[FAST486]
[reactos.git] / lib / fast486 / opgroups.c
index 232eb25..be4d701 100644 (file)
@@ -85,7 +85,7 @@ Fast486ArithmeticOperation(PFAST486_STATE State,
                               || ((Result < FirstValue) && (Result < (SecondValue + Carry)));
             State->Flags.Of = ((FirstValue & SignFlag) == (SecondValue & SignFlag))
                               && ((FirstValue & SignFlag) != (Result & SignFlag));
-            State->Flags.Af = ((((FirstValue & 0x0F) + ((SecondValue + Carry) & 0x0F)) & 0x10) != 0);
+            State->Flags.Af = ((FirstValue ^ SecondValue ^ Result) & 0x10) != 0;
 
             break;
         }
@@ -98,10 +98,12 @@ Fast486ArithmeticOperation(PFAST486_STATE State,
             Result = (FirstValue - SecondValue - Carry) & MaxValue;
 
             /* Update CF, OF and AF */
-            State->Flags.Cf = FirstValue < (SecondValue + Carry);
+            State->Flags.Cf = Carry
+                              ? (FirstValue <= SecondValue)
+                              : (FirstValue < SecondValue);
             State->Flags.Of = ((FirstValue & SignFlag) != (SecondValue & SignFlag))
                               && ((FirstValue & SignFlag) != (Result & SignFlag));
-            State->Flags.Af = (FirstValue & 0x0F) < ((SecondValue + Carry) & 0x0F);
+            State->Flags.Af = ((FirstValue ^ SecondValue ^ Result) & 0x10) != 0;
 
             break;
         }
@@ -293,7 +295,7 @@ Fast486RotateOperation(PFAST486_STATE State,
 
 FAST486_OPCODE_HANDLER(Fast486OpcodeGroup8082)
 {
-    UCHAR Immediate, Dummy, Value;
+    UCHAR Immediate, Value;
     FAST486_MOD_REG_RM ModRegRm;
     BOOLEAN AddressSize = State->SegmentRegs[FAST486_REG_CS].Size;
 
@@ -313,7 +315,7 @@ FAST486_OPCODE_HANDLER(Fast486OpcodeGroup8082)
     }
 
     /* Read the operands */
-    if (!Fast486ReadModrmByteOperands(State, &ModRegRm, &Dummy, &Value))
+    if (!Fast486ReadModrmByteOperands(State, &ModRegRm, NULL, &Value))
     {
         /* Exception occurred */
         return FALSE;
@@ -349,7 +351,7 @@ FAST486_OPCODE_HANDLER(Fast486OpcodeGroup81)
 
     if (OperandSize)
     {
-        ULONG Immediate, Value, Dummy;
+        ULONG Immediate, Value;
 
         /* Fetch the immediate operand */
         if (!Fast486FetchDword(State, &Immediate))
@@ -359,7 +361,7 @@ FAST486_OPCODE_HANDLER(Fast486OpcodeGroup81)
         }
 
         /* Read the operands */
-        if (!Fast486ReadModrmDwordOperands(State, &ModRegRm, &Dummy, &Value))
+        if (!Fast486ReadModrmDwordOperands(State, &ModRegRm, NULL, &Value))
         {
             /* Exception occurred */
             return FALSE;
@@ -376,7 +378,7 @@ FAST486_OPCODE_HANDLER(Fast486OpcodeGroup81)
     }
     else
     {
-        USHORT Immediate, Value, Dummy;
+        USHORT Immediate, Value;
 
         /* Fetch the immediate operand */
         if (!Fast486FetchWord(State, &Immediate))
@@ -386,7 +388,7 @@ FAST486_OPCODE_HANDLER(Fast486OpcodeGroup81)
         }
 
         /* Read the operands */
-        if (!Fast486ReadModrmWordOperands(State, &ModRegRm, &Dummy, &Value))
+        if (!Fast486ReadModrmWordOperands(State, &ModRegRm, NULL, &Value))
         {
             /* Exception occurred */
             return FALSE;
@@ -432,10 +434,10 @@ FAST486_OPCODE_HANDLER(Fast486OpcodeGroup83)
     if (OperandSize)
     {
         ULONG Immediate = (ULONG)((LONG)ImmByte); // Sign extend
-        ULONG Value, Dummy;
+        ULONG Value;
 
