS: */
+        case 0x65:
+        {
+            if (!(State->PrefixFlags & SOFT386_PREFIX_SEG))
+            {
+                State->PrefixFlags |= SOFT386_PREFIX_SEG;
+                State->SegmentOverride = SOFT386_REG_GS;
+                Valid = TRUE;
+            }
+
+            break;
+        }
+
+        /* OPSIZE */
+        case 0x66:
+        {
+            if (!(State->PrefixFlags & SOFT386_PREFIX_OPSIZE))
+            {
+                State->PrefixFlags |= SOFT386_PREFIX_OPSIZE;
+                Valid = TRUE;
+            }
+
+            break;
+        }
+
+        /* ADSIZE */
+        case 0x67:
+        {
+            if (!(State->PrefixFlags & SOFT386_PREFIX_ADSIZE))
+            {
+                State->PrefixFlags |= SOFT386_PREFIX_ADSIZE;
+                Valid = TRUE;
+            }
+            break;
+        }
+
+        /* LOCK */
+        case 0xF0:
+        {
+            if (!(State->PrefixFlags & SOFT386_PREFIX_LOCK))
+            {
+                State->PrefixFlags |= SOFT386_PREFIX_LOCK;
+                Valid = TRUE;
+            }
+
+            break;
+        }
+
+        /* REPNZ */
+        case 0xF2:
+        {
+            /* Mutually exclusive with REP */
+            if (!(State->PrefixFlags
+                & (SOFT386_PREFIX_REPNZ | SOFT386_PREFIX_REP)))
+            {
+                State->PrefixFlags |= SOFT386_PREFIX_REPNZ;
+                Valid = TRUE;
+            }
+
+            break;
+        }
+
+        /* REP / REPZ */
+        case 0xF3:
+        {
+            /* Mutually exclusive with REPNZ */
+            if (!(State->PrefixFlags
+                & (SOFT386_PREFIX_REPNZ | SOFT386_PREFIX_REP)))
+            {
+                State->PrefixFlags |= SOFT386_PREFIX_REP;
+                Valid = TRUE;
+            }
+
+            break;
+        }
+    }
+
+    if (!Valid)
+    {
+        /* Clear all prefixes */
+        State->PrefixFlags = 0;
+
+        /* Throw an exception */
+        Soft386Exception(State, SOFT386_EXCEPTION_UD);
+        return FALSE;
+    }
+
+    return TRUE;
+}
diff --git a/lib/soft386/opcodes.h b/lib/soft386/opcodes.h
index 9377e29f0b5..25fec0002f4 100644
--- a/lib/soft386/opcodes.h
+++ b/lib/soft386/opcodes.h
@@ -11,12 +11,24 @@
 
 /* DEFINES ********************************************************************/
 
+#ifndef FASTCALL
+#define FASTCALL __fastcall
+#endif
+
 #define SOFT386_NUM_OPCODE_HANDLERS 256
 
-typedef BOOLEAN (__fastcall *SOFT386_OPCODE_HANDLER_PROC)(PSOFT386_STATE);
+typedef BOOLEAN (FASTCALL *SOFT386_OPCODE_HANDLER_PROC)(PSOFT386_STATE, UCHAR);
 
 extern
 SOFT386_OPCODE_HANDLER_PROC
 Soft386OpcodeHandlers[SOFT386_NUM_OPCODE_HANDLERS];
 
+BOOLEAN
+FASTCALL
+Soft386OpcodePrefix
+(
+    PSOFT386_STATE State,
+    UCHAR Opcode
+);
+
 #endif // _OPCODES_H_
diff --git a/lib/soft386/soft386.c b/lib/soft386/soft386.c
index 4f74158cdaa..adbb426c43f 100644
--- a/lib/soft386/soft386.c
+++ b/lib/soft386/soft386.c
@@ -37,7 +37,7 @@ VOID
 NTAPI
 Soft386ExecutionControl(PSOFT386_STATE State, INT Command)
 {
-    BYTE Opcode;
+    UCHAR Opcode;
     INT ProcedureCallCount = 0;
 
     /* Main execution loop */
@@ -51,7 +51,7 @@ Soft386ExecutionControl(PSOFT386_STATE State, INT Command)
         if (Soft386OpcodeHandlers[Opcode] != NULL)
         {
             /* Call the opcode handler */
-            Soft386OpcodeHandlers[Opcode](State);
+            Soft386OpcodeHandlers[Opcode](State, Opcode);
         }
         else
         {