diff --git a/scripts/update_asm_instructions.py b/scripts/update_asm_instructions.py
index 9c1f9df..537ceda 100755
--- a/scripts/update_asm_instructions.py
+++ b/scripts/update_asm_instructions.py
@@ -244,10 +244,10 @@ class Instruction(object):
                     raise RuntimeError(msg.format(byte))
 
         for i, cond in enumerate(conds):
-            if types[i] == "register" and cond.startswith("ix"):
-                ret.insert(0, "INST_IX_PREFIX")
-            elif types[i] == "register" and cond.startswith("iy"):
-                ret.insert(0, "INST_IY_PREFIX")
+            if types[i] == "register" and cond[0] == "i":
+                prefix = "INST_I{0}_PREFIX".format(cond[1].upper())
+                if ret[0] != prefix:
+                    ret.insert(0, prefix)
             elif types[i] == "indexed":
                 ret.insert(0, "INST_INDEX_PREFIX({0})".format(i))
                 ret.append("INST_INDEX({0}).offset".format(i))
diff --git a/src/assembler/instructions.inc.c b/src/assembler/instructions.inc.c
index 41bda1a..350716b 100644
--- a/src/assembler/instructions.inc.c
+++ b/src/assembler/instructions.inc.c
@@ -7,7 +7,7 @@
     `make` should trigger a rebuild when it is modified; if not, use:
     `python scripts/update_asm_instructions.py`.
 
-    @AUTOGEN_DATE Mon May 18 04:41:13 2015 UTC
+    @AUTOGEN_DATE Mon May 18 05:16:08 2015 UTC
 */
 
 /* @AUTOGEN_INST_BLOCK_START */
@@ -115,9 +115,9 @@ INST_FUNC(add)
         if (INST_REG(0) == REG_HL && INST_REG(1) == REG_HL)
             INST_RETURN(1, 0x29)
         if (INST_REG(0) == REG_IX && INST_REG(1) == REG_IX)
-            INST_RETURN(3, INST_IX_PREFIX, INST_IX_PREFIX, 0x29)
+            INST_RETURN(2, INST_IX_PREFIX, 0x29)
         if (INST_REG(0) == REG_IY && INST_REG(1) == REG_IY)
-            INST_RETURN(3, INST_IY_PREFIX, INST_IY_PREFIX, 0x29)
+            INST_RETURN(2, INST_IY_PREFIX, 0x29)
         if (INST_REG(0) == REG_HL && INST_REG(1) == REG_SP)
             INST_RETURN(1, 0x39)
         if (INST_REG(0) == REG_IX && INST_REG(1) == REG_SP)
@@ -442,15 +442,15 @@ INST_FUNC(ld)
         if (INST_REG(0) == REG_H && INST_REG(1) == REG_H)
             INST_RETURN(1, 0x64)
         if (INST_REG(0) == REG_IXH && INST_REG(1) == REG_IXH)
-            INST_RETURN(3, INST_IX_PREFIX, INST_IX_PREFIX, 0x64)
+            INST_RETURN(2, INST_IX_PREFIX, 0x64)
         if (INST_REG(0) == REG_IYH && INST_REG(1) == REG_IYH)
-            INST_RETURN(3, INST_IY_PREFIX, INST_IY_PREFIX, 0x64)
+            INST_RETURN(2, INST_IY_PREFIX, 0x64)
         if (INST_REG(0) == REG_H && INST_REG(1) == REG_L)
             INST_RETURN(1, 0x65)
         if (INST_REG(0) == REG_IXH && INST_REG(1) == REG_IXL)
-            INST_RETURN(3, INST_IX_PREFIX, INST_IX_PREFIX, 0x65)
+            INST_RETURN(2, INST_IX_PREFIX, 0x65)
         if (INST_REG(0) == REG_IYH && INST_REG(1) == REG_IYL)
-            INST_RETURN(3, INST_IY_PREFIX, INST_IY_PREFIX, 0x65)
+            INST_RETURN(2, INST_IY_PREFIX, 0x65)
         if (INST_REG(0) == REG_L && INST_REG(1) == REG_A)
             INST_RETURN(1, 0x6F)
         if (INST_REG(0) == REG_IXL && INST_REG(1) == REG_A)
@@ -484,15 +484,15 @@ INST_FUNC(ld)
         if (INST_REG(0) == REG_L && INST_REG(1) == REG_H)
             INST_RETURN(1, 0x6C)
         if (INST_REG(0) == REG_IXL && INST_REG(1) == REG_IXH)
-            INST_RETURN(3, INST_IX_PREFIX, INST_IX_PREFIX, 0x6C)
+            INST_RETURN(2, INST_IX_PREFIX, 0x6C)
         if (INST_REG(0) == REG_IYL && INST_REG(1) == REG_IYH)
-            INST_RETURN(3, INST_IY_PREFIX, INST_IY_PREFIX, 0x6C)
+            INST_RETURN(2, INST_IY_PREFIX, 0x6C)
         if (INST_REG(0) == REG_L && INST_REG(1) == REG_L)
             INST_RETURN(1, 0x6D)
         if (INST_REG(0) == REG_IXL && INST_REG(1) == REG_IXL)
-            INST_RETURN(3, INST_IX_PREFIX, INST_IX_PREFIX, 0x6D)
+            INST_RETURN(2, INST_IX_PREFIX, 0x6D)
         if (INST_REG(0) == REG_IYL && INST_REG(1) == REG_IYL)
-            INST_RETURN(3, INST_IY_PREFIX, INST_IY_PREFIX, 0x6D)
+            INST_RETURN(2, INST_IY_PREFIX, 0x6D)
         if (INST_REG(0) == REG_A && INST_REG(1) == REG_IX)
             INST_RETURN(3, INST_IX_PREFIX, 0xED, 0x57)
         if (INST_REG(0) == REG_A && INST_REG(1) == REG_IY)