Add some documentation about the guest memory

docs/cpu.md

@@ -1,5 +1,47 @@
 # 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
@@ -8,12 +50,12 @@ TODO
 
 ### PowerPC
 
-The processor in the 360 is a 64-bit PowerPC chip running with 32-bit memory
+The processor in the 360 is a 64-bit PowerPC chip running in 32-bit mode.
-addresses. This can make some of the documentation a little confusing as most
+Programs are still allowed to use 64-bit PowerPC instructions, and registers
-of the PowerPC docs only have a 'you're in 32 or 64' style. The CPU is largely
+are 64-bit as well, but 32-bit instructions will run in 32-bit mode.
-similar to the PPC part in the PS3, so Cell documents often line up for the
+The CPU is largely similar to the PPC part in the PS3, so Cell documents
-core instructions. The 360 adds some additional AltiVec instructions, though,
+often line up for the core instructions. The 360 adds some additional AltiVec
-which are only documented in a few places (like the gcc source code, etc).
+instructions, though,which are only documented in a few places (like the gcc source code, etc).
 
 * [Free60 Info](http://www.free60.org/Xenon_\(CPU\))
 * [Power ISA docs](https://www.power.org/wp-content/uploads/2012/07/PowerISA_V2.06B_V2_PUBLIC.pdf) (aka 'PowerISA')