diff --git a/reactos/subsystems/mvdm/ntvdm/dos/dos32krnl/himem.h b/reactos/subsystems/mvdm/ntvdm/dos/dos32krnl/himem.h
index eadd7d45bd0..0976ce35556 100644
--- a/reactos/subsystems/mvdm/ntvdm/dos/dos32krnl/himem.h
+++ b/reactos/subsystems/mvdm/ntvdm/dos/dos32krnl/himem.h
@@ -13,8 +13,8 @@
 /* DEFINITIONS ****************************************************************/
 
 #define XMS_ADDRESS     0x110000        // Just above HMA
-#define XMS_BLOCKS      0x3BC0          // XMS_ADDRESS + (XMS_BLOCKS * XMS_BLOCK_SIZE) == 16 MB
 #define XMS_BLOCK_SIZE  1024
+#define XMS_BLOCKS      ((MAX_ADDRESS - XMS_ADDRESS) / XMS_BLOCK_SIZE)
 #define XMS_MAX_HANDLES 16              // Specification: min 1, max 128, default 32
 
 #define XMS_STATUS_SUCCESS              0x00
diff --git a/reactos/subsystems/mvdm/ntvdm/emulator.h b/reactos/subsystems/mvdm/ntvdm/emulator.h
index 56ee6975110..2b2a56c79c4 100644
--- a/reactos/subsystems/mvdm/ntvdm/emulator.h
+++ b/reactos/subsystems/mvdm/ntvdm/emulator.h
@@ -23,6 +23,7 @@
 #define MAX_SEGMENT 0xFFFF
 #define MAX_OFFSET  0xFFFF
 #define MAX_ADDRESS 0x1000000 // 16 MB of RAM; see also: kernel32/client/vdm.c!BaseGetVdmConfigInfo
+C_ASSERT(0x100000 <= MAX_ADDRESS);  // A minimum of 1 MB is required for PC emulation.
 
 #define SEG_OFF_TO_PTR(seg, off)    \
     (PVOID)((ULONG_PTR)BaseAddress + TO_LINEAR((seg), (off)))