1 files changed, 45 insertions, 0 deletions
diff --git a/subx/014index_addressing.cc b/subx/014index_addressing.cc
new file mode 100644
index 00000000..1615e652
--- /dev/null
+++ b/subx/014index_addressing.cc
@@ -0,0 +1,45 @@
+//: operating on memory at the address provided by some register plus optional scale and offset
+
+:(scenario add_r32_to_mem_at_r32_with_sib)
+% Reg[3].i = 0x10;
+% Reg[0].i = 0x60;
+% SET_WORD_IN_MEM(0x60, 1);
+# op  ModR/M  SIB   displacement  immediate
+  01  1c      20                             # add EBX (reg 3) to *EAX (reg 0)
+# SIB in binary: 00 (scale 1) 100 (no index) 000 (base EAX)
+# See Table 2-3 of the Intel programming manual.
++run: add reg 3 to effective address
++run: effective address is mem at address 0x60 (reg 0)
++run: storing 0x00000011
+
+:(before "End Mod 0 Special-cases")
+case 4:
+  // exception: SIB addressing
+  uint8_t sib = next();
+  uint8_t base = sib&0x7;
+  uint8_t index = (sib>>3)&0x7;
+  if (index == ESP) {
+    // ignore index and scale
+    trace(2, "run") << "effective address is mem at address 0x" << std::hex << Reg[base].u << " (reg " << NUM(base) << ")" << end();
+    result = reinterpret_cast<int32_t*>(&Mem.at(Reg[base].u));
+  }
+  else {
+    uint8_t scale = (1 << (sib>>6));
+    uint32_t addr = Reg[base].u + Reg[index].u*scale;
+    trace(2, "run") << "effective address is mem at address 0x" << std::hex << addr << " (reg " << NUM(base) << " + reg " << NUM(index) << " * " << NUM(scale) << ")" << end();
+    result = reinterpret_cast<int32_t*>(&Mem.at(addr));
+  }
+  break;
+
+:(scenario add_r32_to_mem_at_base_plus_index)
+% Reg[3].i = 0x10;  // source
+% Reg[0].i = 0x5e;  // dest base
+% Reg[1].i = 0x2;  // dest index
+% SET_WORD_IN_MEM(0x60, 1);
+# op  ModR/M  SIB   displacement  immediate
+  01  1c      08                             # add EBX (reg 3) to *(EAX+ECX)
+# SIB in binary: 00 (scale 1) 001 (index ECX) 000 (base EAX)
+# See Table 2-3 of the Intel programming manual.
++run: add reg 3 to effective address
++run: effective address is mem at address 0x60 (reg 0 + reg 1 * 1)
++run: storing 0x00000011