diff --git a/src/hulk/usb/xhci.d b/src/hulk/usb/xhci.d
index 5df6a61..1eaf5ef 100644
--- a/src/hulk/usb/xhci.d
+++ b/src/hulk/usb/xhci.d
@@ -16,19 +16,105 @@ struct XHCI
 {
     /**
      * Located at PCI base address.
+     *
+     * All are read-only.
      */
     struct CapabilityRegisters
     {
+        /**
+         * Offset from beginning of capability registers to beginning of
+         * operational registers.
+         */
         ubyte capability_length;
+
         ubyte _reserved01;
+
+        /**
+         * Interface version number supported (psuedo-BSD).
+         */
         ushort hci_version;
-        uint[3] hcs_params;
+
+        /**
+         * 7:0 number of device slots (MaxSlots)
+         * 18:8 number of interrupters (MaxIntrs)
+         * 23:19 reserved
+         * 31:24 number of ports (MaxPorts)
+         */
+        uint hcs_params1;
+
+        /**
+         * 3:0 isochronous scheduling threshold (IST)
+         * 7:4 event ring segment table max (ERST Max)
+         * 20:8 reserved
+         * 25:21 max scratchpad buffers (high 5 bits; xHCI v1.1+)
+         * 26 scratchpad restore (SPR)
+         * 31:27 max scratchpad buffers (low 5 bits)
+         */
+        uint hcs_params2;
+
+        /**
+         * 7:0 U1 device exit latency
+         * 15:8 reserved
+         * 31:16 U2 device exit latency
+         */
+        uint hcs_params3;
+
+        /**
+         * 0 64-bit addressing capability (AC64)
+         * 1 BW negotiation capability (BNC)
+         * 2 context size (CSZ)
+         * 3 port power control (PPC)
+         * 4 port indicators (PIND)
+         * 5 light HC reset capability (LHRC)
+         * 6 latency tolerance messaging capability (LTC)
+         * 7 no secondary SID support (NSS)
+         * 8 parse all event data (PAE; xHCI v1.1+)
+         * 9 stopped - short packet capability (SPC; xHCI v1.1+)
+         * 10 stopped EDTLA capability (SEC; xHCI v1.1+)
+         * 11 continuous frame ID capability (CFC; xHCI v1.1+)
+         * 15:12 maximum primary stream array size (MaxPSASize)
+         * 31:16 xHCI extended capabilities pointer (xECP)
+         *   offset in 32-bit words from base of capability registers to base
+         *   of capabilities list
+         */
         uint hcc_params1;
+
+        /**
+         * 1:0 reserved
+         * 31:2 doorbell array offset in 32-bit words
+         *   offset in 32-bit words from base of capability registers to the
+         *   doorbell array
+         */
         uint doorbell_offset;
+
+        /**
+         * 4:0 reserved
+         * 31:5 runtime register space offset
+         */
         uint rts_offset;
+
+        /**
+         * 0 U3 entry capability (U3C)
+         * 1 ConfigEP command max exit latency too large (CMC)
+         * 2 force save context capability (FSC)
+         * 3 compliance transition capability (CTC)
+         * 4 large ESIT payload capability (LEC)
+         * 5 configuration information capability (CIC)
+         * 6 extended TBC capability (ETC)
+         * 7 extended TBC TRB status capability (ETC_TSC)
+         * 8 get/set extended property capability (GSC; xHCI v1.1+)
+         * 9 virtualization based trusted I/O capability (VTC; xHCI v1.2+)
+         * 31:10 reserved
+         */
         uint hcc_params2;
+
+        /**
+         * 11:0 reserved
+         * 31:12 VTIO register space offset (xHCI v1.2+)
+         */
+        uint vtios_offset;
     }
-    static assert(CapabilityRegisters.sizeof == 0x20u);
+    static assert(CapabilityRegisters.sizeof == 0x24u);
 
     /**
      * Located at offset capability_length from PCI base address.