1 files changed, 62 insertions, 1 deletions
diff --git a/asm/intel64/codes.cpp b/asm/intel64/codes.cpp
index a1d9e87..66a08dd 100644
--- a/asm/intel64/codes.cpp
+++ b/asm/intel64/codes.cpp
@@ -1,7 +1,12 @@
 #include "codes.h"
 
+#include <exception>
+#include <unordered_map>
+
+using namespace std::string_literals;
+
 // REX prefix: 0b0100WRXB
-std::vector<uint8_t> REX(std::string s) {
+std::vector<uint8_t> REX(const std::string& s) {
  uint8_t result{0b01000000};
  if (s == "W")
   result |= 0b00001000;
@@ -15,3 +20,59 @@ std::vector<uint8_t> REX(std::string s) {
  return { result }; 
 }
 
+namespace {
+
+ std::unordered_map<std::string, size_t> IndexOfRegister{
+  {"al", 0}, {"ah", 4},
+  {"bl", 3}, {"bh", 7},
+  {"cl", 1}, {"ch", 5},
+  {"dl", 2}, {"dh", 6},
+
+  {"ax", 0}, {"sp", 4},
+  {"bx", 3}, {"bp", 7},
+  {"cx", 1}, {"si", 5},
+  {"dx", 2}, {"di", 6},
+
+  {"eax", 0}, {"esp", 4},
+  {"ebx", 3}, {"ebp", 7},
+  {"ecx", 1}, {"esi", 5},
+  {"edx", 2}, {"edi", 6},
+ };
+
+}
+
+// Manual, page 530
+// Reg + Reg/Memory
+uint8_t ModRM(const std::string& reg, const std::string& rm) {
+ // TODO: extend
+ uint8_t result{0b11000000};
+
+ auto index1{ IndexOfRegister.find(reg) };
+ if (index1 == IndexOfRegister.end())
+  throw std::runtime_error("Unknown register for arg1: "s + reg);
+
+ result |= (index1->second << 3);
+
+ auto index2{ IndexOfRegister.find(rm) };
+ if (index2 == IndexOfRegister.end())
+  throw std::runtime_error("Unknown register for arg2: "s + rm);
+
+ result |= index2->second;
+
+ return result;
+}
+
+#if 0
+ prefixes{
+  "lock", 0xf0,
+
+  // branch hint
+  0x2e, "branch not taken"
+  0x3e, "branch taken"
+
+  0x66, "operand size override" // switch between 16 and 32 bit operands
+  0x67, "address size override" // switch between 16 and 32 bit addresses
+ };
+ };
+#endif
+