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Updated Cxbx Reloaded overview (markdown)

Margen67 2019-05-13 18:30:11 -07:00
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Cxbx-Reloaded-overview.md

@ -1,14 +1,14 @@
Here some technical details on how Cxbx-Reloaded works.
Here's some technical details on how Cxbx-Reloaded works.
# Executable launch modes
Cxbx-Reloaded is a single executable which operates in two different modes:
1. If launched normally (without any command-line arguments), it will show a user-interface then wait for user inputs.
* However, if drag and drop an xbe file on top of executable. It will automatic run xbe file.
* However, if you drag and drop an xbe file on top of the executable, it will automatic run said xbe file.
2. To emulate Xbox software, Cxbx-Reloaded relaunches itself (with the path to the Xbox executable to run on the command-line).
The emulated Xbox screen is then put over the user-interface.
At the end of emulation, the second instance terminates and the user-interface reappears.
* If want to start standalone emulation. Then the following argument should be input `> cxbx.exe /load "path to xbe"` via command prompt.
* If you want to start standalone emulation, than the following argument should be input `cxbx.exe /load "path to xbe"` via command prompt.
If another process launches Cxbx-Reloaded (for instance, a user starts a shortcut via Windows Explorer) with a command-line argument, there's no parent process, so when emulation ends in this scenario, the Cxbx-Reloaded process terminates entirely.
@ -22,26 +22,26 @@ Cxbx-Reloaded currently runs on 64-bit Windows 7 and up.
## CPU 'emulation'
Cxbx-Reloaded emulates the Xbox using direct code execution.
This means most of the CPU instructions the Xbox would execute, are executed directly on the host machine, without any modification or interpretation.
This means most of the CPU instructions the Xbox would execute are executed directly on the host machine without any modification or interpretation.
_This restricts running Cxbx-Reloaded to CPU's that are compatible with the Xbox Pentium 3 Coppermine CPU!_
The advantage of this approach is that Cxbx doesn't need to emulate the CPU at all, and offers native, 'faster-than-real-hardware' emulation speeds.
Admittedly, executing Xbox code as-is does bring with it a few complexities:
* some opcodes are prohibited to be executed in user-mode on Windows
* Some opcodes are prohibited to be executed in user-mode on Windows
* IN/OUT opcodes must be emulated
* CPU-specific opcodes (like CPUID) still need to be emulated
* the FS segment layout differs between Xbox and Windows
* mapping physical to virtual addresses (using the MMU) isn't allowed
* The FS segment layout differs between Xbox and Windows
* Mapping physical to virtual addresses using the MMU isn't allowed
(More on this later, under _[Hardware accesses](#hardware-accesses)_ and _[Contiguous memory](#contiguous-memory)_.)
## Xbox Kernel (BIOS)
Cxbx-Reloaded contains it's own implementation of the Xbox kernel, which means there's no need to acquire a dump from your Xbox.
In it's current form, the Cxbx-Reloaded kernel forwards many of the 378 kernel API's towards the host Windows kernel API's.
In it's current form, the Cxbx-Reloaded kernel forwards many of the 378 kernel APIs towards the host Windows kernel APIs.
__This restricts running Cxbx-Reloaded to operating systems that contain the Win32 API's, so far Windows only!__
__This restricts running Cxbx-Reloaded to operating systems that contain the Win32 APIs, so far Windows only!__
Because many of the kernel API's are just forwards, some aspects of the Xbox kernel are not emulated well enough.
@ -71,7 +71,7 @@ APU and JIT LLE are disabled due to being unimplemented.
For now, Cxbx-Reloaded only reliably supports HLE and LLE GPU.
## Emulating Xbox memory layout
Cxbx-Reloaded emulates the Xbox memory layout using a trick that allows so-called 'fast path' memory access, by mimicking the memory layout of the Xbox on the host OS.
Cxbx-Reloaded emulates the Xbox memory layout using a trick that allows 'fast path' memory access by mimicking the memory layout of the Xbox on the host OS.
This involves the following steps:
@ -87,20 +87,18 @@ The downside to this is that the Cxbx-Reloaded is quite big (more than 67 MiB cu
Since Cxbx-Reloaded launches itself a second time for actual emulation, memory-requirements on the host are doubled to at least 256 MiB of memory.
Expect swapping when the host has less than 1 GiB of physical memory installed.
## Hardware accesses
Most Xbox hardware is mapped from 0xF0000000 to 0xFFFFFFFF.
Cxbx-Reloaded reserves this memory-region using so-called 'guard-pages' which guarantees this memory range isn't used for anything else, and causes so-called access violations (or more accurately, guard-page exceptions).
Cxbx-Reloaded reserves this memory-region using so-called 'guard-pages' which guarantees this memory range isn't used for anything else, and causes so-called access violations,or more accurately, guard-page exceptions.
When Cxbx-Reloaded emulates an Xbox instruction that accesses a hardware component (reading or writing to one of the MMIO memory-addresses), this access is trapped using a so-called 'exception handler'.
When Cxbx-Reloaded emulates an Xbox instruction that accesses a hardware component (reading or writing to one of the MMIO memory-addresses), this access is trapped using an 'exception handler'.
This allows us to capture and emulate hardware accesses that aren't present on the host.
When Cxbx-Reloaded operates entirely in HLE mode which is the current default, most if not all hardware accesses are prevented entirely.
So normally, this is mostly a moot point, and you won't see slow-down in emulation because of this.
Normally, this is mostly a moot point, and you won't see slow-down in emulation because of this.
However, when it happens, exception handling is slow, as it incurs a context-switch (storing and restoring the entire state of a thread) and has to figure out which address needs which handling.
@ -133,11 +131,11 @@ Xbox renders using the Nvidia NV2A GPU.
Most software written for the Xbox was developed with some version of the official "Xbox Development Kit" (XDK for short), which offered Direct3D 8 like features.
Therefore, Cxbx-Reloaded uses Direct3D 8.1 as its rendering API, although it's planned to port to Direct3D 9 (for HLE) and OpenGL (for LLE).
Therefore, Cxbx-Reloaded uses Direct3D 9 as its rendering API.
Vulkan is currently not under consideration.
Some features the NV2A offered are unavailable on Direct3D 8 or even 9, so for those Cxbx-Reloaded tries to convert it to be host-compatible.
Some features the NV2A offered are unavailable on Direct3D 8 and 9; For those Cxbx-Reloaded tries to convert it to be host-compatible.
For example, some texture-formats like P8, R8B8 and others are not available on host Direct3D, and are thus converted into ARGB, the most widely supported texture format.