Digital Foundry: How Does Xbox 360 Backwards Compatibility on Xbox One Actually Work?

[DonFerrari]

Mr Puggsly said:

DonFerrari said:

But not pure emulation. Still doesn't explain why it is an emulation case by case.

Well an emulator can override some settings but isnt changing the code of the game per se.

Hence, its emulation but the emulator has some features. Calling it a port though is inaccurate because its running the 360 game.

I accept port being inaccurate, but I don't think anyone can say it's pure BC or emulation either.

drkohler said:

DonFerrari said:

And that needs to redownload the game, severely improves graphics and just work on specific games cause of reason?

First of all, when you play a BC game, there is not a single line of X360 code involved. Not a single bit. You redownload "the game" because components in the run time system (might) have been improved since.

Here's a simple example (for illustration only, not real functions or memnonics). Suppose you disassemble some x360 game code and you come across something like:

PUSH $500

PUSH $300

CALL $ACFE000

Now this is the end for us, no outsider knows what that mystery procedure call does. But not for MS. Since they have all the x360 source code, they know these three lines resolve to a call of a procedure named CreateShadowBuffer(xsize,ysize)

So MS instantly knows the game allocates memory for a 1280*768 buffer. It can now replace these lines with the corresponding X1 procedure CreateSBufSize(1920,1080)

And they have automatically increased the resolution to 1920*1080. Of course this doesn't magically create better shadows, but it will not break the game at all (the game still uses the 1280,768 sizes for all calculationssince those are variables you have to find in your disassembly. Until MS finds the variables that are set to 1280,768 and replaces them with 1920,1080 and presto your BC game has better shadows. 

Apparently there are enough "MS grunt workers" around to look for stuff like this and improve the code bit by bit, hence you redownload the runtime system to profit from those finds. Since every x360 game uses different "ideas for programming" (for lack of a better word), every BC game comes with its own run time system. Which, again, contains zero x360 code lines (but an entire x360 os written in Jaguar code, for starters).

Appreciate the explain. But at least where I come from when you write an emulator it basically run like 90% of the code of the system it's emulating as is. That is my hard time with this "emulation" that have to be made one by one. Even if the code or the inner core are secrets to MS only and they are the ones that need to make the emulator, doesn't make sense to have the core changing all the time.