Cannot use "g" as an arg constraint because

some memory references aren't actually valid and gcc is a bit too smart in parsing (%0) and a bit too dumb in knowing which memory refs are not valid. Using "r" seems to constrain gcc to use a register as in (%eax) and not substitute a memory reference like (-36(%ebp)). No one really understands these things.

Cannot use "g" as an arg constraint because
3163085a · rsc · 5e03dd86 · 3163085a
--- a/x86.h
+++ b/x86.h
@@ -50,7 +50,7 @@ lgdt(struct segdesc *p, int size)
  pd[1] = (uint)p;
  pd[2] = (uint)p >> 16;

-  asm volatile("lgdt (%0)" : : "g" (pd));
+  asm volatile("lgdt (%0)" : : "r" (&pd));
 }

 struct gatedesc;