diff --git a/src/pciinit.c b/src/pciinit.c
index 71177bc..39c8f0f 100644
--- a/src/pciinit.c
+++ b/src/pciinit.c
@@ -126,16 +126,28 @@
         /* default memory mappings */
         for (i = 0; i < PCI_NUM_REGIONS; i++) {
             int ofs;
-            u32 val, size;
-
+            u32 old, val, mask, size;
             if (i == PCI_ROM_SLOT)
                 ofs = PCI_ROM_ADDRESS;
             else
                 ofs = PCI_BASE_ADDRESS_0 + i * 4;
-            pci_config_writel(bdf, ofs, 0xffffffff);
+
+            old = pci_config_readl(bdf, ofs);
+            if (i == PCI_ROM_SLOT) {
+                mask = PCI_ROM_ADDRESS_MASK;
+                pci_config_writel(bdf, ofs, mask);
+            } else {
+                if (val & PCI_BASE_ADDRESS_SPACE_IO)
+                    mask = PCI_BASE_ADDRESS_IO_MASK;
+                else
+                    mask = PCI_BASE_ADDRESS_MEM_MASK;
+                pci_config_writel(bdf, ofs, ~0);
+            }
             val = pci_config_readl(bdf, ofs);
+            pci_config_writel(bdf, ofs, old);
+
             if (val != 0) {
-                size = (~(val & ~0xf)) + 1;
+                size = (~(val & mask)) + 1;
                 if (val & PCI_BASE_ADDRESS_SPACE_IO)
                     paddr = &pci_bios_io_addr;
                 else if (size >= 0x04000000)