diff --git a/CHERICC_Fat.bsv b/CHERICC_Fat.bsv
index 6484ecf..5d69b6a 100644
--- a/CHERICC_Fat.bsv
+++ b/CHERICC_Fat.bsv
@@ -928,6 +928,7 @@ instance CHERICap #(CapMem, OTypeW, FlagsW, CapAddressW, CapW, TSub#(MW, 3));
     capMem.address = address;
     return pack(capMem);
   endfunction
+  function addAddrUnsafe (cap, inc) = setAddrUnsafe(cap, getAddr(cap) + signExtend(inc));
   function getOffset = error("getOffset not implemented for CapMem");
   function modifyOffset = error("modifyOffset not implemented for CapMem");
   function getBase = error("getBase not implemented for CapMem");
@@ -935,7 +936,7 @@ instance CHERICap #(CapMem, OTypeW, FlagsW, CapAddressW, CapW, TSub#(MW, 3));
   function getLength = error("getLength not implemented for CapMem");
   function isInBounds = error("isInBounds not implemented for CapMem");
   function setBoundsCombined = error("setBoundsCombined not implemented for CapMem");
-  function nullWithAddr(addr) = setAddrUnsafe(packCap(null_cap), addr);
+  function nullWithAddr(Bit#(CapAddressW) addr) = setAddrUnsafe(packCap(null_cap), addr);
   function almightyCap;
     CapReg res = almightyCap;
     return cast(res);
@@ -1062,6 +1063,8 @@ instance CHERICap #(CapReg, OTypeW, FlagsW, CapAddressW, CapW, TSub#(MW, 3));
     return setCapPointer(cap, zeroExtend(address));
   endfunction
 
+  function addAddrUnsafe (cap, inc) = setAddrUnsafe(cap, getAddr(cap) + signExtend(inc));
+
   function getOffset = error("getOffset not implemented for CapReg");
   function modifyOffset = error("modifyOffset not implemented for CapReg");
   function getBase = error("getBase not implemented for CapReg");
@@ -1181,6 +1184,8 @@ instance CHERICap #(CapPipe, OTypeW, FlagsW, CapAddressW, CapW, TSub#(MW, 3));
     return cap;
   endfunction
 
+  function addAddrUnsafe (cap, inc) = setAddrUnsafe(cap, getAddr(cap) + signExtend(inc));
+
   function getOffset (x) = getOffsetFat(x.capFat, x.tempFields);
 
   function Exact#(CapPipe) modifyOffset (CapPipe cap, Bit#(CapAddressW) offset, Bool doInc);
diff --git a/CHERICap.bsv b/CHERICap.bsv
index 51339d3..499f376 100644
--- a/CHERICap.bsv
+++ b/CHERICap.bsv
@@ -124,6 +124,8 @@ typeclass CHERICap#(type t, numeric type ot, numeric type flg, numeric type n, n
   function Exact#(t) setAddr (t cap, Bit#(n) addr);
   // Set the address of the capability. Result assumed to be representable
   function t setAddrUnsafe (t cap, Bit#(n) addr);
+  // Add to the address of the capability. Result assumed to be representable
+  function t addAddrUnsafe (t cap, Bit#(maskable_bits) inc);
 
   // Get the offset of the capability
   function Bit#(n) getOffset (t cap) = getAddr(cap) - getBase(cap);