Simulating memory

The memory is the simplest. It stores the program and any data the program works on. The CPU needs a way to read the program and the data from the memory and write results to it.

public class Memory {
    
    private int[] mem = new int[65536];
    
    public int read(int address) {
        return mem[address];
    }
    
    public void write(int address, int data) {
        mem[address] = data;
    }
    
}

As you can see, the memory can only store numbers. That’s generally the case with everything a computer does: Everything needs to be encoded into numbers because that is the only kind of thing it can work with. We will see how to encode text and more complex structures, but for now we will just work with numbers directly.

In the example, the memory is initialized as an array with 2¹⁶ = 65536 cells. That is exactly 64 kB.¹ Why this number? Traditionally, 8 bit microcomputers actually had 16 bit of address space. Think of the memory as a long street with houses numbered from 0 to some upper limit. With 16 bit, you can have 2¹⁶ houses.

To read one of the memory cells, we call the read method with the house number as the parameter. It means: Give me the number that “lives” in that house number, or address.

To write into one of the memory cells, we call the write method with the address we want to write into and the number we want to move inside. Note that only one number can “live” in each memory cell. When we write a second one to that same address, the previous one is overwritten and forgotten.

Let’s add one more method to the memory class for convenience. Assume we’re loading a program from a harddisk or an image from an SD card. In this case, we need an easy way to load a whole sequence numbers into memory. Let’s call this method load.

    public void load(int address, int[] data) {
        System.arraycopy(data, 0, mem, address, data.length);
    }

This loads the data given as a parameter into memory, starting at the specified address.

The technical term for this kind of bulk loading from a storage device is Direct Memory Access or DMA, but let’s not worry about that too much now.

Let’s look at the CPU class next.