Added opcode OP_DUP_*

Duplicates the nth datum off the stack, pushing it to the top.  Useful
for operations such as MOV which eat the stack.

src/inst.h

@@ -40,6 +40,10 @@ typedef enum
   OP_MOV_HWORD,
   OP_MOV_WORD,
 
+  OP_DUP_BYTE,
+  OP_DUP_HWORD,
+  OP_DUP_WORD,
+
   // Boolean operations
   OP_NOT_BYTE,
   OP_NOT_HWORD,
@@ -108,6 +112,9 @@ inst_t *insts_read_bytecode_file(FILE *, size_t *);
 #define INST_PUSH_REG(TYPE, REG) \
   ((inst_t){.opcode = OP_PUSH_REGISTER_##TYPE, .operand = D##TYPE(REG)})
 
+#define INST_DUP(TYPE, OP) \
+  ((inst_t){.opcode = OP_DUP_##TYPE, .operand = DWORD(OP)})
+
 #define INST_NOT(TYPE) ((inst_t){.opcode = OP_NOT_##TYPE})
 #define INST_OR(TYPE)  ((inst_t){.opcode = OP_OR_##TYPE})
 #define INST_AND(TYPE) ((inst_t){.opcode = OP_AND_##TYPE})