Simpletron Question

[fb]

Hi everyone. I have the following code. It was a question out of C++
how to program. I get a warning about "possible unreachable code" in
the RunCode() function, but I don't see any problem with it...

I was hoping for a better way of exiting the case statements below. I
used exit() for "divide by zero" and "invalid instruction". I can't get
a try/catch to work. Probably from a lack of experience. I have a
fairly strong C background if that help any.

Also, everything is very global. If you have any suggestions as to how
I might improve information hiding, feel free to point them out.

Thanks for your time...

//Simpletron Code Listing

#include <iostream>
#include <iomanip>
#include <cstdio> //For EXIT_SUCCESS and EXIT_FAILURE

//From iostream
using std::cout;
using std::endl;
using std::cin;
using std::ios;

//From iomanip
using std::setfill;
using std::setw;
using std::setiosflags;
using std::resetiosflags;

//Input/Output operations:
const int READ = 10;
const int WRITE = 11;

//Load and store operations:
const int LOAD = 20;
const int STORE = 21;

//Arithmetic operations:
const int ADD = 30;
const int SUBTRACT = 31;
const int DIVIDE = 32;
const int MULTIPLY = 33;

//Transfer-of-control operations:
const int BRANCH = 40;
const int BRANCHNEG = 41;
const int BRANCHZERO = 42;
const int HALT = 43;

//Generic Operations
const int NOP = 00;

//General size boundaries
const int MEMSIZE = 100;

//variables
int Accumulator = 0; //Accumulator Register
int counter = 0; //Counter to keep track of location in
memory that
//contains the instruction being performed
int OperationCode = 0; //Current operation being performed
int Operand = 0; //Memory location of opcode being acted on
int InstructionRegister = 0; //next instruction to be split and run

int Memory[MEMSIZE] = {0}; //My memory array -- zeroed out

void WelcomeHeader(void);
int ReadInput(void);
int RunCode(void);

int main(void)
{

WelcomeHeader();
ReadInput();
RunCode();

return EXIT_SUCCESS;
}

void WelcomeHeader(void)
{
cout << "*** Welcome to Simpletron ***\n\n"
<< "*** Please enter your program one instruction ***\n"
<< "*** (or data word) at a time. I will type the ***\n"
<< "*** location number and a question mark (?). ***\n"
<< "*** You then type the word for that location. ***\n"
<< "*** Type the sentinel -99999 to stop entering ***\n"
<< "*** your program. ***\n" << endl;
}

int ReadInput(void)
{
while(counter < 100)
{
if (counter < 10) //Formatting...Add a zero to make it line up
cout << "0";
cout << counter << " ? ";
cin >> Memory[counter];
if((Memory[counter] < 10000) && (Memory[counter] > -10000))
counter++;
else if(Memory[counter] == -99999){
Memory[counter] = 0;
break;
}
}
return EXIT_SUCCESS;
}

int RunCode(void)
{
counter = 0;
for(;{
InstructionRegister = Memory[counter];
OperationCode = InstructionRegister / 100;
Operand = InstructionRegister % 100;
switch(OperationCode){
case READ:
cout << "Enter Integer: " << std::flush;
cin >> Memory[Operand];
counter++;
break;
case NOP:
counter++;
break;
case WRITE:
cout << Memory[Operand] << endl;
counter++;
break;
case LOAD:
Accumulator = Memory[Operand];
counter++;
break;
case STORE:
Memory[Operand] = Accumulator;
counter++;
break;
case ADD:
Accumulator += Memory[Operand];
counter++;
break;
case SUBTRACT:
Accumulator -= Memory[Operand];
counter++;
break;
case MULTIPLY:
Accumulator *= Memory[Operand];
counter++;
break;
case DIVIDE:
if (Memory[Operand] == 0){
cout << "Division by zero error. Program Terminated." << endl;
exit(1);
break;
}
Accumulator /= Memory[Operand];
counter++;
break;
case BRANCH: //Unconditional Jump (aka: goto)
counter = Operand;
counter++;
break;
case BRANCHNEG:
if(Accumulator < 0)
counter = Operand;
counter++;
break;
case BRANCHZERO:
if(Accumulator = 0)
counter = Operand;
counter++;
break;
case HALT:
cout << "*** Simpletron execution terminated ***" << endl;
return EXIT_SUCCESS;
break;
default:
cout << "ERROR: Invalid SML instruction -- Terminating" << endl;
exit(1);
break;
} //end switch
}//end for
return EXIT_SUCCESS; //Is this unreachable?
}//end RunCode

 

[Victor Bazarov]

Hi everyone. I have the following code. It was a question out of C++ how
to program. I get a warning about "possible unreachable code" in the
RunCode() function, but I don't see any problem with it...

The "problem" is that the loop in it never ends. It's like you'd write

void function() {
while (true) {
}
DoSomething();
}

'DoSomething' will never be reached because the loop before it is infinite.

[...]

V

 

[Alf P. Steinbach]

* fb:

Hi everyone. I have the following code. It was a question out of C++
how to program. I get a warning about "possible unreachable code" in
the RunCode() function, but I don't see any problem with it...

The return statement is after a 'for(;' loop that never exits
(except via function 'return' or program termination 'exit()'), so
it will never be executed.

Also note the bug where you assign, '=', instead of compare, '=='.

I was hoping for a better way of exiting the case statements below.

Case statement is OK here, but a table-driven approach would be
cleaner and would scale much better to more op-codes; i.e., a table of
op-codes and corresponding function pointers.

I used exit() for "divide by zero" and "invalid instruction". I can't get
a try/catch to work. Probably from a lack of experience. I have a
fairly strong C background if that help any.

'exit' is a bit drastic as response to bad user data.

That also goes for using an exception.

Instead simply use a function return code.

Also, everything is very global. If you have any suggestions as to how
I might improve information hiding, feel free to point them out.

Hint: C++ is C with classes (that was the language's first name).