Question

My work on a 6502 emulator is continuing. I’ve run into some quirks with the embedded processor emulation wherein I need to load a 6502 binary file in an array starting at 0x1000.

Of course, loading a file is easy. But the offset part is isn’t. In C, I can do it easily.

Here’s the code for loading a file I’m using right now:

  def loadbinary(filename)
    @prog = File.open(filen, "rb") { |io| io.read }
    @imagesize = @prog.size
  end

If the load is loaded at 0x1000, the preceding space can be empty (before 0x1000).

Any ideas?

For anyone interesting, this is for the 6502.rb project I have on GitHub

Thanks.

Answer 1

You’ll probably need to allocate an array of n bytes, where n is the size of addressable RAM you want to emulate. From there you can start storing your data read in from the file into “RAM”.

This is what I’d start with:

def loadbinary(filename)
  @prog = File.read(filename, 'rb')
  @imagesize = @prog.bytesize
end

RAM_SIZE = 1024 * 64
RAM = Array.new(RAM_SIZE)

loadbinary('/path/to/foo')
RAM[0x1000, @imagesize] = @prog.bytes.to_a

Note: You can’t use String.size to get the length of the string, because it allows for multibyte characters. Instead, to get the number of bytes, you need bytesize, which, according to the documentation:

Returns the length of str in bytes.

Similarly, we can’t try to convert @prog into an array using split('') because that allows for multibyte characters too. Instead, we can get the bytes and turn that into an array. It could be done using unpack or bytes.to_a.