JIT documentation!

docs/cpu.md

@@ -1,15 +1,70 @@
 # CPU Documentation
 
-## Code
+## The JIT
 
-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.
+![JIT Diagram](images/CPU-JIT.png?raw=true)
 
-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.
+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](../src/xenia/cpu/ppc/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](../src/xenia/cpu/ppc/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](../src/xenia/cpu/backend/x64/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](../src/xenia/cpu/backend/x64/x64_backend.cc#L389).
+Registers are stored on the stack as defined by [StackLayout::Thunk](../src/xenia/cpu/backend/x64/x64_stack_layout.h#L96)
+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_](../src/xenia/cpu/backend/x64/x64_backend.cc#L57).
+
+#### 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