4051

subx: Move register direct mode before indirect in the exposition.

1 files changed, 81 insertions, 0 deletions

diff --git a/subx/012indirect_addressing.cc b/subx/012indirect_addressing.cc
new file mode 100644
index 00000000..ee56b2e1
--- /dev/null
+++ b/subx/012indirect_addressing.cc
@@ -0,0 +1,81 @@
+//: operating on memory at the address provided by some register
+
+:(scenario add_r32_to_mem_at_r32)
+% Reg[3].i = 0x10;
+% Reg[0].i = 0x60;
+# word in addresses 0x60-0x63 has value 1
+% Mem.at(0x60) = 1;
+# op  ModR/M  SIB   displacement  immediate
+  01  18                                     # add EBX (reg 3) to *EAX (reg 0)
++run: add reg 3 to effective address
++run: effective address is mem at address 0x60 (reg 0)
++run: storing 0x00000011
+
+:(before "End Mod Special-cases")
+case 0:
+  // mod 0 is usually indirect addressing
+  switch (rm) {
+  default:
+    trace(2, "run") << "effective address is mem at address 0x" << std::hex << Reg[rm].u << " (reg " << NUM(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;
+
+//:
+
+:(scenario add_mem_at_r32_to_r32)
+% Reg[0].i = 0x60;
+% Reg[3].i = 0x10;
+% Mem.at(0x60) = 1;
+# op  ModR/M  SIB   displacement  immediate
+  03  18                                      # add *EAX (reg 0) to EBX (reg 3)
++run: add effective address to reg 3
++run: effective address is mem at address 0x60 (reg 0)
++run: storing 0x00000011
+
+:(before "End Single-Byte Opcodes")
+case 0x03: {  // add r/m32 to r32
+  uint8_t modrm = next();
+  uint8_t arg1 = (modrm>>3)&0x7;
+  trace(2, "run") << "add effective address to reg " << NUM(arg1) << end();
+  const int32_t* arg2 = effective_address(modrm);
+  BINARY_ARITHMETIC_OP(+, Reg[arg1].i, *arg2);
+  break;
+}
+
+//:: subtract
+
+:(scenario sub_r32_from_mem_at_r32)
+% Reg[0].i = 0x60;
+% Mem.at(0x60) = 10;
+% Reg[3].i = 1;
+# op  ModRM   SIB   displacement  immediate
+  29  18                                      # subtract EBX (reg 3) from *EAX (reg 0)
++run: subtract reg 3 from effective address
++run: effective address is mem at address 0x60 (reg 0)
++run: storing 0x00000009
+
+//:
+
+:(scenario sub_mem_at_r32_from_r32)
+% Reg[0].i = 0x60;
+% Mem.at(0x60) = 1;
+% Reg[3].i = 10;
+# op  ModRM   SIB   displacement  immediate
+  2b  18                                      # subtract *EAX (reg 0) from EBX (reg 3)
++run: subtract effective address from reg 3
++run: effective address is mem at address 0x60 (reg 0)
++run: storing 0x00000009
+
+:(before "End Single-Byte Opcodes")
+case 0x2b: {  // subtract r/m32 from r32
+  uint8_t modrm = next();
+  uint8_t arg1 = (modrm>>3)&0x7;
+  trace(2, "run") << "subtract effective address from reg " << NUM(arg1) << end();
+  const int32_t* arg2 = effective_address(modrm);
+  BINARY_ARITHMETIC_OP(-, Reg[arg1].i, *arg2);
+  break;
+}