diff -uNr firmware-orig/hvmloader/acpi/build.c firmware/hvmloader/acpi/build.c
--- firmware-orig/hvmloader/acpi/build.c	2008-07-10 17:42:53.984872925 +0100
+++ firmware/hvmloader/acpi/build.c	2008-07-10 17:44:52.936306925 +0100
@@ -70,12 +70,7 @@
 
 static int construct_bios_info_table(uint8_t *buf)
 {
-    struct bios_info {
-        uint8_t  com1_present:1;
-        uint8_t  com2_present:1;
-        uint8_t  hpet_present:1;
-        uint32_t pci_min, pci_len;
-    } *bios_info = (struct bios_info *)buf;
+    struct bios_info *bios_info = (struct bios_info *)buf;
 
     memset(bios_info, 0, sizeof(*bios_info));
 
@@ -86,6 +81,7 @@
 
     bios_info->pci_min = PCI_MEMBASE;
     bios_info->pci_len = PCI_MEMSIZE;
+    bios_info->xen_pfiob = 0xdead;
 
     return align16(sizeof(*bios_info));
 }
diff -uNr firmware-orig/hvmloader/config.h firmware/hvmloader/config.h
--- firmware-orig/hvmloader/config.h	2008-07-10 17:42:53.988873175 +0100
+++ firmware/hvmloader/config.h	2008-07-10 15:10:54.480290200 +0100
@@ -31,4 +31,7 @@
 #define ROMBIOS_PHYSICAL_ADDRESS      0x000F0000
 #define SCRATCH_PHYSICAL_ADDRESS      0x00010000
 
+/* Xen Platform Device */
+#define PFFLAG_ROM_LOCK 1 /* Sets whether ROM memory area is RW or RO */
+
 #endif /* __HVMLOADER_CONFIG_H__ */
diff -uNr firmware-orig/hvmloader/hvmloader.c firmware/hvmloader/hvmloader.c
--- firmware-orig/hvmloader/hvmloader.c	2008-07-10 17:42:53.988873175 +0100
+++ firmware/hvmloader/hvmloader.c	2008-07-10 17:44:09.333581925 +0100
@@ -105,6 +105,8 @@
 
 static enum { VGA_none, VGA_std, VGA_cirrus } virtual_vga = VGA_none;
 
+static uint16_t xen_platform_io_base;
+
 static void
 init_hypercalls(void)
 {
@@ -149,6 +151,7 @@
     uint32_t base, devfn, bar_reg, bar_data, bar_sz, cmd;
     uint16_t class, vendor_id, device_id;
     unsigned int bar, pin, link, isa_irq;
+    uint32_t xen_pf_devfn = 0xff;
 
     /* Resources assignable to PCI devices via BARs. */
     struct resource {
@@ -213,6 +216,10 @@
                 pci_writew(devfn, 0x42, 0x8000); /* enable IDE1 */
             }
             break;
+        case 0xff80:
+            if ( (vendor_id == 0x5853) && (device_id == 0x0001) )
+                xen_pf_devfn = devfn;
+            break;
         }
 
         /* Map the I/O memory and port resources. */
@@ -284,6 +291,10 @@
         }
 
         base = (resource->base + bar_sz - 1) & ~(bar_sz - 1);
+        
+        if ( devfn == xen_pf_devfn && resource == &io_resource )
+            xen_platform_io_base = (uint16_t) base;
+
         bar_data |= base;
         base += bar_sz;
 
@@ -527,6 +538,9 @@
                ROMBIOS_PHYSICAL_ADDRESS,
                ROMBIOS_PHYSICAL_ADDRESS + rombios_sz - 1);
 
+    /* Tell rombios the platform device base io port so it can write-protect ROM once its done self-modifying */
+    ((struct bios_info *)ACPI_PHYSICAL_ADDRESS)->xen_pfiob = xen_platform_io_base;
+
     printf("Invoking ROMBIOS ...\n");
     return 0;
 }
diff -uNr firmware-orig/hvmloader/util.h firmware/hvmloader/util.h
--- firmware-orig/hvmloader/util.h	2008-07-10 17:42:54.004874175 +0100
+++ firmware/hvmloader/util.h	2008-07-10 16:18:05.742877675 +0100
@@ -137,6 +137,14 @@
 /* Prepare the 32bit BIOS */
 void highbios_setup(void);
 
+struct bios_info {
+    uint8_t  com1_present:1;
+    uint8_t  com2_present:1;
+    uint8_t  hpet_present:1;
+    uint32_t pci_min, pci_len;
+    uint16_t xen_pfiob;
+};
+
 #define isdigit(c) ((c) >= '0' && (c) <= '9')
 
 #endif /* __HVMLOADER_UTIL_H__ */
diff -uNr firmware-orig/rombios/rombios.c firmware/rombios/rombios.c
--- firmware-orig/rombios/rombios.c	2008-07-10 17:42:54.064877925 +0100
+++ firmware/rombios/rombios.c	2008-07-10 17:35:28.549034925 +0100
@@ -275,6 +275,31 @@
   mov ?1*4+2, ax
 MEND
 
+/* bios_info is the first structure at ACPI_PHYSICAL_ADDRESS(0xEA000)
+   and the offset of .xen_pfiob is 0x0C */
+   
+#define ACPI_PFIOB_OFFSET 0xA00C
+
+MACRO ROM_WRITE_ENABLE
+    push ds
+    mov ax,#((ACPI_PHYSICAL_ADDRESS >> 4) & $F000)
+    mov ds,ax
+    mov ax,#0
+    mov dx,ACPI_PFIOB_OFFSET
+    out dx,al
+    pop ds
+MEND
+
+MACRO ROM_WRITE_DISABLE
+    push ds
+    mov ax,#((ACPI_PHYSICAL_ADDRESS >> 4) & $F000)
+    mov ds,ax
+    mov ax,#1    
+    mov dx,ACPI_PFIOB_OFFSET
+    out dx,al
+    pop ds
+MEND
+
 ASM_END
 
 typedef unsigned char  Bit8u;
@@ -10155,6 +10180,10 @@
   call tcpa_post_part2
 #endif
 
+#ifdef HVMASSIST
+  ROM_WRITE_DISABLE
+#endif 
+
   ;; Start the boot sequence.   See the comments in int19_relocated 
   ;; for why we use INT 18h instead of INT 19h here.
   int  #0x18