Adding two numbers
Let’s begin with an extremely simple program. It will do nothing but add two numbers. In Java, it would look like this:
public class SimpleExample {
public static void main(String[] args) {
System.out.println(3 + 4);
}
}
We want to write it in machine language though.
Now this is a little more complicated that simply writing 3 + 4 because
traditionally, CPUs had no op codes that take two operands at the same time.
Instead, each op code only takes one operand — or none, as in the case
of NOP and HALT. To work with multiple operands, the CPU has it’s own tiny
memory called registers. A register can only hold a single data item at a time.
The register we will use is called accumulator. It is used for most arithmetic
and logical operations.
Let’s add it to the Cpu class:
public class Cpu {
private int acc;
public int getAcc() {
return acc;
}
// ...
}
We also need two more opcodes:
- One for moving the first number into the accumulator:
MOV - One for adding the second number
ADD
So the whole program in machine language (using mnemonics) would be:
MOV A, 3 ;load the number 3 into the accumulator
ADD 4 ;add the number 4 to the contents of the accumulator
HLT
The opcodes for MOV and ADD are 0x3e and 0xc6, respectively. The numbers
you want to move or add must follow the opcode immediately.
So altogether our program looks like this: 0x3e, 0x03, 0xc6, 0x04, 0x76.
Let’s enhance our Simulation class to print the contents of the accumulator
at the end of the program, so that we can see the actual result of the
calculation.
public class Simulation {
public static void main(String[] args) {
Memory mem = new Memory();
System.out.println("Loading program into memory.");
mem.load(0, new int[]{0x3e, 0x03, 0xc6, 0x04, 0x76});
Cpu cpu = new Cpu();
System.out.println("Attaching memory to the CPU");
cpu.attachMemory(mem);
System.out.println("Running the program");
cpu.run();
System.out.println("Finished, accumulator containts " + cpu.getAcc());
}
}
Try this now and see if it prints the correct result.