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xenia/docs/cpu.md

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CPU Documentation

The JIT

JIT Diagram

The JIT is the core of Xenia. It translates Xenon PowerPC code into native code runnable on the host computer.

There are 3 phases to translation:

  1. Translation to IR (intermediate representation)
  2. IR compilation/optimization
  3. Backend emission

PowerPC instructions are translated to Xenia's intermediate representation format in src/xenia/cpu/ppc/ppc_emit_*.cc (e.g. processor control is done in ppc_emit_control.cc). HIR opcodes are relatively simple opcodes such that any host can define an implementation.

After the HIR is generated, it is ran through a compiler to prep it for generation. The compiler is ran in a series of passes, the order of which is defined in ppc_translator.cc. Some passes are essential to the successful generation, while others are merely for optimization purposes. Compiler passes are defined in src/xenia/cpu/compiler/passes with descriptive class names.

Finally, the backend consumes the HIR and emits code that runs natively on the host. Currently, the only backend that exists is the x64 backend, with all the emission done in x64_sequences.cc.

ABI

Xenia guest functions are not directly callable, but rather must be called through APIs provided by Xenia. Xenia will first execute a thunk to transition the host context to a state dependent on the JIT backend, and that will call the guest code.

x64

Transition thunks defined in x64_backend.cc. Registers are stored on the stack as defined by StackLayout::Thunk for later transitioning back to the host.

Some registers are reserved for usage by the JIT to store temporary variables. See: X64Emitter::gpr_reg_map_ and X64Emitter::xmm_reg_map_.

Integer Registers

Register Usage
RAX Scratch
RBX JIT temp
RCX Scratch
RDX Scratch
RSP Stack Pointer
RBP Unused
RSI PowerPC Context
RDI Virtual Memory Base
R8-R11 Unused (parameters)
R12-R15 JIT temp

Floating Point Registers

Register Usage
XMM0-XMM5 Scratch
XMM6-XMM15 JIT temp

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

The 0xE0000000-0xFFFFFFFF range is mapped to physical memory with a single 4 KB page offset. On Windows, memory mappings must be aligned to 64 KB, so the offset has to be added when guest addresses are converted to host addresses in the translated CPU code. This can't be faked other ways because calculations involving the offset are built into games - see the following sequence:

srwi      r9, r10, 20       # r9 = r10 >> 20
clrlwi    r10, r10, 3       # r10 = r10 & 0x1FFFFFFF (physical address)
addi      r11, r9, 0x200
rlwinm    r11, r11, 0,19,19 # r11 = r11 & 0x1000
add       r11, r11, r10     # add 1 page to addresses > 0xE0000000

# r11 = addess passed to GPU

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