2. How to emulate a machine

Note: This is only a sketch. The most likely this doesn't contain all the data you need to write an emulator.

Emulation techniques

Two ways: Low-level emulation (LLE) and High-level emulation (HLE)

LLE

• Simulates the behavior of the hardware to be emulated
• Requires fast processor
• Can be achieved via hardware or software
• First PS3 systems emulated PS2 this way.

HLE

• Simulates functions of the hardware
• Easier to program than LLE
• Java Virtual Machine is an example of HLE
  • Java code doesn't complied and run on the host machine
  • Host runs a theoretical Java machine (where Java code is ran and compiled
• Three methods to do this: Interpreting, Dynamic Recompiling and Lists interception.

Interpreting

• Emulator executes the application line-by-line.
• It's done by mimicking what each instruction is supposed to do.

Dynamic Recompiling

• The emulator looks at chunks of the application's processors instructions.
  • looks if this can be optimized to run better on host machine.
• This is opposed to running each instruction one by one.
  • Results in lookup overhead penalties.

List interception

• Some hardware require the main processor to send command lists.
  • Co-processors, like GPU and audio chip.
• These are a series of instructions that tell the co-processor what to do.
• The emulator can intercept command list and turn it into something the host computer can process a similar co-processor.
• For example:
  1. Command lists going to the emulated system's GPU
  2. Intercepted and turned into DirectX or OpenGL command for the hosts' video card to process.

Which one to use?

LLE

Pros:

• Low-lever more accurate

Cons:

• Hardware isn't always feasible as it adds cost to the system
• Often limited to either
  • much older systems
  • prototype emulators that aregetting a handle on things, or
  • lesser of a system like an I/O controller.

HLE

Pros:

• allows a system with complete hardware to be emulated on something only a bit more powerful.
• It may also allow one without intimate knowledge of the hardware to emulate it.
• It may not be able to emulate special features specific to the hardware etc.
  • that developers used to do something beyond the norm
  • However, nowadays developers use industry-standard API, which:
    • makes easier to emulate a system.
    • quite possible run the application with better performance than the original.
    • This is the approach most emulators of modern systems are taking.
• modern emulators use HLE, (which makes better support). Cons:
• Requires intimate knowledge of the system or its parts.
  • May not be possible if documentation is for its scarce
• Requires much powerful system than the emulated machine.