         /* Read the operands */
-        if (!Fast486ReadModrmDwordOperands(State, &ModRegRm, &Dummy, &Value))
+        if (!Fast486ReadModrmDwordOperands(State, &ModRegRm, NULL, &Value))
         {
             /* Exception occurred */
             return FALSE;
@@ -453,10 +455,10 @@ FAST486_OPCODE_HANDLER(Fast486OpcodeGroup83)
     else
     {
         USHORT Immediate = (USHORT)((SHORT)ImmByte); // Sign extend
-        USHORT Value, Dummy;
+        USHORT Value;
 
         /* Read the operands */
-        if (!Fast486ReadModrmWordOperands(State, &ModRegRm, &Dummy, &Value))
+        if (!Fast486ReadModrmWordOperands(State, &ModRegRm, NULL, &Value))
         {
             /* Exception occurred */
             return FALSE;
@@ -527,7 +529,7 @@ FAST486_OPCODE_HANDLER(Fast486OpcodeGroup8F)
 
 FAST486_OPCODE_HANDLER(Fast486OpcodeGroupC0)
 {
-    UCHAR Dummy, Value, Count;
+    UCHAR Value, Count;
     FAST486_MOD_REG_RM ModRegRm;
     BOOLEAN AddressSize = State->SegmentRegs[FAST486_REG_CS].Size;
 
@@ -547,7 +549,7 @@ FAST486_OPCODE_HANDLER(Fast486OpcodeGroupC0)
     }
 
     /* Read the operands */
-    if (!Fast486ReadModrmByteOperands(State, &ModRegRm, &Dummy, &Value))
+    if (!Fast486ReadModrmByteOperands(State, &ModRegRm, NULL, &Value))
     {
         /* Exception occurred */
         return FALSE;
@@ -593,10 +595,10 @@ FAST486_OPCODE_HANDLER(Fast486OpcodeGroupC1)
 
     if (OperandSize)
     {
-        ULONG Dummy, Value;
+        ULONG Value;
 
         /* Read the operands */
-        if (!Fast486ReadModrmDwordOperands(State, &ModRegRm, &Dummy, &Value))
+        if (!Fast486ReadModrmDwordOperands(State, &ModRegRm, NULL, &Value))
         {
             /* Exception occurred */
             return FALSE;
@@ -614,10 +616,10 @@ FAST486_OPCODE_HANDLER(Fast486OpcodeGroupC1)
     }
     else
     {
-        USHORT Dummy, Value;
+        USHORT Value;
 
         /* Read the operands */
-        if (!Fast486ReadModrmWordOperands(State, &ModRegRm, &Dummy, &Value))
+        if (!Fast486ReadModrmWordOperands(State, &ModRegRm, NULL, &Value))
         {
             /* Exception occurred */
             return FALSE;
@@ -728,7 +730,7 @@ FAST486_OPCODE_HANDLER(Fast486OpcodeGroupC7)
 
 FAST486_OPCODE_HANDLER(Fast486OpcodeGroupD0)
 {
-    UCHAR Dummy, Value;
+    UCHAR Value;
     FAST486_MOD_REG_RM ModRegRm;
     BOOLEAN AddressSize = State->SegmentRegs[FAST486_REG_CS].Size;
 
@@ -741,7 +743,7 @@ FAST486_OPCODE_HANDLER(Fast486OpcodeGroupD0)
     }
 
     /* Read the operands */
-    if (!Fast486ReadModrmByteOperands(State, &ModRegRm, &Dummy, &Value))
+    if (!Fast486ReadModrmByteOperands(State, &ModRegRm, NULL, &Value))
     {
         /* Exception occurred */
         return FALSE;
@@ -776,10 +778,10 @@ FAST486_OPCODE_HANDLER(Fast486OpcodeGroupD1)
 
     if (OperandSize)
     {
-        ULONG Dummy, Value;
+        ULONG Value;
 
