diff --git a/system/ohci.c b/system/ohci.c
index e9fe5de..beaacd5 100644
--- a/system/ohci.c
+++ b/system/ohci.c
@@ -21,7 +21,7 @@
 
 // Values defined by the OHCI specification.
 
-// HcRevision register
+// HcControl register
 
 #define OHCI_CTRL_CBSR          0x00000003      // Control Bulk Service Ratio
 #define OHCI_CTRL_CBSR0         0x00000000      // Control Bulk Service Ratio 0
@@ -592,7 +592,7 @@ bool ohci_init(uintptr_t base_addr, usb_hcd_t *hcd)
 
     // Initialise the interrupt ED and TD for each keyboard interface and find the minimum interval.
     int min_interval = OHCI_MAX_INTERVAL;
-    ohci_ed_t *last_kbd_ed = NULL;
+    uint32_t intr_head_ed = 0;
     for (int kbd_idx = 0; kbd_idx < num_keyboards; kbd_idx++) {
         usb_ep_t *kbd = &keyboards[kbd_idx];
 
@@ -604,8 +604,8 @@ bool ohci_init(uintptr_t base_addr, usb_hcd_t *hcd)
         build_ohci_td(kbd_td, OHCI_TD_DP_IN | OHCI_TD_DT_USE_TD | OHCI_TD_DT_0 | OHCI_TD_DI_NO_DLY, kbd_rpt, sizeof(hid_kbd_rpt_t));
         build_ohci_ed(kbd_ed, ohci_ed_control(kbd), kbd_td+0, kbd_td+1);
 
-        kbd_ed->next_ed = (uintptr_t)last_kbd_ed;
-        last_kbd_ed = kbd_ed;
+        kbd_ed->next_ed = intr_head_ed;
+        intr_head_ed = (uintptr_t)kbd_ed;
 
         if (kbd->interval < min_interval) {
             min_interval = kbd->interval;
@@ -614,7 +614,7 @@ bool ohci_init(uintptr_t base_addr, usb_hcd_t *hcd)
 
     // Initialise the interrupt table.
     for (int i = 0; i < OHCI_MAX_INTERVAL; i += min_interval) {
-        ws->hcca.intr_head_ed[i] = (uintptr_t)last_kbd_ed;
+        ws->hcca.intr_head_ed[i] = intr_head_ed;
     }
     write32(&op_regs->control, OHCI_CTRL_HCFS_RUN | OHCI_CTRL_CLE | OHCI_CTRL_PLE | OHCI_CTRL_CBSR0);
     flush32(&op_regs->interrupt_status, ~0);
diff --git a/system/xhci.c b/system/xhci.c
index 972f9d2..8991d3f 100644
--- a/system/xhci.c
+++ b/system/xhci.c
@@ -102,8 +102,7 @@
 
 // Add Context flags
 
-#define XHCI_CONTEXT_A0                 (1 << 0)
-#define XHCI_CONTEXT_A1                 (1 << 1)
+#define XHCI_CONTEXT_A(n)               (1 << (n))
 
 // Port Speed values
 
@@ -737,7 +736,7 @@ static bool assign_address(const usb_hcd_t *hcd, const usb_hub_t *hub, int port_
     // Prepare the input context for the ADDRESS_DEVICE command.
 
     xhci_ctrl_context_t *ctrl_context = (xhci_ctrl_context_t *)ws->input_context_addr;
-    ctrl_context->add_context_flags = XHCI_CONTEXT_A0 | XHCI_CONTEXT_A1;
+    ctrl_context->add_context_flags = XHCI_CONTEXT_A(0) | XHCI_CONTEXT_A(1);
 
     xhci_slot_context_t *slot_context = (xhci_slot_context_t *)(ws->input_context_addr + ws->context_size);
     slot_context->params1 = 1 << 27 | usb_to_xhci_speed(device_speed) << 20;
@@ -828,7 +827,7 @@ static bool configure_interrupt_endpoint(workspace_t *ws, const usb_ep_t *ep, in
     // CONFIGURE_ENDPOINT command before issuing the command.
 
     xhci_ctrl_context_t *ctrl_context = (xhci_ctrl_context_t *)ws->input_context_addr;
-    ctrl_context->add_context_flags = XHCI_CONTEXT_A0 | 1 << ep_id;
+    ctrl_context->add_context_flags = XHCI_CONTEXT_A(0) | XHCI_CONTEXT_A(ep_id);
 
     xhci_slot_context_t *slot_context = (xhci_slot_context_t *)(ws->input_context_addr + ws->context_size);
     slot_context->params1           = ep_id << 27 | hub_flag << 26 | (slot_context->params1 & 0x00ffffff);