Dynamic Recompiler on a Chip8 Emulator
Tags: programming, tutorial, opensource, showdev
Published: 2022-11-02 03:14:18


Emulating a computer system on a different hardware platform is a task that requires a lot of processing power and memory. One way to improve the performance of an emulator is through the use of dynamic recompilation, also known as dynamic binary translation. In this article, i will explore how to implement a dynamic recompiler using Dyna/Rec JIT on a Chip 8 emulator.

The term "dynamic recompilation" abbreviated as "Dynarec" refers to a method used by emulators to quicken CPU emulation. Since nothing is actually being recompiled when anything is dynamically translated, the name dynamic binary translator would be a better fit (static binary translators do exist, but they aren't frequently utilized in chip8 emulation).

Currently, Chip8 emulates the core that is present in the RAM using a tool known as a cached interpreter. Cached interpreters continuously pick up a single instruction, decode it, and execute it. For a variety of reasons, this is quite sluggish.

Therefore, even if you've ran an instruction before, you still need to re retrieve and re decode it.

Even if you've ran it previously, you must load and then write back numerous registers for each simulated instruction, which leaves very little area for the target code to be optimized.


Although some of those issues can be lessened with more sophisticated interpreter approaches (a threaded interpreter, for instance, can decode once regardless of how frequently an instruction is run), binary translation still outperforms them all.


Basic blocks, which are merely linearly executing sequences of code, are the building blocks of binary translation (no branches).

The binary translator (BT ) accepts a basic block and performs analysis to determine which registers are being used and in how many.

(The number and types of registers currently in use. the number of cycles required. Can there be any compiler-like optimizations, etc.) and after that replicates the target code in native code. Thus, when running PCSX2, it analyzes the provided MIPS code and produces equivalent x86 code. There is a significant reduction in overhead compared to interpreters because this code doesn't need to be re-fetched or re-decoded, and the execution phase is optimized.



What is Dynamic Recompilation?

Dynamic recompilation is a technique used by emulators to speed up CPU emulation. It works by analyzing the code executed by the guest machine and generating native code for the host machine on the fly. This approach allows for faster execution than traditional interpretation methods, which require retrieving and decoding instructions repeatedly.

In the context of Chip 8 emulation, dynamic recompilation can significantly enhance the performance of the emulator. The Chip 8 processor has a simple architecture, making it an ideal candidate for dynamic recompilation. By using Dyna/Rec JIT, we can generate optimized native code for the host machine, resulting in faster emulation speeds.

How Does Dyna/Rec JIT Work?

Dyna/Rec JIT is a dynamic binary translator that converts machine code from one architecture to another. It consists of two main components: the binary translator (BT) and the just-in-time (JIT) compiler.

The BT takes a basic block of code (a sequence of instructions that executes linearly without branches) and performs analysis to determine which registers are used and how often. Based on this information, the BT generates target code in native code. The JIT compiler further optimizes the generated code by applying various techniques such as loop unrolling, dead code elimination, and register allocation.