diff --git a/doc/harp_iset.tex b/doc/harp_iset.tex
index ef8918ae..4cd881f4 100644
--- a/doc/harp_iset.tex
+++ b/doc/harp_iset.tex
@@ -32,7 +32,7 @@ An \texttt{ArchID} uniquely identifies a HARP ISA.
 
 \section{Architecture Identifier String (\texttt{ArchID})}
 The best way to understand the multifaceted parameterizablity of the HARP ISAs is to study the architecture identifier strings used to uniquely identify a single HARP instruction set architecture.
-We'll start by breaking down Harptool's default \texttt{ArchID}: \texttt{8w32/32/8}:
+We'll start by breaking down Harptool's default \texttt{ArchID}: \texttt{8w32/32/8/8}:
 
 \begin{center}
 \begin{tabular}{rl}
@@ -43,10 +43,11 @@ We'll start by breaking down Harptool's default \texttt{ArchID}: \texttt{8w32/32
 \texttt{32}&32 general-purpose registers per lane\\
 \texttt{32}&32 predicate registers per lane\\
 \texttt{8} &8 SIMD lanes\\
+\texttt{8} &8 warps (thread groups)\\
 \end{tabular}
 \end{center}
 
-All ArchIDs have a similar format, although the SIMD lanes field can be omitted, as object files are still fully compatible even if the number of lanes changes. 
+All ArchIDs have a similar format, although the final two fields can be omitted, as object files are still fully compatible even if the dimensions of the core change. 
 
 \section{HarpTool}
 The assembler/linker/emulator/disassembler program for HARP is called HarpTool.
diff --git a/src/core.cpp b/src/core.cpp
index f775ec68..8bdf0a0c 100644
--- a/src/core.cpp
+++ b/src/core.cpp
@@ -32,7 +32,7 @@ void Harp::reg_doWrite(Word cpuId, Word regNum) {
 Core::Core(const ArchDef &a, Decoder &d, MemoryUnit &mem, Word id):
   a(a), iDec(d), mem(mem)
 {
-  for (unsigned i = 0; i < 8; ++i)
+  for (unsigned i = 0; i < a.getNWarps(); ++i)
     w.push_back(Warp(this));
 
   w[0].activeThreads = 1;
diff --git a/src/harptool.cpp b/src/harptool.cpp
index 8d82bfec..10efd1a5 100644
--- a/src/harptool.cpp
+++ b/src/harptool.cpp
@@ -49,7 +49,7 @@ HarpToolMode findMode(int argc, char** argv) {
 }
 
 int asm_main(int argc, char **argv) {
-  string archString("8w32/32/8"), outFileName("a.out.HOF"),
+  string archString("8w32/32/8/8"), outFileName("a.out.HOF"),
          inFileName(argv[argc-1]);
   bool showHelp;  
 
@@ -136,7 +136,7 @@ int asm_main(int argc, char **argv) {
 
 int disasm_main(int argc, char **argv) {
   bool showHelp;
-  string outFileName("a.out.s"), archString("8w32/32/8");
+  string outFileName("a.out.s"), archString("8w32/32/8/8");
   
 
   /* Get command line arguments. */
@@ -206,7 +206,7 @@ int disasm_main(int argc, char **argv) {
 }
 
 int emu_main(int argc, char **argv) {
-  string archString("8w32/32/8"), imgFileName("a.out.bin");
+  string archString("8w32/32/8/8"), imgFileName("a.out.bin");
   bool showHelp;
 
   /* Read the command line arguments. */
@@ -249,7 +249,8 @@ int emu_main(int argc, char **argv) {
 
 int ld_main(int argc, char **argv) {
   bool showHelp, mustResolveRefs(true);
-  string outFileName("a.out.bin"), archString("8w32/32/8"), formatString("bin");
+  string outFileName("a.out.bin"), archString("8w32/32/8/8"),
+         formatString("bin");
   Size nObjects;
   Addr binOffset(0);
 
