I am currently working on a very basic assembler. The assembler needs to take in assembly instructions and output 16-bit binary instructions for use with a computer we are making.
My design strategy has been to create a Command class, that has 3 child classes. There is one for each type of command: A-commands, C-commands, and L-commands. To identify the type of command I am working with, I have included a string command_type that is either "A", "C", or "L" respectively.
EDIT:
I am still having a lot of trouble figuring out how to properly derive these classes. Basically, A and L commands should have a "symbol" string, which represents an integer value that needs to be converted, while C commands have "dest","comp", and "jump" values that also must be accessed, however they do not have "symbol" values.
Command.h
#include <fstream>
#include <string>
class Command {
std::string command_type = "";
protected:
void set_commandType(std::string x){command_type = x;}
public:
Command();
virtual ~Command();
std::string commandType() const {return command_type;}
};
class A_COMMAND : public Command
{
std::string symbol;
public:
A_COMMAND(std::string s);
std::string get_symbol(){return symbol;}; //Returns the symbol or decimal Xxx of the current command @Xxx or (Xxx) . Should be called only when commandType() is A_COMMAND or L_COMMAND.
};
class C_COMMAND : public Command
{
std::string comp;
std::string dest;
std::string jump;
public:
C_COMMAND(std::string s, std::string d, std::string j);
std::string get_comp(){return comp;}; //Returns the comp mnemonic in the current C-command (28 possibilities). Should be called only when commandType() is C_COMMAND.
std::string get_dest(){return dest;}; //Returns the dest mnemonic in the current C-command (8 possibilities). Should be called only when commandType() is C_COMMAND.
std::string get_jump(){return jump;}; //Returns the jump mnemonic in the current C-command (8 possibilities). Should be called only when commandType() is C_COMMAND.
};
class L_COMMAND : public Command
{
std::string symbol;
public:
L_COMMAND(std::string s);
std::string get_symbol(){return symbol;}; //Returns the symbol or decimal Xxx of the current command @Xxx or (Xxx) . Should be called only when commandType() is A_COMMAND or L_COMMAND.
};
Command.cpp
#include "Command.h"
//---------------------------------------------
//A-Command functions
Command::Command(){}
A_COMMAND::A_COMMAND(std::string s) : symbol(s)
{
set_commandType("A");
}
//---------------------------------------------
//C-Command functions
C_COMMAND::C_COMMAND(std::string c, std::string d, std::string j) : comp(c), dest(d), jump(j)
{
set_commandType("C");
}
//---------------------------------------------
//L-Command functions
L_COMMAND::L_COMMAND(std::string s) : symbol(s)
{
set_commandType("L");
}
I have a Parser.cpp and Parser.h that process the input and are responsible for creating a deque of commands:
Parser.h
#include "Command.h"
#include <vector>
#include <deque>
class Parser {
private:
std::deque<Command> commands;
public:
Parser(std::vector<std::string>);
bool hasMoreCommands() //are there more commands in the input?
{
if(commands.size() != 0)
return true;
else
return false;
}
void advance(){commands.pop_front();} //move to next command, should only work if hasMoreCommands returns false}
Command currentCommand(){return commands.front();}
std::vector<std::string> translateCommands(); //convert commands into binary strings
};
Parser.cpp
#include "Parser.h"
#include "Command.h"
#include <vector>
#include <iostream>
#include <string>
#include <unordered_map>
bool inList(std::string& str, std::vector<std::string> list) //check if a given string contains one of the elements in the comp, dest, jump vectors. if so extract string for use in constructor
{
for(auto i = list.begin(); i!=list.end(); ++i)
{
std::size_t found = str.find(*i);
if(found!=std::string::npos)
{
return true;
}
}
return false;
}
Parser::Parser(std::vector<std::string> input) {
std::vector<std::string> dest_list = {"","M","D","MD","A","AM","AD","AMD"}; //all possible dests
std::vector<std::string> comp_list = {"0","1","D","A","!D","!A","-D","-A","D+1","A+1","D-1","A-1","D+A","D-A","A-D","D&A","D|A","M","!M","-M","M+1","M-1","D+M","D-M","M-D","D&M","D|M"}; //all possible comps
std::vector<std::string> jump_list = {"","JGT","JEQ","JGE","JLT","JNE","JLE","JMP"}; //all possible jumps
std::string dest, comp, jump;
std::deque<Command> commands;
for(std::vector<std::string>::const_iterator i = input.begin(); i != input.end(); ++i)
{
std::string line = *i;
if(*line.begin()=='@') //A-command
{
A_COMMAND command(line.substr(1));
std::cout << "Command type: " << command.commandType() << "\n";
std::cout << "symbol: " << command.get_symbol() << "\n";
commands.push_back(command);
}
else if(*line.begin()=='(' && *line.rbegin() == ')' && line.size() > 2) //L-command
{
L_COMMAND command(line.substr(1, line.size() - 2));
std::cout << "Command type: " << command.commandType() << "\n";
std::cout << "symbol: " << command.get_symbol() << "\n";
commands.push_back(command); }
else
{
std::string rhs = line;
std::string dest_string = "";
std::string comp_string = "";
std::string jump_string = "";
size_t equals_pos = line.find('='); //position of = in string, if present
size_t semi_pos = line.find(';'); //position of ; in string, if present
if(equals_pos != line.npos) //if there is an = then we have a dest
{
dest_string = line.substr(0,equals_pos);
rhs = line.substr(equals_pos+1);
}
if(semi_pos != line.npos) //jump
{
comp_string = rhs.substr(0,semi_pos);
jump_string = rhs.substr(semi_pos+1);
}
else //no jump
{
comp_string = rhs;
}
//now confirm if inputs are valid
if(inList(dest_string, dest_list))
dest = dest_string;
else
std::cout << "invalid dest \n";
if(inList(comp_string, comp_list))
comp = comp_string;
else
std::cout << "invalid comp \n";
if(inList(jump_string, jump_list))
jump = jump_string;
else
std::cout << "invalid jump \n";
C_COMMAND command(comp, dest, jump);
std::cout << "Command type: " << command.commandType() << "\n";
std::cout << "dest: " << command.get_dest() << "\n";
std::cout << "comp: " << command.get_comp() << "\n";
std::cout << "jump: " << command.get_jump() << "\n";
commands.push_back(command);
}
}
}
My main.cpp loads the input, and passes it through the parser. The problem I have is that I cannot do anything with the input.
I have tried to write a function like so:
string translateLine(Command command, Code code) //Code is a table for translating the command
{
string output;
if(command.commandType() == "A")
{
string symbol = parser.currentCommand().get_symbol();
cout << symbol << endl;
//perform binary conversion
}
/*else if(command.commandType() == "C")
{
string dest = command.get_dest();
}*/
//shouldn't be any L commands in symbol-less version
else
{
std::cout << "unexpected command value \n";
}
return output;
}
But as soon as I call get_symbol(), the compiler doesn't recognize the function. I know that this is because the base Command doesn't have a get_symbol() function, but I can't figure out how to correctly add the functions to the base class and derive them to the lower 3. I can't just make the pure virtual because not all of the functions are used in each class. How can I correctly accomplish this?
