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1 files changed, 63 insertions, 0 deletions

diff --git a/subx/021byte_addressing.cc b/subx/021byte_addressing.cc
new file mode 100644
index 00000000..84a51779
--- /dev/null
+++ b/subx/021byte_addressing.cc
@@ -0,0 +1,63 @@
+//:
+
+:(before "End Initialize Op Names(name)")
+put(name, "88", "copy r8 (lowermost byte of r32) to r8/m8-at-r32");
+
+:(scenario copy_r8_to_mem_at_r32)
+% Reg[EBX].i = 0x224488ab;
+% Reg[EAX].i = 0x2000;
+== 0x1
+# op  ModR/M  SIB   displacement  immediate
+  88  18                                      # copy just the lowermost byte of EBX to the byte at *EAX
+# ModR/M in binary: 00 (indirect mode) 011 (src EBX) 000 (dest EAX)
+== 0x2000
+f0 cc bb aa  # 0xf0 with more data in following bytes
++run: copy lowermost byte of EBX to r8/m8-at-r32
++run: effective address is 0x2000 (EAX)
++run: storing 0xab
+% CHECK_EQ(0xaabbccab, read_mem_u32(0x2000));
+
+:(before "End Single-Byte Opcodes")
+case 0x88: {  // copy r8 to r/m8
+  const uint8_t modrm = next();
+  const uint8_t rsrc = (modrm>>3)&0x7;
+  trace(90, "run") << "copy lowermost byte of " << rname(rsrc) << " to r8/m8-at-r32" << end();
+  // use unsigned to zero-extend 8-bit value to 32 bits
+  uint8_t* dest = reinterpret_cast<uint8_t*>(effective_address(modrm));
+  *dest = Reg[rsrc].u;
+  trace(90, "run") << "storing 0x" << HEXBYTE << NUM(*dest) << end();
+  break;
+}
+
+//:
+
+:(before "End Initialize Op Names(name)")
+put(name, "8a", "copy r8/m8-at-r32 to r8 (lowermost byte of r32)");
+
+:(scenario copy_mem_at_r32_to_r8)
+% Reg[EBX].i = 0xaabbcc0f;  // one nibble each of lowest byte set to all 0s and all 1s, to maximize value of this test
+% Reg[EAX].i = 0x2000;
+== 0x1
+# op  ModR/M  SIB   displacement  immediate
+  8a  18                                      # copy just the byte at *EAX to lowermost byte of EBX (clearing remaining bytes)
+# ModR/M in binary: 00 (indirect mode) 011 (dest EBX) 000 (src EAX)
+== 0x2000  # data segment
+ab ff ff ff  # 0xab with more data in following bytes
++run: copy r8/m8-at-r32 to lowermost byte of EBX
++run: effective address is 0x2000 (EAX)
++run: storing 0xab
+# remaining bytes of EBX are *not* cleared
++run: EBX now contains 0xaabbccab
+
+:(before "End Single-Byte Opcodes")
+case 0x8a: {  // copy r/m8 to r8
+  const uint8_t modrm = next();
+  const uint8_t rdest = (modrm>>3)&0x7;
+  trace(90, "run") << "copy r8/m8-at-r32 to lowermost byte of " << rname(rdest) << end();
+  // use unsigned to zero-extend 8-bit value to 32 bits
+  const uint8_t* src = reinterpret_cast<uint8_t*>(effective_address(modrm));
+  trace(90, "run") << "storing 0x" << HEXBYTE << NUM(*src) << end();
+  *reinterpret_cast<uint8_t*>(&Reg[rdest].u) = *src;  // assumes host is little-endian
+  trace(90, "run") << rname(rdest) << " now contains 0x" << HEXWORD << Reg[rdest].u << end();
+  break;
+}