diff --git a/src/include/archdef.h b/src/include/archdef.h
index b7cf97e7..2384d55a 100644
--- a/src/include/archdef.h
+++ b/src/include/archdef.h
@@ -28,7 +28,9 @@ namespace Harp {
       if (!iss || sep != '/') { extent = EXT_REGS; return; }
       iss >> sep >> nThds;
       if (!iss || sep != '/') { extent = EXT_PREGS; return; }
-      extent = EXT_THDS;
+      iss >> sep >> nWarps;
+      if (!iss || sep != '/') { extent = EXT_THDS; return; }
+      extent = EXT_WARPS;
     }
 
     operator std::string () const {
@@ -40,14 +42,25 @@ namespace Harp {
       if (extent >= EXT_REGS    ) oss << nRegs;
       if (extent >= EXT_PREGS   ) oss << '/' << nPRegs;
       if (extent >= EXT_THDS    ) oss << '/' << nThds;
+      if (extent >= EXT_WARPS    ) oss << '/' << nWarps;
 
       return oss.str();
     }
 
     bool operator==(const ArchDef &r) const {
-      return (extent == r.extent)   && (wordSize == r.wordSize) && 
-             (encChar == r.encChar) && (nRegs == r.nRegs) &&
-             (nPRegs == r.nPRegs)   && (nThds == r.nThds);
+      Extent minExtent(r.extent > extent ? extent : r.extent);
+
+      // Can't be equal if we can't specify a binary encoding at all.
+      if (minExtent < EXT_PREGS) return false;
+      
+      if (minExtent >= EXT_WORDSIZE) { if (wordSize!=r.wordSize) return false; }
+      if (minExtent >= EXT_ENC     ) { if (encChar != r.encChar) return false; }
+      if (minExtent >= EXT_REGS    ) { if (nRegs   !=   r.nRegs) return false; }
+      if (minExtent >= EXT_PREGS   ) { if (nPRegs  !=  r.nPRegs) return false; }
+      if (minExtent >= EXT_THDS    ) { if (nThds   !=   r.nThds) return false; }
+      if (minExtent >= EXT_WARPS   ) { if (nWarps  !=  r.nWarps) return false; }
+
+      return true;
     }
 
     bool operator!=(const ArchDef &r) const { return !(*this == r); }
@@ -71,14 +84,20 @@ namespace Harp {
     ThdNum getNThds() const {
       if (extent < EXT_THDS) throw Undefined(); else return nThds;
     }
+
+    ThdNum getNWarps() const {
+      if (extent < EXT_WARPS) throw Undefined(); else return nWarps;
+    }
     
   private:
-    enum { 
-      EXT_NULL, EXT_WORDSIZE, EXT_ENC, EXT_REGS, EXT_PREGS, EXT_THDS
-    } extent;
+    enum Extent { 
+      EXT_NULL, EXT_WORDSIZE, EXT_ENC, EXT_REGS, EXT_PREGS, EXT_THDS, EXT_WARPS
+    };
+
+    Extent extent;
 
     Size wordSize;
-    ThdNum nThds;
+    ThdNum nThds, nWarps;
     RegNum nRegs, nPRegs;
     char encChar;
   };
diff --git a/src/obj.cpp b/src/obj.cpp
index 19fb3feb..bdb66edc 100644
--- a/src/obj.cpp
+++ b/src/obj.cpp
@@ -657,7 +657,8 @@ Obj *HOFReader::read(std::istream &input) {
   string archString(inputString(input));
   ArchDef fileArch(archString);
   if (fileArch != arch) {
-    cout << "File arch does not match reader arch in HOFReader::read().\n";
+    cout << "File arch " << archString << " does not match reader arch "
+         << string(arch) << " in HOFReader::read().\n";
     exit(1);
   }
 
diff --git a/src/test/Makefile b/src/test/Makefile
index 2c2c8b75..9e69e374 100644
--- a/src/test/Makefile
+++ b/src/test/Makefile
@@ -2,7 +2,7 @@ HARPLD  = ../harptool -L
 HARPAS  = ../harptool -A
 HARPEM  = ../harptool -E
 HARPDIS = ../harptool -D
-4BARCH  = 4b16/16/2
+4BARCH  = 4b16/16/2/1
 
 all: simple.bin sieve.bin 2thread.bin simple.4b.bin sieve.4b.bin 2thread.4b.bin bubble.bin bubble.4b.bin dotprod.bin dotprod.4b.bin matmul.bin matmul.4b.bin \
  matmul-mt.s lfsr.bin diverge.bin