GameRoy: JIT Compilation in High-Accuracy Game Boy Emulation

I've thought of doing one of these for my favourite 8-bit computer, though never actually got round to actually trying it.

How wide is a bank number? Even if it's the full 8 bits, that's only 24 bits, which is nothing these days. 64 MB for the full table, assuming 32 bits is enough to cover every offset into the JIT buffer. (Assuming max 4 GB of JIT'd code. If that isn't, perhaps 40 bits would be? 128 MB table, max 1 TByte of JIT'd code.) Would you need a hash table rather than an array?

The typical hello world is actually a CHIP 8 emulator, which is a type of VM that ran on an old calculator, the specifics of which elude me. It's got a tiny memory space, simple instructions and a handful of features, so it's the sort of thing you can get up and "running" with minimal effort. It lets you explore the high level concepts involved in running a "guest" system on some hosts, and tickles the challenge of synchronizing the timing and dealing with video and sound, without being overwhelming the way a physical console can be.

> how people start with game emulators?

I had some Computer Organization and Architecture classes in university, and a previously-existing burning desire to understand emulation. I decided that the NES would be a good target; it was the oldest game hardware that I had personal interest in. In 2008, I found as much info as I could about how to structure an emulator, and technical documents about the NES, and started building. I got something basic working in a couple months, and came back to it every couple of years, either to add functionality or experiment with optimizations.

> what are the basic concepts?

A CPU goes through a repeating fetch->decode->execute loop, and mostly communicates with the rest of the system over a "bus", which has some number of address lines (in the NES, 16 address lines provide access to 2^16 or 65,536 addresses), some number of data lines (in the NES, 8 lines for 8-bit values), and some number of control lines (read/write, ready, and others that vary system-to-system). Different devices are connected at different address ranges. They could be RAM, ROM, I/O hardware, etc. So when a CPU is booting, the system is usually designed to expose some ROM at the first address the CPU fetches from.

> what is the easiest hello-world game to start w/?

Chip-8, probably. Space Invaders is a good one too; simple, but using a CPU related to those in a lot of other systems, and a bit less of a toy-scale system than Chip-8.

The maximum supported size is 512 banks, so 25 bits. But searching now, I don't think there are any games bigger than 1 MiB in size (64 banks), so 22 bits, or just 16 MiB or 32 MiB of lookup table. Yeah, maybe it is not so bad to replace the HashMap with an array, especially when I am losing a significant amount of time doing HashMap look ups already.

But of course I would like to avoid using that amount of memory if possible. Maybe I will also take a look at perfect hash mapping someday.

I feel like I've seen the trick of estimating upcoming interrupts before somewhere...was it mednafen or higan?

Anyways cool post!

How come you don't consider conditional branches to be terminating instructions of your basic blocks?