I'm trying to write a chip8 emulator. I'm at the step where I load the rom into the memory. According to the documentation each instruction is 2 bytes and max memory addressed is 4K. So I define the memory as an array of ushorts.

struct Chip8
{
	ushort[4096] memory;
...

To load in the rom I do this

	void read(string rom)
	{
		import std.file : exists, getSize, read;

		if (exists(rom))
		{
			writeln("Loading the Rom");
			auto romSize = getSize(rom);
			writeln("The Rom size is : ", romSize);
			if (romSize > this.memory.length - memStart)
				writefln("Rom Size is too big! romSize = %s MemSize = %s", romSize,
					this.memory.length - memStart);

			else
			{
								
				// is it possible to to!int[] or do I have to use a cast here?
				this.memory[memStart..memStart + romSize] = cast(ushort[])read(rom);

			}
		}
		else
		{
			writeln("Cannot read ", rom);
		}

	}

But I get a range violation error.

core.exception.RangeError@source\chip.d(85): Range violation

I don't understand why? According to Windows the file is 478 bytes. memStart is 0x200. 0x200 + 478 = 990 which is well within the 4096Kb array which I created.

writeln (getSize(rom))

reports 478 bytes but since you work with ushorts (why? as far as I can see, this is a 8 bit machine) you convert the read(rom) into ushorts, which is only half in size:

writeln (cast(ushort[])read(rom));

gives you 478/2 = 239 bytes

this.memory[memStart..memStart + romSize] = cast(ushort[])read(rom);

fails because both ranges have different sizes.