diff --git a/subx/000organization.cc b/subx/000organization.cc
index 320c0d4b..f729b17e 100644
--- a/subx/000organization.cc
+++ b/subx/000organization.cc
@@ -115,6 +115,7 @@ int main(int argc, char* argv[]) {
   // run on a 32-bit system
   assert(sizeof(int) == 4);
   assert(sizeof(float) == 4);
+  assert_little_endian();
 
   // End One-time Setup
 
@@ -138,3 +139,12 @@ int main(int argc, char* argv[]) {
 void reset() {
   // End Reset
 }
+
+void assert_little_endian() {
+  const int x = 1;
+  const char* y = reinterpret_cast<const char*>(&x);
+  if (*y != 1) {
+    cerr << "the SubX VM only runs on little-endian processors. Do you have Intel (or AMD or Atom) inside?\n";
+    exit(1);
+  }
+}
diff --git a/subx/010core.cc b/subx/010core.cc
index 87c06d90..a1d61ba8 100644
--- a/subx/010core.cc
+++ b/subx/010core.cc
@@ -40,6 +40,7 @@ SF = ZF = OF = false;
   /* arg1 and arg2 must be signed */ \
   int64_t tmp = arg1 op arg2; \
   arg1 = arg1 op arg2; \
+  trace(2, "run") << "storing 0x" << std::hex << arg1 << end(); \
   SF = (arg1 < 0); \
   ZF = (arg1 == 0); \
   OF = (arg1 != tmp); \
@@ -48,6 +49,7 @@ SF = ZF = OF = false;
 #define BINARY_BITWISE_OP(op, arg1, arg2) { \
   /* arg1 and arg2 must be unsigned */ \
   arg1 = arg1 op arg2; \
+  trace(2, "run") << "storing 0x" << std::hex << arg1 << end(); \
   SF = (arg1 >> 31); \
   ZF = (arg1 == 0); \
   OF = false; \
diff --git a/subx/011add.cc b/subx/011add.cc
new file mode 100644
index 00000000..b84797d1
--- /dev/null
+++ b/subx/011add.cc
@@ -0,0 +1,46 @@
+:(scenario add_r32_to_rm32)
+% Reg[3].i = 0x10;
+% Reg[0].i = 0x60;
+# word in addresses 0x60-0x63 has value 1
+% Mem[0x60] = 1;
+# op  ModR/M  SIB   displacement  immediate
+  01  18                                     # add EBX to *EAX
++run: add register 3 to effective address
++run: effective address is memory at address 0x60 (register 0)
++run: storing 0x11
+
+:(before "End Single-Byte Opcodes")
+case 0x01: {  // add r32 to r/m32
+  uint8_t modrm = next();
+  uint8_t arg2 = (modrm>>3)&0x7;
+  trace(2, "run") << "add register " << static_cast<int>(arg2) << " to effective address" << end();
+  int32_t* arg1 = effective_address(modrm);
+  BINARY_ARITHMETIC_OP(+, *arg1, Reg[arg2].i);
+  break;
+}
+
+:(code)
+// Implement tables 2-2 and 2-3 in the Intel manual, Volume 2.
+// We return a pointer so that instructions can write to multiple bytes in
+// 'Mem' at once.
+int32_t* effective_address(uint8_t modrm) {
+  uint8_t mod = (modrm>>6);
+  // ignore middle 3 'register opcode' bits
+  uint8_t rm = modrm & 0x7;
+  int32_t* result = 0;
+  switch (mod) {
+    case 0:
+      // mod 0 is usually indirect addressing
+      switch (rm) {
+      default:
+        trace(99, "run") << "effective address is memory at address 0x" << std::hex << Reg[rm].u << " (register " << static_cast<int>(rm) << ")" << end();
+        assert(Reg[rm].u + sizeof(int32_t) <= Mem.size());
+        result = reinterpret_cast<int32_t*>(&Mem.at(Reg[rm].u));  // rely on the host itself being in little-endian order
+        break;
+      // End Mod 0 Special-Cases
+      }
+      break;
+    // End Mod Special-Cases
+  }
+  return result;
+}