diff --git a/bios/vxtx.asm b/bios/vxtx.asm
index 0324905b..5577e3d0 100644
--- a/bios/vxtx.asm
+++ b/bios/vxtx.asm
@@ -54,6 +54,17 @@ int_13_handler:
     pop ax
 
     iret
+    
+int_15_handler:
+    cmp ah, 0x88
+    jne .error
+    mov ax, 64 ; Extended memory in KB.
+    clc
+    iret
+.error:
+    mov ah, 0x86
+    stc
+    iret
 
 int_19_handler:
     push bp
@@ -77,6 +88,9 @@ install_handlers:
     mov word [INT_13_OFFSET], int_13_handler
     mov [INT_13_OFFSET+2], cs
 
+    mov word [INT_15_OFFSET], int_15_handler
+    mov [INT_15_OFFSET+2], cs
+    
     mov word [INT_19_OFFSET], int_19_handler
     mov [INT_19_OFFSET+2], cs
     
@@ -95,6 +109,7 @@ install_handlers:
 ;;;;;;;;;;;;;;;;;; Data ;;;;;;;;;;;;;;;;;;
 
 INT_13_OFFSET  equ 0x4C
+INT_15_OFFSET  equ 0x54
 INT_19_OFFSET  equ 0x64
 
 db 'VXTX - VirtualXT BIOS Extensions', 0xA
diff --git a/bios/vxtx.bin b/bios/vxtx.bin
index faa2daf6..954a04f7 100644
Binary files a/bios/vxtx.bin and b/bios/vxtx.bin differ
diff --git a/lib/vxt/system.c b/lib/vxt/system.c
index 75337ff3..7b8d9596 100644
--- a/lib/vxt/system.c
+++ b/lib/vxt/system.c
@@ -381,7 +381,7 @@ VXT_API vxt_byte vxt_system_read_byte(CONSTP(vxt_system) s, vxt_pointer addr) {
 		
 	if (addr >= 0x100000) {
 		addr -= 0x100000;
-		return (addr >= UMA_SIZE) ? 0xFF : s->ext_mem[addr];
+		return (addr >= EXT_MEM_SIZE) ? 0xFF : s->ext_mem[addr];
 	}
 
 	CONSTSP(vxt_peripheral) dev = s->devices[s->mem_map[addr >> 4]];
@@ -394,7 +394,7 @@ VXT_API void vxt_system_write_byte(CONSTP(vxt_system) s, vxt_pointer addr, vxt_b
 		
 	if (addr >= 0x100000) {
 		addr -= 0x100000;
-		if (addr < UMA_SIZE)
+		if (addr < EXT_MEM_SIZE)
 			s->ext_mem[addr] = data;
 		return;
 	}
diff --git a/lib/vxt/system.h b/lib/vxt/system.h
index 4a562c51..0ab58903 100644
--- a/lib/vxt/system.h
+++ b/lib/vxt/system.h
@@ -29,9 +29,11 @@
 
 #define MAX_TIMERS 256
 #define INT64 long long
-#define UMA_SIZE (0x10000 - 16)
 #define PERIPHERAL_SIGNATURE 0xFAF129C3
 
+ // Set this to 64K (not 64-16) because anything less causes problem with himem.sys.
+#define EXT_MEM_SIZE 0x10000
+
 #define VERIFY_PERIPHERAL(p, r)										\
 	if (((struct peripheral*)(p))->sig != PERIPHERAL_SIGNATURE) {	\
 		VXT_LOG("Invalid peripheral!");								\
@@ -61,7 +63,7 @@ struct system {
 
    vxt_byte io_map[VXT_IO_MAP_SIZE];
    vxt_byte mem_map[VXT_MEM_MAP_SIZE];
-   vxt_byte ext_mem[UMA_SIZE];
+   vxt_byte ext_mem[EXT_MEM_SIZE];
 
    vxt_allocator *alloc;
    struct cpu cpu;