diff --git a/subx/021byte_addressing.cc b/subx/021byte_addressing.cc
index cffc4050..fba9a827 100644
--- a/subx/021byte_addressing.cc
+++ b/subx/021byte_addressing.cc
@@ -48,7 +48,7 @@ put_new(Name, "88", "copy r8 to r8/m8-at-r32");
   88  18                                      # copy BL to the byte at *EAX
 # ModR/M in binary: 00 (indirect mode) 011 (src BL) 000 (dest EAX)
 == 0x2000
-f0 cc bb aa  # 0xf0 with more data in following bytes
+f0 cc bb aa
 +run: copy BL to r8/m8-at-r32
 +run: effective address is 0x00002000 (EAX)
 +run: storing 0xab
@@ -114,3 +114,39 @@ case 0x8a: {  // copy r/m8 to r8
 +run: storing 0x44
 # ensure ESI is unchanged
 % CHECK_EQ(Reg[ESI].u, 0xaabbccdd);
+
+//:
+
+:(before "End Initialize Op Names")
+put_new(Name, "c6", "copy imm8 to r8/m8-at-r32 (mov)");
+
+:(scenario copy_imm8_to_mem_at_r32)
+% Reg[EAX].i = 0x2000;
+== 0x1
+# op  ModR/M  SIB   displacement  immediate
+  c6  00                          dd          # copy to the byte at *EAX
+# ModR/M in binary: 00 (indirect mode) 000 (unused) 000 (dest EAX)
+== 0x2000
+f0 cc bb aa
++run: copy imm8 to r8/m8-at-r32
++run: effective address is 0x00002000 (EAX)
++run: storing 0xdd
+% CHECK_EQ(0xaabbccdd, read_mem_u32(0x2000));
+
+:(before "End Single-Byte Opcodes")
+case 0xc6: {  // copy imm8 to r/m8
+  const uint8_t modrm = next();
+  const uint8_t src = next();
+  trace(90, "run") << "copy imm8 to r8/m8-at-r32" << end();
+  trace(90, "run") << "imm8 is 0x" << HEXWORD << src << end();
+  const uint8_t subop = (modrm>>3)&0x7;  // middle 3 'reg opcode' bits
+  if (subop != 0) {
+    cerr << "unrecognized subop for opcode c6: " << NUM(subop) << " (only 0/copy currently implemented)\n";
+    exit(1);
+  }
+  // use unsigned to zero-extend 8-bit value to 32 bits
+  uint8_t* dest = reinterpret_cast<uint8_t*>(effective_byte_address(modrm));
+  *dest = src;
+  trace(90, "run") << "storing 0x" << HEXBYTE << NUM(*dest) << end();
+  break;
+}
diff --git a/subx/031check_operands.cc b/subx/031check_operands.cc
index 1038bca9..4fd500fa 100644
--- a/subx/031check_operands.cc
+++ b/subx/031check_operands.cc
@@ -228,6 +228,7 @@ void init_permitted_operands() {
   //  imm32 imm8  disp32 |disp16  disp8 subop modrm
   //  0     1     0      |0       0     1     1
   put(Permitted_operands, "c1", 0x23);  // combine
+  put(Permitted_operands, "c6", 0x23);  // copy
 
   //// Class P: op, ModR/M, subop (not r32) and imm32
   //  imm32 imm8  disp32 |disp16  disp8 subop modrm
diff --git a/subx/056write.subx b/subx/056write.subx
index 395d33b5..8f5de6e9 100644
--- a/subx/056write.subx
+++ b/subx/056write.subx
@@ -107,9 +107,9 @@ $clear-stream:loop:
     39/compare                      3/mod/direct    0/rm32/EAX    .           .             .           1/r32/ECX   .               .                 # compare EAX with ECX
     7d/jump-if-greater-or-equal  $clear-stream:end/disp8
     # *EAX = 0
-    c7          0/subop/copy        0/mod/direct    0/rm32/EAX    .           .             .           .           .               0/imm32           # copy to *EAX
-    # EAX += 4
-    81          0/subop/add         3/mod/direct    0/rm32/EAX    .           .             .           .           .               4/imm32           # add to EAX
+    c6          0/subop/copy        0/mod/direct    0/rm32/EAX    .           .             .           .           .               0/imm8            # copy byte to *EAX
+    # ++EAX
+    40/increment-EAX
     eb/jump  $clear-stream:loop/disp8
 $clear-stream:end:
     # . restore registers
diff --git a/subx/apps/crenshaw2-1 b/subx/apps/crenshaw2-1
index 5a28117c..efb462ec 100755
Binary files a/subx/apps/crenshaw2-1 and b/subx/apps/crenshaw2-1 differ
diff --git a/subx/apps/crenshaw2-1b b/subx/apps/crenshaw2-1b
index 46a26183..e6dceafa 100755
Binary files a/subx/apps/crenshaw2-1b and b/subx/apps/crenshaw2-1b differ
diff --git a/subx/apps/factorial b/subx/apps/factorial
index 679feb71..ac02aca4 100755
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diff --git a/subx/apps/handle b/subx/apps/handle
index 7bc762e4..17cdcc8f 100755
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diff --git a/subx/apps/hex b/subx/apps/hex
index 93c9f27f..8680176c 100755
Binary files a/subx/apps/hex and b/subx/apps/hex differ