         /* Read the operands */
-        if (!Fast486ReadModrmDwordOperands(State, &ModRegRm, &Dummy, &Value))
+        if (!Fast486ReadModrmDwordOperands(State, &ModRegRm, NULL, &Value))
         {
             /* Exception occurred */
             return FALSE;
@@ -793,10 +795,10 @@ FAST486_OPCODE_HANDLER(Fast486OpcodeGroupD1)
     }
     else
     {
-        USHORT Dummy, Value;
+        USHORT Value;
 
         /* Read the operands */
-        if (!Fast486ReadModrmWordOperands(State, &ModRegRm, &Dummy, &Value))
+        if (!Fast486ReadModrmWordOperands(State, &ModRegRm, NULL, &Value))
         {
             /* Exception occurred */
             return FALSE;
@@ -812,7 +814,7 @@ FAST486_OPCODE_HANDLER(Fast486OpcodeGroupD1)
 
 FAST486_OPCODE_HANDLER(Fast486OpcodeGroupD2)
 {
-    UCHAR Dummy, Value;
+    UCHAR Value;
     FAST486_MOD_REG_RM ModRegRm;
     BOOLEAN AddressSize = State->SegmentRegs[FAST486_REG_CS].Size;
 
@@ -825,7 +827,7 @@ FAST486_OPCODE_HANDLER(Fast486OpcodeGroupD2)
     }
 
     /* Read the operands */
-    if (!Fast486ReadModrmByteOperands(State, &ModRegRm, &Dummy, &Value))
+    if (!Fast486ReadModrmByteOperands(State, &ModRegRm, NULL, &Value))
     {
         /* Exception occurred */
         return FALSE;
@@ -863,10 +865,10 @@ FAST486_OPCODE_HANDLER(Fast486OpcodeGroupD3)
 
     if (OperandSize)
     {
-        ULONG Dummy, Value;
+        ULONG Value;
 
         /* Read the operands */
-        if (!Fast486ReadModrmDwordOperands(State, &ModRegRm, &Dummy, &Value))
+        if (!Fast486ReadModrmDwordOperands(State, &ModRegRm, NULL, &Value))
         {
             /* Exception occurred */
             return FALSE;
@@ -884,10 +886,10 @@ FAST486_OPCODE_HANDLER(Fast486OpcodeGroupD3)
     }
     else
     {
-        USHORT Dummy, Value;
+        USHORT Value;
 
         /* Read the operands */
-        if (!Fast486ReadModrmWordOperands(State, &ModRegRm, &Dummy, &Value))
+        if (!Fast486ReadModrmWordOperands(State, &ModRegRm, NULL, &Value))
         {
             /* Exception occurred */
             return FALSE;
@@ -907,7 +909,7 @@ FAST486_OPCODE_HANDLER(Fast486OpcodeGroupD3)
 
 FAST486_OPCODE_HANDLER(Fast486OpcodeGroupF6)
 {
-    UCHAR Dummy, Value = 0;
+    UCHAR Value = 0;
     FAST486_MOD_REG_RM ModRegRm;
     BOOLEAN AddressSize = State->SegmentRegs[FAST486_REG_CS].Size;
 
@@ -920,7 +922,7 @@ FAST486_OPCODE_HANDLER(Fast486OpcodeGroupF6)
     }
 
     /* Read the operands */
-    if (!Fast486ReadModrmByteOperands(State, &ModRegRm, &Dummy, &Value))
+    if (!Fast486ReadModrmByteOperands(State, &ModRegRm, NULL, &Value))
     {
         /* Exception occurred */
         return FALSE;
@@ -1010,8 +1012,17 @@ FAST486_OPCODE_HANDLER(Fast486OpcodeGroupF6)
         /* DIV */
         case 6:
         {
-            UCHAR Quotient = State->GeneralRegs[FAST486_REG_EAX].LowWord / Value;
-            UCHAR Remainder = State->GeneralRegs[FAST486_REG_EAX].LowWord % Value;
+            UCHAR Quotient, Remainder;
+
+            if (Value == 0)
+            {
+                /* Divide error */
+                Fast486Exception(State, FAST486_EXCEPTION_DE);
+                return FALSE;
+            }
+
+            Quotient = State->GeneralRegs[FAST486_REG_EAX].LowWord / Value;
+            Remainder = State->GeneralRegs[FAST486_REG_EAX].LowWord % Value;
 
