diff --git a/libudis86/decode.c b/libudis86/decode.c
index 2e147de..e179a36 100644
--- a/libudis86/decode.c
+++ b/libudis86/decode.c
@@ -277,21 +277,23 @@ modrm(struct ud * u)
 
 
 static unsigned int
-resolve_operand_size( const struct ud * u, unsigned int s )
+resolve_operand_size(const struct ud* u, enum ud_operand_size osize)
 {
-    switch ( s ) 
-    {
-    case SZ_V:
-        return ( u->opr_mode );
-    case SZ_Z:  
-        return ( u->opr_mode == 16 ) ? 16 : 32;
-    case SZ_Y:
-        return ( u->opr_mode == 16 ) ? 32 : u->opr_mode;
-    case SZ_RDQ:
-        return ( u->dis_mode == 64 ) ? 64 : 32;
-    default:
-        return s;
-    }
+  switch (osize) {
+  case SZ_V:
+    return u->opr_mode;
+  case SZ_Z:  
+    return u->opr_mode == 16 ? 16 : 32;
+  case SZ_Y:
+    return u->opr_mode == 16 ? 32 : u->opr_mode;
+  case SZ_RDQ:
+    return u->dis_mode == 64 ? 64 : 32;
+  case SZ_X:
+    UD_ASSERT(u->vex_op != 0);
+    return (P_VEXL(u->itab_entry->prefix) && vex_l(u)) ?  SZ_QQ : SZ_DQ;
+  default:
+    return osize;
+  }
 }
 
 
@@ -391,8 +393,7 @@ decode_reg(struct ud *u,
     case REGCLASS_GPR : reg = decode_gpr(u, size, num); break;
     case REGCLASS_MMX : reg = UD_R_MM0  + (num & 7); break;
     case REGCLASS_XMM :
-      reg = ((u->vex_op != 0 && P_VEXL(u->itab_entry->prefix) && vex_l(u)) 
-              ? UD_R_YMM0 : UD_R_XMM0) + num;
+      reg = num + (size == SZ_QQ ? UD_R_YMM0 : UD_R_XMM0);
       break;
     case REGCLASS_CR : reg = UD_R_CR0  + num; break;
     case REGCLASS_DB : reg = UD_R_DR0  + num; break;
diff --git a/libudis86/decode.h b/libudis86/decode.h
index 1d3c07d..338ab26 100644
--- a/libudis86/decode.h
+++ b/libudis86/decode.h
@@ -119,8 +119,11 @@ enum ud_operand_size {
     SZ_Q   = 64,
     SZ_T   = 80,
     SZ_O   = 128,
+    SZ_DQ  = 128, /* double quad */
+    SZ_QQ  = 256, /* quad quad */
 
     SZ_Y   = 17,
+    SZ_X   = 18,
 
     /*
      * complex size types, that encode sizes for operands