4051

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

1 files changed, 64 insertions, 0 deletions

diff --git a/subx/011direct_addressing.cc b/subx/011direct_addressing.cc
new file mode 100644
index 00000000..8d280784
--- /dev/null
+++ b/subx/011direct_addressing.cc
@@ -0,0 +1,64 @@
+//: operating directly on a register
+
+:(scenario add_r32_to_r32)
+% Reg[0].i = 0x10;
+% Reg[3].i = 1;
+# op  ModR/M  SIB   displacement  immediate
+  01  d8                                      # add EBX (reg 3) to EAX (reg 0)
++run: add reg 3 to effective address
++run: effective address is reg 0
++run: storing 0x00000011
+
+:(before "End Single-Byte Opcodes")
+case 0x01: {  // add r32 to r/m32
+  uint8_t modrm = next();
+  uint8_t arg2 = (modrm>>3)&0x7;
+  trace(2, "run") << "add reg " << NUM(arg2) << " to effective address" << end();
+  int32_t* arg1 = effective_address(modrm);
+  BINARY_ARITHMETIC_OP(+, *arg1, Reg[arg2].i);
+  break;
+}
+
+:(code)
+// Implement tables 2-2 and 2-3 in the Intel manual, Volume 2.
+// We return a pointer so that instructions can write to multiple bytes in
+// 'Mem' at once.
+int32_t* effective_address(uint8_t modrm) {
+  uint8_t mod = (modrm>>6);
+  // ignore middle 3 'reg opcode' bits
+  uint8_t rm = modrm & 0x7;
+  int32_t* result = 0;
+  switch (mod) {
+  case 3:
+    // mod 3 is just register direct addressing
+    trace(2, "run") << "effective address is reg " << NUM(rm) << end();
+    result = &Reg[rm].i;
+    break;
+  // End Mod Special-cases
+  default:
+    cerr << "unrecognized mod bits: " << NUM(mod) << '\n';
+    exit(1);
+  }
+  return result;
+}
+
+//:: subtract
+
+:(scenario subtract_r32_from_r32)
+% Reg[0].i = 10;
+% Reg[3].i = 1;
+# op  ModR/M  SIB   displacement  immediate
+  29  d8                                      # subtract EBX (reg 3) from EAX (reg 0)
++run: subtract reg 3 from effective address
++run: effective address is reg 0
++run: storing 0x00000009
+
+:(before "End Single-Byte Opcodes")
+case 0x29: {  // subtract r32 from r/m32
+  uint8_t modrm = next();
+  uint8_t arg2 = (modrm>>3)&0x7;
+  trace(2, "run") << "subtract reg " << NUM(arg2) << " from effective address" << end();
+  int32_t* arg1 = effective_address(modrm);
+  BINARY_ARITHMETIC_OP(-, *arg1, Reg[arg2].i);
+  break;
+}