diff --git a/hw/pci.c b/hw/pci.c
index 41e99a9..f9959fc 100644
--- a/hw/pci.c
+++ b/hw/pci.c
@@ -541,19 +541,26 @@ uint32_t pci_default_read_config(PCIDevice *d,
 
 void pci_default_write_config(PCIDevice *d, uint32_t addr, uint32_t val, int l)
 {
-    uint8_t orig[PCI_CONFIG_SPACE_SIZE];
+    uint8_t orig[PCI_CONFIG_HEADER_SIZE];
     int i;
 
-    /* not efficient, but simple */
-    memcpy(orig, d->config, PCI_CONFIG_SPACE_SIZE);
-    for(i = 0; i < l && addr < PCI_CONFIG_SPACE_SIZE; val >>= 8, ++i, ++addr) {
-        uint8_t wmask = d->wmask[addr];
-        d->config[addr] = (d->config[addr] & ~wmask) | (val & wmask);
+    /* not efficient, but simple.  If modifying the header, save it so we
+       can compare its contents later.  */
+    if (addr < sizeof orig) {
+        memcpy(orig, d->config, sizeof orig);
+    }
+
+    for(i = 0; i < l && addr+i < PCI_CONFIG_SPACE_SIZE; val >>= 8, ++i) {
+        uint8_t wmask = d->wmask[addr+i];
+        d->config[addr+i] = (d->config[addr+i] & ~wmask) | (val & wmask);
+    }
+
+    if (addr < sizeof orig) {
+        if (memcmp(orig + PCI_BASE_ADDRESS_0, d->config + PCI_BASE_ADDRESS_0, 24)
+            || ((orig[PCI_COMMAND] ^ d->config[PCI_COMMAND])
+                & (PCI_COMMAND_MEMORY | PCI_COMMAND_IO)))
+            pci_update_mappings(d);
     }
-    if (memcmp(orig + PCI_BASE_ADDRESS_0, d->config + PCI_BASE_ADDRESS_0, 24)
-        || ((orig[PCI_COMMAND] ^ d->config[PCI_COMMAND])
-            & (PCI_COMMAND_MEMORY | PCI_COMMAND_IO)))
-        pci_update_mappings(d);
 }
 
 void pci_data_write(void *opaque, uint32_t addr, uint32_t val, int len)