The nand2tetris computer uses 15 bits for addressing. This means that there is an upper limit of 32768 instructions that we can load in the ROM at a time. When you try to compile, translate and assemble a game of Pong, the CPU emulator says: "Program too large"
Some people in the nand2tetris forums have worked on optimizing the toolchain. The course authors also provided us with a heavily optimized version of Pong. So, it is evident that with enough effort you can fit some really cool stuff into the ROM.
However, I wanted to store even larger programs. I especially wanted to start working on a more sophisticated operating system which would require much more space than we could optimize for. So I decided to forego the optimization "challenge" altogether. Perhaps I'll come back to it later. I moved on to just making the ROM bigger.
We can't simply increase the size of the ROM. We also need to increase the size addressable by our registers. You can't access more than 32K unique locations with only 15 bits to spare. So I changed the architecture from 16-bit to 64-bit. Now, the addressable space for the instruction memory is far more than the 32K I could get with the 15-bit addressing.
2**(16-1) = 32768
2**(64-1) = 9223372036854775808
I had to make some fundamental changes to the architecture and so all the changes in my extension project are incompatible with the original Hack platform.
The A instructions changed in a very simple manner: earlier, we used to have a zero at the MSB and the rest of the bits indicated the value to be put into the A register. The only difference in the 64-bit version is that the size of the address value has increased from 15 bits to 63 bits. The MSB is still zero.
The C instructions also needed to be 64-bit wide. In order to make them wider, I have padded them with zeros to the left. The MSB is hardcoded as 1 to differentiate them from A instructions. New opcodes have been added to support multiplication and division in the CPU emulator instead of the OS.