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1 files changed, 125 insertions, 0 deletions

diff --git a/subx/016index_addressing.cc b/subx/016index_addressing.cc
new file mode 100644
index 00000000..b1b7e563
--- /dev/null
+++ b/subx/016index_addressing.cc
@@ -0,0 +1,125 @@
+//: 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[EBX].i = 0x10;
+% Reg[EAX].i = 0x60;
+== 0x1  # code segment
+# op  ModR/M  SIB   displacement  immediate
+  01  1c      20                             # add EBX to *EAX
+# ModR/M in binary: 00 (indirect mode) 011 (src EBX) 100 (dest in SIB)
+# SIB in binary: 00 (scale 1) 100 (no index) 000 (base EAX)
+== 0x60  # data segment
+01 00 00 00  # 1
++run: add EBX to r/m32
++run: effective address is initially 0x60 (EAX)
++run: effective address is 0x60
++run: storing 0x00000011
+
+:(before "End Mod 0 Special-cases(addr)")
+case 4:  // exception: mod 0b00 rm 0b100 => incoming SIB (scale-index-base) byte
+  addr = effective_address_from_sib(mod);
+  break;
+:(code)
+uint32_t effective_address_from_sib(uint8_t mod) {
+  uint8_t sib = next();
+  uint8_t base = sib&0x7;
+  uint32_t addr = 0;
+  if (base != EBP || mod != 0) {
+    addr = Reg[base].u;
+    trace(90, "run") << "effective address is initially 0x" << std::hex << addr << " (" << rname(base) << ")" << end();
+  }
+  else {
+    // base == EBP && mod == 0
+    addr = imm32();  // ignore base
+    trace(90, "run") << "effective address is initially 0x" << std::hex << addr << " (disp32)" << end();
+  }
+  uint8_t index = (sib>>3)&0x7;
+  if (index == ESP) {
+    // ignore index and scale
+    trace(90, "run") << "effective address is 0x" << std::hex << addr << end();
+  }
+  else {
+    uint8_t scale = (1 << (sib>>6));
+    addr += Reg[index].i*scale;  // treat index register as signed. Maybe base as well? But we'll always ensure it's non-negative.
+    trace(90, "run") << "effective address is 0x" << std::hex << addr << " (after adding " << rname(index) << "*" << NUM(scale) << ")" << end();
+  }
+  return addr;
+}
+
+:(scenario add_r32_to_mem_at_base_r32_index_r32)
+% Reg[EBX].i = 0x10;  // source
+% Reg[EAX].i = 0x5e;  // dest base
+% Reg[ECX].i = 0x2;  // dest index
+== 0x1  # code segment
+# op  ModR/M  SIB   displacement  immediate
+  01  1c      08                             # add EBX to *(EAX+ECX)
+# ModR/M in binary: 00 (indirect mode) 011 (src EBX) 100 (dest in SIB)
+# SIB in binary: 00 (scale 1) 001 (index ECX) 000 (base EAX)
+== 0x60  # data segment
+01 00 00 00  # 1
++run: add EBX to r/m32
++run: effective address is initially 0x5e (EAX)
++run: effective address is 0x60 (after adding ECX*1)
++run: storing 0x00000011
+
+:(scenario add_r32_to_mem_at_displacement_using_sib)
+% Reg[EBX].i = 0x10;  // source
+== 0x1  # code segment
+# op  ModR/M  SIB   displacement  immediate
+  01  1c      25    60 00 00 00              # add EBX to *0x60
+# ModR/M in binary: 00 (indirect mode) 011 (src EBX) 100 (dest in SIB)
+# SIB in binary: 00 (scale 1) 100 (no index) 101 (not EBP but disp32)
+== 0x60  # data segment
+01 00 00 00  # 1
++run: add EBX to r/m32
++run: effective address is initially 0x60 (disp32)
++run: effective address is 0x60
++run: storing 0x00000011
+
+//:
+
+:(scenario add_r32_to_mem_at_base_r32_index_r32_plus_disp8)
+% Reg[EBX].i = 0x10;  // source
+% Reg[EAX].i = 0x59;  // dest base
+% Reg[ECX].i = 0x5;  // dest index
+== 0x1  # code segment
+# op  ModR/M  SIB   displacement  immediate
+  01  5c      08    02                       # add EBX to *(EAX+ECX+2)
+# ModR/M in binary: 01 (indirect+disp8 mode) 011 (src EBX) 100 (dest in SIB)
+# SIB in binary: 00 (scale 1) 001 (index ECX) 000 (base EAX)
+== 0x60  # data segment
+01 00 00 00  # 1
++run: add EBX to r/m32
++run: effective address is initially 0x59 (EAX)
++run: effective address is 0x5e (after adding ECX*1)
++run: effective address is 0x60 (after adding disp8)
++run: storing 0x00000011
+
+:(before "End Mod 1 Special-cases(addr)")
+case 4:  // exception: mod 0b01 rm 0b100 => incoming SIB (scale-index-base) byte
+  addr = effective_address_from_sib(mod);
+  break;
+
+//:
+
+:(scenario add_r32_to_mem_at_base_r32_index_r32_plus_disp32)
+% Reg[EBX].i = 0x10;  // source
+% Reg[EAX].i = 0x59;  // dest base
+% Reg[ECX].i = 0x5;  // dest index
+== 0x1  # code segment
+# op  ModR/M  SIB   displacement  immediate
+  01  9c      08    02 00 00 00              # add EBX to *(EAX+ECX+2)
+# ModR/M in binary: 10 (indirect+disp32 mode) 011 (src EBX) 100 (dest in SIB)
+# SIB in binary: 00 (scale 1) 001 (index ECX) 000 (base EAX)
+== 0x60  # data segment
+01 00 00 00  # 1
++run: add EBX to r/m32
++run: effective address is initially 0x59 (EAX)
++run: effective address is 0x5e (after adding ECX*1)
++run: effective address is 0x60 (after adding disp32)
++run: storing 0x00000011
+
+:(before "End Mod 2 Special-cases(addr)")
+case 4:  // exception: mod 0b10 rm 0b100 => incoming SIB (scale-index-base) byte
+  addr = effective_address_from_sib(mod);
+  break;