             /* Write back the results */
             State->GeneralRegs[FAST486_REG_EAX].LowByte = Quotient;
@@ -1023,8 +1034,17 @@ FAST486_OPCODE_HANDLER(Fast486OpcodeGroupF6)
         /* IDIV */
         case 7:
         {
-            CHAR Quotient = (SHORT)State->GeneralRegs[FAST486_REG_EAX].LowWord / (CHAR)Value;
-            CHAR Remainder = (SHORT)State->GeneralRegs[FAST486_REG_EAX].LowWord % (CHAR)Value;
+            CHAR Quotient, Remainder;
+
+            if (Value == 0)
+            {
+                /* Divide error */
+                Fast486Exception(State, FAST486_EXCEPTION_DE);
+                return FALSE;
+            }
+
+            Quotient = (SHORT)State->GeneralRegs[FAST486_REG_EAX].LowWord / (CHAR)Value;
+            Remainder = (SHORT)State->GeneralRegs[FAST486_REG_EAX].LowWord % (CHAR)Value;
 
             /* Write back the results */
             State->GeneralRegs[FAST486_REG_EAX].LowByte = (UCHAR)Quotient;
@@ -1039,7 +1059,7 @@ FAST486_OPCODE_HANDLER(Fast486OpcodeGroupF6)
 
 FAST486_OPCODE_HANDLER(Fast486OpcodeGroupF7)
 {
-    ULONG Dummy, Value = 0, SignFlag;
+    ULONG Value = 0, SignFlag;
     FAST486_MOD_REG_RM ModRegRm;
     BOOLEAN OperandSize, AddressSize;
 
@@ -1062,7 +1082,7 @@ FAST486_OPCODE_HANDLER(Fast486OpcodeGroupF7)
     if (OperandSize)
     {
         /* 32-bit */
-        if (!Fast486ReadModrmDwordOperands(State, &ModRegRm, &Dummy, &Value))
+        if (!Fast486ReadModrmDwordOperands(State, &ModRegRm, NULL, &Value))
         {
             /* Exception occurred */
             return FALSE;
@@ -1071,7 +1091,7 @@ FAST486_OPCODE_HANDLER(Fast486OpcodeGroupF7)
     else
     {
         /* 16-bit */
-        if (!Fast486ReadModrmWordOperands(State, &ModRegRm, (PUSHORT)&Dummy, (PUSHORT)&Value))
+        if (!Fast486ReadModrmWordOperands(State, &ModRegRm, NULL, (PUSHORT)&Value))
         {
             /* Exception occurred */
             return FALSE;
@@ -1223,6 +1243,13 @@ FAST486_OPCODE_HANDLER(Fast486OpcodeGroupF7)
         /* DIV */
         case 6:
         {
+            if (Value == 0)
+            {
+                /* Divide error */
+                Fast486Exception(State, FAST486_EXCEPTION_DE);
+                return FALSE;
+            }
+
             if (OperandSize)
             {
                 ULONGLONG Dividend = (ULONGLONG)State->GeneralRegs[FAST486_REG_EAX].Long
@@ -1252,6 +1279,13 @@ FAST486_OPCODE_HANDLER(Fast486OpcodeGroupF7)
         /* IDIV */
         case 7:
         {
+            if (Value == 0)
+            {
+                /* Divide error */
+                Fast486Exception(State, FAST486_EXCEPTION_DE);
+                return FALSE;
+            }
+
             if (OperandSize)
             {
                 LONGLONG Dividend = (LONGLONG)State->GeneralRegs[FAST486_REG_EAX].Long
@@ -1284,7 +1318,7 @@ FAST486_OPCODE_HANDLER(Fast486OpcodeGroupF7)
 
 FAST486_OPCODE_HANDLER(Fast486OpcodeGroupFE)
 {
-    UCHAR Dummy, Value;
+    UCHAR Value;
     FAST486_MOD_REG_RM ModRegRm;
     BOOLEAN AddressSize = State->SegmentRegs[FAST486_REG_CS].Size;
 
