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xenia/docs/cpu.md

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CPU Documentation

Code

Xenia uses a dynamic recompiler to recompile PPC instructions to host architecture instructions at runtime. Functions are converted as they are called by the guest/host and stored in a code cache. Currently, the only backend that exists is the x64 backend.

Unfortunately, one problem with this approach is that code usually never ends up in the same spot across reruns due to multithreading. Host code may also morph depending on which spots of a function are called first.

Memory

Xenia defines virtual memory as a mapped range beginning at Memory::virtual_membase(), and physical memory as another mapped range from Memory::physical_membase() (usually 0x100000000 and 0x200000000, respectively). If the default bases are not available, they are shifted left 1 bit until an available range is found.

The guest only has access to these ranges, nothing else.

Map

0x00000000 - 0x3FFFFFFF (1024mb) - virtual 4k pages
0x40000000 - 0x7FFFFFFF (1024mb) - virtual 64k pages
0x80000000 - 0x8BFFFFFF ( 192mb) - xex 64k pages
0x8C000000 - 0x8FFFFFFF (  64mb) - xex 64k pages (encrypted)
0x90000000 - 0x9FFFFFFF ( 256mb) - xex 4k pages
0xA0000000 - 0xBFFFFFFF ( 512mb) - physical 64k pages (overlapped)
0xC0000000 - 0xDFFFFFFF          - physical 16mb pages (overlapped)
0xE0000000 - 0xFFFFFFFF          - physical 4k pages (overlapped)

Virtual pages are usually allocated by NtAllocateVirtualMemory, and physical pages are usually allocated by MmAllocatePhysicalMemoryEx.

Virtual pages mapped to physical memory are also mapped to the physical membase, i.e. virtual 0xA0000000 == physical 0x00000000

Unfortunately, the 0xE0000000-0xFFFFFFFF range is unused in Xenia because it maps to physical memory with a single page offset, which is impossible to do under the Win32 API.

Memory Management

TODO

References

PowerPC

The processor in the 360 is a 64-bit PowerPC chip running in 32-bit mode. Programs are still allowed to use 64-bit PowerPC instructions, and registers are 64-bit as well, but 32-bit instructions will run in 32-bit mode. The CPU is largely similar to the PPC part in the PS3, so Cell documents often line up for the core instructions. The 360 adds some additional AltiVec instructions, though,which are only documented in a few places (like the gcc source code, etc).

x64