diff --git a/macaw-ppc/src/Data/Macaw/PPC/Disassemble.hs b/macaw-ppc/src/Data/Macaw/PPC/Disassemble.hs
index 71459ea3..e9eafcb9 100644
--- a/macaw-ppc/src/Data/Macaw/PPC/Disassemble.hs
+++ b/macaw-ppc/src/Data/Macaw/PPC/Disassemble.hs
@@ -68,7 +68,7 @@ readInstruction mem addr = MM.addrWidthClass (MM.memAddrWidth mem) $ do
               Nothing -> ET.throwError (MM.InvalidInstruction (MM.relativeSegmentAddr addr) contents)
 
 disassembleBlock :: forall ppc s
-                  . (ArchWidth ppc, ArchReg ppc ~ PPCReg ppc, MM.MemWidth (ArchAddrWidth ppc))
+                  . (PPCWidth ppc)
                  => (Value ppc s (BVType (ArchAddrWidth ppc)) -> D.Instruction -> Maybe (PPCGenerator ppc s ()))
                  -> MM.Memory (ArchAddrWidth ppc)
                  -> GenState ppc s
@@ -81,9 +81,6 @@ disassembleBlock lookupSemantics mem gs curIPAddr maxOffset = do
   case readInstruction mem curIPAddr of
     Left err -> failAt gs off curIPAddr (DecodeError err)
     Right (i, bytesRead) -> do
-      -- let nextIP = MM.relativeAddr seg (off + bytesRead)
-      -- let nextIPVal = MC.RelocatableValue undefined nextIP
-
       -- Note: In PowerPC, the IP is incremented *after* an instruction
       -- executes, rather than before as in X86.  We have to pass in the
       -- physical address of the instruction here.
@@ -93,9 +90,6 @@ disassembleBlock lookupSemantics mem gs curIPAddr maxOffset = do
       case lookupSemantics ipVal i of
         Nothing -> failAt gs off curIPAddr (UnsupportedInstruction i)
         Just transformer -> do
-          -- FIXME: Do we need to add failure modes here?  Probably.  There are
-          -- some invalid encodings that we could encounter that would be worth
-          -- failing for.
           egs1 <- liftST $ execGenerator gs $ do
             let line = printf "%s: %s" (show curIPAddr) (show (D.ppInstruction i))
             addStmt (Comment (T.pack  line))
@@ -104,7 +98,7 @@ disassembleBlock lookupSemantics mem gs curIPAddr maxOffset = do
             Left genErr -> failAt gs off curIPAddr (GenerationError i genErr)
             Right gs1 -> undefined
 
-tryDisassembleBlock :: (ArchWidth ppc, ArchReg ppc ~ PPCReg ppc, MM.MemWidth (ArchAddrWidth ppc), KnownNat (RegAddrWidth (PPCReg ppc)))
+tryDisassembleBlock :: (PPCWidth ppc)
                     => (Value ppc s (BVType (ArchAddrWidth ppc)) -> D.Instruction -> Maybe (PPCGenerator ppc s ()))
                     -> MM.Memory (ArchAddrWidth ppc)
                     -> NC.NonceGenerator (ST s) s
@@ -126,7 +120,7 @@ tryDisassembleBlock lookupSemantics mem nonceGen startAddr maxSize = do
 --
 -- Return a list of disassembled blocks as well as the total number of bytes
 -- occupied by those blocks.
-disassembleFn :: (ArchWidth ppc, ArchReg ppc ~ PPCReg ppc, MM.MemWidth (RegAddrWidth (ArchReg ppc)), KnownNat (RegAddrWidth (PPCReg ppc)))
+disassembleFn :: (PPCWidth ppc)
               => proxy ppc
               -> (Value ppc s (BVType (ArchAddrWidth ppc)) -> D.Instruction -> Maybe (PPCGenerator ppc s ()))
               -- ^ A function to look up the semantics for an instruction.  The
diff --git a/macaw-ppc/src/Data/Macaw/PPC/PPCReg.hs b/macaw-ppc/src/Data/Macaw/PPC/PPCReg.hs
index 4ff3a0cf..ba28be47 100644
--- a/macaw-ppc/src/Data/Macaw/PPC/PPCReg.hs
+++ b/macaw-ppc/src/Data/Macaw/PPC/PPCReg.hs
@@ -1,3 +1,4 @@
+{-# LANGUAGE ConstraintKinds #-}
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE GADTs #-}
@@ -12,6 +13,7 @@
 module Data.Macaw.PPC.PPCReg (
   PPCReg(..),
   linuxSystemCallPreservedRegisters,
+  PPCWidth,
   ArchWidth(..)
   ) where
 
@@ -99,8 +101,13 @@ instance (ArchWidth ppc) => HasRepr (PPCReg ppc) TypeRepr where
       PPC_CTR -> BVTypeRepr (pointerNatRepr (Proxy @ppc))
       PPC_CR -> BVTypeRepr n32
 
-instance (MM.MemWidth w, ArchWidth ppc, w ~ MC.RegAddrWidth (PPCReg ppc), 1 <= w)
-       => MC.RegisterInfo (PPCReg ppc) where
+type PPCWidth ppc = (ArchWidth ppc,
+                     MC.ArchReg ppc ~ PPCReg ppc,
+                     MM.MemWidth (MC.RegAddrWidth (MC.ArchReg ppc)),
+                     1 <= MC.RegAddrWidth (PPCReg ppc),
+                     KnownNat (MC.RegAddrWidth (PPCReg ppc)))
+
+instance (PPCWidth ppc) => MC.RegisterInfo (PPCReg ppc) where
   archRegs = ppcRegs
   sp_reg = PPC_GP (D.GPR 1)
   ip_reg = PPC_IP