@@ -1304,7 +1338,7 @@ FAST486_OPCODE_HANDLER(Fast486OpcodeGroupFE)
     }
 
     /* Read the operands */
-    if (!Fast486ReadModrmByteOperands(State, &ModRegRm, &Dummy, &Value))
+    if (!Fast486ReadModrmByteOperands(State, &ModRegRm, NULL, &Value))
     {
         /* Exception occurred */
         return FALSE;
@@ -1363,9 +1397,9 @@ FAST486_OPCODE_HANDLER(Fast486OpcodeGroupFF)
     /* Read the operands */
     if (OperandSize)
     {
-        ULONG Dummy, Value;
+        ULONG Value;
 
-        if (!Fast486ReadModrmDwordOperands(State, &ModRegRm, &Dummy, &Value))
+        if (!Fast486ReadModrmDwordOperands(State, &ModRegRm, NULL, &Value))
         {
             /* Exception occurred */
             return FALSE;
@@ -1400,7 +1434,7 @@ FAST486_OPCODE_HANDLER(Fast486OpcodeGroupFF)
         else if (ModRegRm.Register == 3)
         {
             USHORT Selector;
-            INT Segment = FAST486_REG_DS;
+            FAST486_SEG_REGS Segment = FAST486_REG_DS;
 
             /* Check for the segment override */
             if (State->PrefixFlags & FAST486_PREFIX_SEG)
@@ -1453,7 +1487,7 @@ FAST486_OPCODE_HANDLER(Fast486OpcodeGroupFF)
         else if (ModRegRm.Register == 5)
         {
             USHORT Selector;
-            INT Segment = FAST486_REG_DS;
+            FAST486_SEG_REGS Segment = FAST486_REG_DS;
 
             /* Check for the segment override */
             if (State->PrefixFlags & FAST486_PREFIX_SEG)
@@ -1506,9 +1540,9 @@ FAST486_OPCODE_HANDLER(Fast486OpcodeGroupFF)
     }
     else
     {
-        USHORT Dummy, Value;
+        USHORT Value;
 
-        if (!Fast486ReadModrmWordOperands(State, &ModRegRm, &Dummy, &Value))
+        if (!Fast486ReadModrmWordOperands(State, &ModRegRm, NULL, &Value))
         {
             /* Exception occurred */
             return FALSE;
@@ -1539,11 +1573,14 @@ FAST486_OPCODE_HANDLER(Fast486OpcodeGroupFF)
 
             /* Set the IP to the address */
             State->InstPtr.LowWord = Value;
+
+            /* Clear the top half of EIP */
+            State->InstPtr.Long &= 0xFFFF;
         }
         else if (ModRegRm.Register == 3)
         {
             USHORT Selector;
-            INT Segment = FAST486_REG_DS;
+            FAST486_SEG_REGS Segment = FAST486_REG_DS;
 
             /* Check for the segment override */
             if (State->PrefixFlags & FAST486_PREFIX_SEG)
@@ -1588,6 +1625,8 @@ FAST486_OPCODE_HANDLER(Fast486OpcodeGroupFF)
             /* Set the IP to the address */
             State->InstPtr.LowWord = Value;
 
+            /* Clear the top half of EIP */
+            State->InstPtr.Long &= 0xFFFF;
         }
         else if (ModRegRm.Register == 4)
         {
@@ -1597,7 +1636,7 @@ FAST486_OPCODE_HANDLER(Fast486OpcodeGroupFF)
         else if (ModRegRm.Register == 5)
         {
             USHORT Selector;
-            INT Segment = FAST486_REG_DS;
+            FAST486_SEG_REGS Segment = FAST486_REG_DS;
 
             /* Check for the segment override */
             if (State->PrefixFlags & FAST486_PREFIX_SEG)
@@ -1627,6 +1666,9 @@ FAST486_OPCODE_HANDLER(Fast486OpcodeGroupFF)
 
             /* Set the IP to the address */
             State->InstPtr.LowWord = Value;
+
+            /* Clear the top half of EIP */
+            State->InstPtr.Long &= 0xFFFF;
         }
         else if (ModRegRm.Register == 6)
         {
@@ -1658,6 +1700,312 @@ FAST486_OPCODE_HANDLER(Fast486OpcodeGroupFF)
     return TRUE;
 }
 
