INT 21h: Add option whether to assign MCB ownership or return an erro…

…r if a DOS program attempts to resize a freed memory block. Fix DOS resize to assign ownership even if the MCB is already the size requested to resize to.

CHANGELOG

@@ -1,4 +1,12 @@
 Next:
+  - INT 21h: If a DOS program frees a memory block, and then resizes the
+    freed memory block, reassign ownership of that block to the program
+    as if allocated. This is apparently canonical MS-DOS behavior. Added
+    dosbox.conf option to control whether resizing a freed block silently
+    assigns ownership (default setting) or whether it returns an error.
+    DOS resize memory function for the most part DID assign ownership but
+    not in the case where the size of the MCB was exactly the size requested
+    to resize to. (joncampbell123).
   - Sound Blaster: Fix bug where "force autoinit" prevented Sound Blaster
     playback from working at all, fix for "Jump" by Public NMI (joncampbell123).
   - Debugger UI now shows PIC_FullIndex() and whether or not the CPU is

src/dos/dos.cpp

@@ -140,6 +140,7 @@ bool enable_dbcs_tables = true;
 bool enable_share_exe = true;
 bool enable_filenamechar = true;
 bool shell_keyboard_flush = true;
+bool freed_mcb_allocate_on_resize = true;
 bool enable_network_redirector = true;
 bool force_conversion = false;
 bool hidenonrep = true;
@@ -4110,6 +4111,7 @@ class DOS:public Module_base{
 #endif
 
         dos_sda_size = section->Get_int("dos sda size");
+        freed_mcb_allocate_on_resize = section->Get_bool("resized free memory block becomes allocated");
         shell_keyboard_flush = section->Get_bool("command shell flush keyboard buffer");
 		enable_network_redirector = section->Get_bool("network redirector");
 		enable_dbcs_tables = section->Get_bool("dbcs");

src/dos/dos_memory.cpp

@@ -333,6 +333,7 @@ bool DOS_AllocateMemory(uint16_t * segment,uint16_t * blocks) {
 	return false;
 }
 
+extern bool freed_mcb_allocate_on_resize;
 
 bool DOS_ResizeMemory(uint16_t segment,uint16_t * blocks) {
 	if (segment < DOS_MEM_START+1) {
@@ -349,6 +350,13 @@ bool DOS_ResizeMemory(uint16_t segment,uint16_t * blocks) {
 		return false;
 	}
 
+	if (mcb.GetPSPSeg()==MCB_FREE && !freed_mcb_allocate_on_resize) {
+		*blocks=0;
+		DOS_SetError(DOSERR_MCB_DESTROYED);
+		LOG(LOG_DOSMISC,LOG_DEBUG)("DOS application attempted to resize freed memory block at segment 0x%x",(unsigned int)segment);
+		return false;
+	}
+
 	uint16_t total=mcb.GetSize();
 	DOS_MCB	mcb_next(segment+total);
 
@@ -359,7 +367,10 @@ bool DOS_ResizeMemory(uint16_t segment,uint16_t * blocks) {
 
 	if (*blocks<=total) {
 		if (GCC_UNLIKELY(*blocks==total)) {
-			/* Nothing to do */
+			/* Nothing to do, however if the block is freed, canonical MS-DOS behavior is to assign your PSP segment as if allocated (Incentiv by DID, party '94) */
+			if (mcb.GetPSPSeg()==MCB_FREE && freed_mcb_allocate_on_resize)
+				mcb.SetPSPSeg(dos.psp());
+
 			return true;
 		}
 		/* Shrinking MCB */
@@ -405,6 +416,7 @@ bool DOS_ResizeMemory(uint16_t segment,uint16_t * blocks) {
 		/* adjust type of joined MCB */
 		mcb.SetType(mcb_next.GetType());
 	}
+
 	mcb.SetSize(total);
 	mcb.SetPSPSeg(dos.psp());
 	if (*blocks==total) return true;	/* block fit exactly */

src/dosbox.cpp

@@ -4229,6 +4229,10 @@ void DOSBOX_SetupConfigSections(void) {
     Pbool->Set_help("Enable XMS support.");
     Pbool->SetBasic(true);
 
+    Pbool = secprop->Add_bool("resized free memory block becomes allocated",Property::Changeable::WhenIdle,true);
+    Pbool->Set_help("If set, and the DOS application resizes a freed memory block, that block will be marked as allocated to that program.\n"
+		    "MS-DOS behaves in this manner, apparently.");
+
     Pint = secprop->Add_int("xms handles",Property::Changeable::WhenIdle,0);
     Pint->Set_help("Number of XMS handles available for the DOS environment, or 0 to use a reasonable default");
     Pint->SetBasic(true);