diff --git a/arch/src/x86_64/acpi.rs b/arch/src/x86_64/acpi.rs
index aee840d6a..60e54ccd0 100644
--- a/arch/src/x86_64/acpi.rs
+++ b/arch/src/x86_64/acpi.rs
@@ -306,7 +306,7 @@ pub fn create_acpi_tables(
 
     // 32-bit PCI enhanced configuration mechanism
     mcfg.append(PCIRangeEntry {
-        base_address: layout::MEM_32BIT_DEVICES_START.0,
+        base_address: layout::PCI_MMCONFIG_START.0,
         segment: 0,
         start: 0,
         end: 0xff,
diff --git a/arch/src/x86_64/layout.rs b/arch/src/x86_64/layout.rs
index bbf112f57..735a9eba8 100644
--- a/arch/src/x86_64/layout.rs
+++ b/arch/src/x86_64/layout.rs
@@ -69,9 +69,14 @@ pub const MEM_32BIT_RESERVED_SIZE: GuestUsize = (1024 << 20);
 
 // == Fixed constants within the "32-bit reserved" range ==
 
-// Sub range: 32-bit PCI devices (start: 3GiB, length: 768Mib)
+// Sub range: 32-bit PCI devices (start: 3GiB, length: 640Mib)
 pub const MEM_32BIT_DEVICES_START: GuestAddress = MEM_32BIT_RESERVED_START;
-pub const MEM_32BIT_DEVICES_SIZE: GuestUsize = (768 << 20);
+pub const MEM_32BIT_DEVICES_SIZE: GuestUsize = (640 << 20);
+
+// PCI MMCONFIG space (start: after the device space, length: 256MiB)
+pub const PCI_MMCONFIG_START: GuestAddress =
+    GuestAddress(MEM_32BIT_DEVICES_START.0 + MEM_32BIT_DEVICES_SIZE);
+pub const PCI_MMCONFIG_SIZE: GuestUsize = (256 << 20);
 
 // IOAPIC
 pub const IOAPIC_START: GuestAddress = GuestAddress(0xfec0_0000);