+FAST486_OPCODE_HANDLER(Fast486OpcodeGroup0F00)
+{
+    FAST486_MOD_REG_RM ModRegRm;
+    BOOLEAN AddressSize = State->SegmentRegs[FAST486_REG_CS].Size;
+
+    NO_LOCK_PREFIX();
+    TOGGLE_ADSIZE(AddressSize);
+
+    if (!Fast486ParseModRegRm(State, AddressSize, &ModRegRm))
+    {
+        /* Exception occurred */
+        return FALSE;
+    }
+
+    /* Check which operation this is */
+    switch (ModRegRm.Register)
+    {
+        /* SLDT */
+        case 0:
+        {
+            /* Not recognized in real mode or virtual 8086 mode */
+            if (!(State->ControlRegisters[FAST486_REG_CR0] & FAST486_CR0_PE)
+                || State->Flags.Vm)
+            {
+                Fast486Exception(State, FAST486_EXCEPTION_UD);
+            }
+
+            return Fast486WriteModrmWordOperands(State,
+                                                 &ModRegRm,
+                                                 FALSE,
+                                                 State->Ldtr.Selector);
+        }
+
+        /* STR */
+        case 1:
+        {
+            /* Not recognized in real mode or virtual 8086 mode */
+            if (!(State->ControlRegisters[FAST486_REG_CR0] & FAST486_CR0_PE)
+                || State->Flags.Vm)
+            {
+                Fast486Exception(State, FAST486_EXCEPTION_UD);
+            }
+
+            return Fast486WriteModrmWordOperands(State,
+                                                 &ModRegRm,
+                                                 FALSE,
+                                                 State->TaskReg.Selector);
+        }
+
+        /* LLDT */
+        case 2:
+        {
+            USHORT Selector;
+            FAST486_SYSTEM_DESCRIPTOR GdtEntry;
+
+            /* Not recognized in real mode or virtual 8086 mode */
+            if (!(State->ControlRegisters[FAST486_REG_CR0] & FAST486_CR0_PE)
+                || State->Flags.Vm)
+            {
+                Fast486Exception(State, FAST486_EXCEPTION_UD);
+            }
+
+            /* This is a privileged instruction */
+            if (Fast486GetCurrentPrivLevel(State) != 0)
+            {
+                Fast486Exception(State, FAST486_EXCEPTION_GP);
+                return FALSE;
+            }
+
+            if (!Fast486ReadModrmWordOperands(State,
+                                              &ModRegRm,
+                                              NULL,
+                                              &Selector))
+            {
+                /* Exception occurred */
+                return FALSE;
+            }
+
+            /* Make sure the GDT contains the entry */
+            if (GET_SEGMENT_INDEX(Selector) >= (State->Gdtr.Size + 1))
+            {
+                Fast486ExceptionWithErrorCode(State, FAST486_EXCEPTION_GP, Selector);
+                return FALSE;
+            }
+
+            /* Read the GDT */
+            if (!Fast486ReadLinearMemory(State,
+                                         State->Gdtr.Address
+                                         + GET_SEGMENT_INDEX(Selector),
+                                         &GdtEntry,
+                                         sizeof(GdtEntry)))
+            {
+                /* Exception occurred */
+                return FALSE;
+            }
+
+            if (GET_SEGMENT_INDEX(Selector) == 0)
+            {
+                Fast486Exception(State, FAST486_EXCEPTION_GP);
+                return FALSE;
+            }
+
+            if (!GdtEntry.Present)
+            {
+                Fast486ExceptionWithErrorCode(State, FAST486_EXCEPTION_NP, Selector);
+                return FALSE;
+            }
+
+            if (GdtEntry.Signature != FAST486_LDT_SIGNATURE)
+            {
+                /* This is not a LDT descriptor */
+                Fast486ExceptionWithErrorCode(State, FAST486_EXCEPTION_GP, Selector);
+                return FALSE;
+            }
+
+            /* Update the LDTR */
+            State->Ldtr.Selector = Selector;
+            State->Ldtr.Base = GdtEntry.Base | (GdtEntry.BaseMid << 16) | (GdtEntry.BaseHigh << 24);
+            State->Ldtr.Limit = GdtEntry.Limit | (GdtEntry.LimitHigh << 16);
+            if (GdtEntry.Granularity) State->Ldtr.Limit <<= 12;
+
+            return TRUE;
+        }
+
+        /* LTR */
+        case 3:
+        {
+            USHORT Selector;
+            FAST486_SYSTEM_DESCRIPTOR GdtEntry;
+
+            /* Not recognized in real mode or virtual 8086 mode */
+            if (!(State->ControlRegisters[FAST486_REG_CR0] & FAST486_CR0_PE)
+                || State->Flags.Vm)
+            {
+                Fast486Exception(State, FAST486_EXCEPTION_UD);
+            }
+
+            /* This is a privileged instruction */
+            if (Fast486GetCurrentPrivLevel(State) != 0)
+            {
+                Fast486Exception(State, FAST486_EXCEPTION_GP);
+                return FALSE;
+            }
+
+            if (!Fast486ReadModrmWordOperands(State,
+                                              &ModRegRm,
+                                              NULL,
+                                              &Selector))
+            {
+                /* Exception occurred */
+                return FALSE;
+            }
+
+            /* Make sure the GDT contains the entry */
+            if (GET_SEGMENT_INDEX(Selector) >= (State->Gdtr.Size + 1))
+            {
+                Fast486ExceptionWithErrorCode(State, FAST486_EXCEPTION_GP, Selector);
+                return FALSE;
+            }
+
+            /* Read the GDT */
+            if (!Fast486ReadLinearMemory(State,
+                                         State->Gdtr.Address
+                                         + GET_SEGMENT_INDEX(Selector),
+                                         &GdtEntry,
