diff --git a/src/hulk/usb/xhci.d b/src/hulk/usb/xhci.d
index c7ecde2..8792e2f 100644
--- a/src/hulk/usb/xhci.d
+++ b/src/hulk/usb/xhci.d
@@ -1,3 +1,11 @@
+/**
+ * eXtensible Host Controller Interface for Universal Serial Bus
+ *
+ * Documentation:
+ * - https://wiki.osdev.org/EXtensible_Host_Controller_Interface
+ * - https://www.intel.com/content/dam/www/public/us/en/documents/technical-specifications/extensible-host-controler-interface-usb-xhci.pdf
+ */
+
 module hulk.usb.xhci;
 
 import hulk.pci;
@@ -5,6 +13,67 @@ import hulk.hurl.a1;
 
 struct XHCI
 {
+    /**
+     * Located at PCI base address.
+     */
+    struct CapabilityRegisters
+    {
+        ubyte capability_length;
+        ubyte _reserved01;
+        ushort hci_version;
+        uint[3] hcs_params;
+        uint hcc_params1;
+        uint doorbell_offset;
+        uint rts_offset;
+        uint hcc_params2;
+    }
+    static assert(CapabilityRegisters.sizeof == 0x20u);
+
+    /**
+     * Located at offset capability_length from PCI base address.
+     */
+    struct OperationalRegisters
+    {
+        uint usb_command;
+        uint usb_status;
+        uint page_size;
+        uint[2] _reserved01;
+        uint dn_ctrl;
+        ulong cr_ctrl;
+        uint[4] _reserved02;
+        ulong dcbaap;
+        uint config;
+        uint _reserved03;
+    }
+    static assert(OperationalRegisters.sizeof == 0x40u);
+
+    /**
+     * Located at offset 0x400 from start of Operational Registers.
+     */
+    struct PortRegisters
+    {
+        uint portsc;
+        uint portpmsc;
+        uint portli;
+        uint porthlpmc;
+    }
+    static assert(PortRegisters.sizeof == 0x10u);
+
+    struct RuntimeRegisters
+    {
+        uint mfindex;
+        uint[7] _reserved01;
+        uint ir;
+    }
+
+    struct DoorbellRegister
+    {
+        ubyte target;
+        ubyte _reserved01;
+        ushort task_id;
+    }
+    static assert(DoorbellRegister.sizeof == 4u);
+
     void initialize(Pci.Device * pci_device)
     {
     }