Dynamically allocate the second half of the SFT table, zero memory to…

… avoid problems. Windows 3.1 is still pefectly happy with it

CHANGELOG

@@ -1,4 +1,9 @@
 Next:
+  - DOS kernel: Remove fixed hacks for SFT and dynamically allocate the
+    second SFT table for the last N - 5 file handles. Add code to zero
+    SFT table memory to ensure that random data doesn't cause issues.
+    Windows 3.1 is still perfectly fine with it, so the SFT table update
+    and cleanup is complete. (joncampbell123).
   - DOS kernel: Move the magic "CON" strings into the first SFT and
     mimic the 5 default open file handles that every DOS process starts
     with: CON CON CON AUX PRN. Windows 3.1 is still happy with it, which

src/dos/dos_classes.cpp

@@ -126,7 +126,7 @@ void DOS_InfoBlock::SetLocation(uint16_t segment) {
 	sSave(sDIB,nulString[6],(uint8_t)0x20);
 	sSave(sDIB,nulString[7],(uint8_t)0x20);
 
-#if 1 // FIXME: The new code imported from DOSBox Staging is causing Windows 3.1 to crash on exit, so this old code remains active until that can be fixed
+#if 1
 	/* Create a fake SFT, so programs think there are 100 file handles.
 	 * NTS: The INFO block is only given 0x20 paragraphs (0x200 = 512 bytes) which is only enough for a 5-entry SFT table,
 	 *      however that's what MS-DOS does anyway. For any value of FILES you specify, the first SFT is 5 entries and the
@@ -136,9 +136,16 @@ void DOS_InfoBlock::SetLocation(uint16_t segment) {
 	 *      The table must be 5, or else it extends into other DOS kernel structures and memory corruption might happen. */
 	if (DOS_FILES < 6) E_Exit("[dos] files= setting is too small");
 
+	/* Secondary table */
+	unsigned int tbl2_seg = DOS_GetMemory((SftHeaderSize+(SftEntrySize*(DOS_FILES-5))+0xFu)/16u,"SFT secondary table");
+
 	uint16_t sftOffset=0xCC; /* this offset is fixed according to MS-DOS 5.0, MS-DOS 6.22, Windows 95, etc. */
+
+	/* zero out the table */
+	for (unsigned int i=sftOffset;i < 0x200;i++) real_writeb(segment,i,0);
+
 	sSave(sDIB,firstFileTable,RealMake(segment,sftOffset));
-	real_writed(segment,sftOffset+0x00,RealMake(segment+0x26,0));	//Next File Table
+	real_writed(segment,sftOffset+0x00,RealMake(tbl2_seg,0));	//Next File Table
 	real_writew(segment,sftOffset+0x04,5);				//File Table supports 5 files
 
 	if ((sftOffset+SftHeaderSize+(5*SftEntrySize)) > 0x200) E_Exit("Primary SFT is too big");
@@ -156,10 +163,10 @@ void DOS_InfoBlock::SetLocation(uint16_t segment) {
 	real_writed(segment,sftOffset+SftHeaderSize+(SftEntrySize*3)+0x20,0x20585541); // AUX
 	real_writed(segment,sftOffset+SftHeaderSize+(SftEntrySize*4)+0x20,0x204e5250); // PRN
 
-	// FIXME: This must be segment+0x26 (DOS_CONSTRING_SEG-2) because some magic "CON" strings must be written there
-	//        to make Windows 3.1 happy.
-	real_writed(segment+0x26,0x00,0xffffffff);			//Last File Table
-	real_writew(segment+0x26,0x04,DOS_FILES-5);			//File Table supports DOS_FILES-5 files
+	/* zero out the table */
+	for (unsigned int i=0;i < (SftHeaderSize+(SftEntrySize*(DOS_FILES-5)));i++) real_writeb(tbl2_seg,i,0);
+	real_writed(tbl2_seg,0x00,0xFFFFFFFFu);
+	real_writew(tbl2_seg,0x04,DOS_FILES-5);
 
 #else
     /* Imported from dosbox-staging/dosbox-staging#3680 */