+                                         sizeof(GdtEntry)))
+            {
+                /* Exception occurred */
+                return FALSE;
+            }
+
+            if (GET_SEGMENT_INDEX(Selector) == 0)
+            {
+                Fast486Exception(State, FAST486_EXCEPTION_GP);
+                return FALSE;
+            }
+
+            if (!GdtEntry.Present)
+            {
+                Fast486ExceptionWithErrorCode(State, FAST486_EXCEPTION_NP, Selector);
+                return FALSE;
+            }
+
+            if (GdtEntry.Signature != FAST486_TSS_SIGNATURE)
+            {
+                /* This is not a TSS descriptor */
+                Fast486ExceptionWithErrorCode(State, FAST486_EXCEPTION_GP, Selector);
+                return FALSE;
+            }
+
+            /* Update the TR */
+            State->TaskReg.Selector = Selector;
+            State->TaskReg.Base = GdtEntry.Base | (GdtEntry.BaseMid << 16) | (GdtEntry.BaseHigh << 24);
+            State->TaskReg.Limit = GdtEntry.Limit | (GdtEntry.LimitHigh << 16);
+            if (GdtEntry.Granularity) State->TaskReg.Limit <<= 12;
+            State->TaskReg.Busy = TRUE;
+
+            return TRUE;
+        }
+
+        /* VERR/VERW */
+        case 4:
+        case 5:
+        {
+            USHORT Selector;
+            FAST486_GDT_ENTRY GdtEntry;
+
+            /* Not recognized in real mode or virtual 8086 mode */
+            if (!(State->ControlRegisters[FAST486_REG_CR0] & FAST486_CR0_PE)
+                || State->Flags.Vm)
+            {
+                Fast486Exception(State, FAST486_EXCEPTION_UD);
+            }
+
+            /* This is a privileged instruction */
+            if (Fast486GetCurrentPrivLevel(State) != 0)
+            {
+                Fast486Exception(State, FAST486_EXCEPTION_GP);
+                return FALSE;
+            }
+
+            if (!Fast486ReadModrmWordOperands(State,
+                                              &ModRegRm,
+                                              NULL,
+                                              &Selector))
+            {
+                /* Exception occurred */
+                return FALSE;
+            }
+
+            if (!(Selector & SEGMENT_TABLE_INDICATOR))
+            {
+                /* Make sure the GDT contains the entry */
+                if (GET_SEGMENT_INDEX(Selector) >= (State->Gdtr.Size + 1))
+                {
+                    /* Clear ZF */
+                    State->Flags.Zf = FALSE;
+                    return TRUE;
+                }
+
+                /* Read the GDT */
+                if (!Fast486ReadLinearMemory(State,
+                                             State->Gdtr.Address
+                                             + GET_SEGMENT_INDEX(Selector),
+                                             &GdtEntry,
+                                             sizeof(GdtEntry)))
+                {
+                    /* Exception occurred */
+                    return FALSE;
+                }
+            }
+            else
+            {
+                /* Make sure the LDT contains the entry */
+                if (GET_SEGMENT_INDEX(Selector) >= (State->Ldtr.Limit + 1))
+                {
+                    /* Clear ZF */
+                    State->Flags.Zf = FALSE;
+                    return TRUE;
+                }
+
+                /* Read the LDT */
+                if (!Fast486ReadLinearMemory(State,
+                                             State->Ldtr.Base
+                                             + GET_SEGMENT_INDEX(Selector),
+                                             &GdtEntry,
+                                             sizeof(GdtEntry)))
+                {
+                    /* Exception occurred */
+                    return FALSE;
+                }
+            }
+
+            /* Set ZF if it is valid and accessible */
+            State->Flags.Zf = GdtEntry.Present // must be present
+                               && GdtEntry.SystemType // must be a segment
+                               && (((ModRegRm.Register == 4)
+                               /* code segments are only readable if the RW bit is set */
+                               && (!GdtEntry.Executable || GdtEntry.ReadWrite))
+                               || ((ModRegRm.Register == 5)
+                               /* code segments are never writable, data segments are writable when RW is set */
+                               && (!GdtEntry.Executable && GdtEntry.ReadWrite)))
+                               /*
+                                * for segments other than conforming code segments,
+                                * both RPL and CPL must be less than or equal to DPL
+                                */
+                               && ((!GdtEntry.Executable || !GdtEntry.DirConf)
+                               && ((GET_SEGMENT_RPL(Selector) <= GdtEntry.Dpl)
+                               && (Fast486GetCurrentPrivLevel(State) <= GdtEntry.Dpl)))
+                               /* for conforming code segments, DPL must be less than or equal to CPL */
+                               && ((GdtEntry.Executable && GdtEntry.DirConf)
+                               && (GdtEntry.Dpl <= Fast486GetCurrentPrivLevel(State)));
+
+
+            return TRUE;
+        }
+
+        /* Invalid */
+        default:
+        {
+            Fast486Exception(State, FAST486_EXCEPTION_UD);
+            return FALSE;
+        }
+    }
+}
+
 FAST486_OPCODE_HANDLER(Fast486OpcodeGroup0F01)
 {
     UCHAR TableReg[6];
@@ -1813,7 +2161,7 @@ FAST486_OPCODE_HANDLER(Fast486OpcodeGroup0F01)
         /* LMSW */
         case 6:
         {
-            USHORT MachineStatusWord, Dummy;
+            USHORT MachineStatusWord;
 
