Get a bit, extract a value, or set a bit from a number

/*Bit Getting/Setting*/
//Macros//
#define EXTRACT_BITS_RL(the_val, bits_start, bits_len) ((the_val >> (bits_start - 1)) & ((1 << bits_len) - 1))
#define MODIFY_BIT_RL(the_val, bit_num, bit_val) (bit_val == 0 ? (the_val & (~(1 << (bit_num - 1)))) : (the_val | (1 << (bit_num - 1))))
//End Macros//
//Globals//
int iVal = 0, iShift = 0, iBitVal = 0, iMenu = 1;
//End Globals//
//Prototyes//
void ShowVal();
void ShowMenu();
//End Prototypes//
int main()
	{
		while(iMenu < 4) //Better way of avoiding GOTOs
			{
				if(iMenu == 1)
					{
						cout << "Input Test Value (0 to 255): ";
						cin >> iVal;
					}
				if(iMenu == 0)
					{
						cout << "Enter Bit Position To Be Modified (1 to 8): ";
						cin >> iShift;
						cout << "Enter New Bit Value (0 or 1): ";
						cin >> iBitVal;
						iVal = MODIFY_BIT_RL(iVal, iShift, iBitVal); //Modify the number's bits
					}
				if(iMenu == 2)
					{
						ShowVal();
					}
				if(iMenu == 3)
					{
						cout << "Enter Start: ";
						cin >> iShift;
						cout << "Enter Length: ";
						cin >> iBitVal;
						cout << "Extracted Value: " << EXTRACT_BITS_RL(iVal, iShift, iBitVal) << "\n";
					}
				ShowMenu();
			}
	return(0);
	}
void ShowVal()
	{
		cout << "Binary Values (Right To Left) for \"" << iVal << "\"\n";
		for(int I = 1; I < 9; I++)
			{
				cout << I << ": " << EXTRACT_BITS_RL(iVal, I, 1) << "\n"; //Show bit position (Right to Left)
			}
	}
void ShowMenu()
	{
		//Display menu
		cout << "\n->Program Menu:\n0: Modify \"" << iVal << "\"\n1: Choose New Number\n2: Show Values for \"" << iVal << "\"\n3: Extract Value\n4: Quit\nChoose: ";
		cin >> iMenu;
		cout << "\n";
	}