Xbox 360 Backward Compatibility

2022-06-22T12:51:16Z

XENON: Just added some clarity and fixed a typo

Xbox 360 Backward Compatibility, also known as '''FU''' or '''Fusion''' is Microsoft's original Xbox emulator for the Xbox 360.

The emulator binary is called xefu.xex. The first resource is xb1krnl which is a modified version of [[Kernel|xboxkrnl.exe]].

=== Modifications to xboxkrnl.exe ===

The IDEXPRDT section has been dropped, additionally the extra data from the MS-DOS header is gone.

==== Guest to host communication ====

The entrypoint of the kernel looks like:

<pre>
80030878: 56 push %esi
80030879: 57 push %edi
8003087a: 8d 05 4c ac 02 80 lea 0x8002ac4c,%eax
80030880: 0f 3f (bad)
80030882: 04 20
80030884: 8d 05 6c ac 02 80 lea 0x8002ac6c,%eax
8003088a: 0f 3f (bad)
8003088c: 04 20
8003088e: 8d 05 8c ac 02 80 lea 0x8002ac8c,%eax
80030894: 0f 3f (bad)
80030896: 04 21
80030898: 8d 05 70 94 01 80 lea 0x80019470,%eax
...
</pre>

According to [https://web.archive.org/web/20070216172548/https://www.symantec.com/avcenter/reference/Virtual_Machine_Threats.pdf this document by symantec] (Page 5, Left-hand-side) the patterns <code>0F 3F x1 x2</code> and <code>0F C7 C8 y1 y2</code> are used for communication with the host.

{| class="wikitable"
! x1 !! x2 !! Notes
|-
| 0x04 || 0x20 || Seems to use eax (address) as parameter? eax points to a zero terminated list of pointers into the kernel memory [7 elements]
|-
| 0x04 || 0x21 || Seems to use eax (address) as parameter? " [4 elements]
|-
| 0x04 || 0x22 || Seems to use eax (address) as parameter? Seems to be some call to that address?!
|-
| 0x04 || 0x23 || Seems to use eax (address) as parameter?
|-
| 0x04 || 0x24 || Seems to use eax (address) as parameter?
|-
| 0x04 || 0x35 || Seems to use eax (address) as parameter?
|-
| 0x04 || 0x50 || Seems to use eax (address) as parameter? " [3 elements]
|-
| colspan="3" | Cleaner list starts here {{FIXME}}
|-
| 0x04 || 0x20 ||
|-
| 0x04 || 0x20 ||
|-
| 0x04 || 0x21 ||
|-
| 0x04 || 0x22 ||
|-
| 0x04 || 0x23 ||
|-
| 0x04 || 0x24 ||
|-
| 0x04 || 0x35 ||
|-
| 0x04 || 0x50 ||
|-
| 0x06 || 0x00 || Seems to use eax (address) and ecx (size) as parameter? Memory is 0x00 filled before. location is 0x8002b420, size would be 0x3000
|-
| 0x06 || 0x02 ||
|-
| 0x06 || 0x20 || Some call or callback registration to the address pointed to by eax
|-
| 0x06 || 0x21 ||
|-
| 0x06 || 0x22 ||
|-
| 0x06 || 0x23 || Some call or callback registration to the address pointed to by eax
|-
| 0x06 || 0x24 ||
|-
| 0x06 || 0x25 ||
|-
| 0x06 || 0x26 ||
|-
| 0x06 || 0x27 ||
|-
| 0x06 || 0x28 ||
|-
| 0x06 || 0x29 ||
|-
| 0x06 || 0x40 ||
|}

== References and links ==
* [http://support.xbox.com/en-US/legacy-devices/original-console/play-original-games Official compatibility list by Microsoft]
* [http://michaelbrundage.com/project/xbox-360-emulator/ Michael Brundages (Microsoft) page about the original Xbox emulator in the Xbox 360]
** [http://michaelbrundage.com/note/2005/05/15/xbox-360-emulator/ More information about the original Xbox emulator in the Xbox 360]
* [https://randomascii.wordpress.com/2019/03/20/exercises-in-emulation-xbox-360s-fma-instruction/ Blog post about FMA math emulation by Bruce Dawson (Microsoft)]
* [https://www.youtube.com/watch?v=Da_ont-2AG0 Modern Vintage Gamer: Revisiting Original Xbox Backward Compatibility on the Xbox 360]

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Xbox 360 Backward Compatibility