             /* This is a privileged instruction */
             if (Fast486GetCurrentPrivLevel(State) != 0)
@@ -1823,7 +2171,7 @@ FAST486_OPCODE_HANDLER(Fast486OpcodeGroup0F01)
             }
 
             /* Read the new Machine Status Word */
-            if (!Fast486ReadModrmWordOperands(State, &ModRegRm, &Dummy, &MachineStatusWord))
+            if (!Fast486ReadModrmWordOperands(State, &ModRegRm, NULL, &MachineStatusWord))
             {
                 /* Exception occurred */
                 return FALSE;
@@ -1928,10 +2276,10 @@ FAST486_OPCODE_HANDLER(Fast486OpcodeGroup0FBA)
 
     if (OperandSize)
     {
-        ULONG Dummy, Value;
+        ULONG Value;
 
         /* Read the value */
-        if (!Fast486ReadModrmDwordOperands(State, &ModRegRm, &Dummy, &Value))
+        if (!Fast486ReadModrmDwordOperands(State, &ModRegRm, NULL, &Value))
         {
             /* Exception occurred */
             return FALSE;
@@ -1968,10 +2316,10 @@ FAST486_OPCODE_HANDLER(Fast486OpcodeGroup0FBA)
     }
     else
     {
-        USHORT Dummy, Value;
+        USHORT Value;
 
         /* Read the value */
-        if (!Fast486ReadModrmWordOperands(State, &ModRegRm, &Dummy, &Value))
+        if (!Fast486ReadModrmWordOperands(State, &ModRegRm, NULL, &Value))
         {
             /* Exception occurred */
             return FALSE;