[aandrejevic] 67647: [FAST486] The LMSW instruction doesn't #GP when the program tries to return to real mode. Rather, it just doesn't happen (the PE bit remains 1).

Author: aandrejevic
Date: Sun May 10 23:50:33 2015
New Revision: 67647

URL: http://svn.reactos.org/svn/reactos?rev=67647&view=rev
Log:
[FAST486]
The LMSW instruction doesn't #GP when the program tries to return to real mode.
Rather, it just doesn't happen (the PE bit remains 1).


Modified:
    trunk/reactos/lib/fast486/opgroups.c

Modified: trunk/reactos/lib/fast486/opgroups.c
URL:
http://svn.reactos.org/svn/reactos/trunk/reactos/lib/fast486/opgroups.c?rev…
==============================================================================
--- trunk/reactos/lib/fast486/opgroups.c	[iso-8859-1] (original)
+++ trunk/reactos/lib/fast486/opgroups.c	[iso-8859-1] Sun May 10 23:50:33 2015
@@ -2239,11 +2239,14 @@
         {
             USHORT MachineStatusWord;
 
-            /* This is a privileged instruction */
-            if (Fast486GetCurrentPrivLevel(State) != 0)
-            {
-                Fast486Exception(State, FAST486_EXCEPTION_GP);
-                return;
+            if (State->ControlRegisters[FAST486_REG_CR0] & FAST486_CR0_PE)
+            {
+                /* This is a privileged instruction */
+                if (Fast486GetCurrentPrivLevel(State) != 0)
+                {
+                    Fast486Exception(State, FAST486_EXCEPTION_GP);
+                    return;
+                }
             }
 
             /* Read the new Machine Status Word */
@@ -2253,16 +2256,8 @@
                 return;
             }
 
-            /* This instruction cannot be used to return to real mode */
-            if ((State->ControlRegisters[FAST486_REG_CR0] & FAST486_CR0_PE)
-                && !(MachineStatusWord & FAST486_CR0_PE))
-            {
-                Fast486Exception(State, FAST486_EXCEPTION_GP);
-                return;
-            }
-
-            /* Set the lowest 4 bits */
-            State->ControlRegisters[FAST486_REG_CR0] &= 0xFFFFFFF0;
+            /* Set the lowest 4 bits, but never clear bit 0 */
+            State->ControlRegisters[FAST486_REG_CR0] &= 0xFFFFFFF1;
             State->ControlRegisters[FAST486_REG_CR0] |= MachineStatusWord & 0x0F;
 
             break;