mirror of https://github.com/nmlgc/ReC98.git
231 lines
9.1 KiB
Markdown
231 lines
9.1 KiB
Markdown
## Welcome!
|
||
|
||
If we've seen you doing any kind of reverse-engineering or modding work on the
|
||
PC-98 Touhou games before, you might have already been [invited as a
|
||
collaborator][2]. In that case, feel free to create separate branches for your
|
||
work directly in this repository – this will immediately inform anyone who
|
||
watches this repo or subscribed to a webhook. If you prefer, you can still
|
||
use your own fork though.
|
||
|
||
### What can I do on these separate branches?
|
||
|
||
Anything – reverse-engineering and decompilation of original ZUN code (which
|
||
then could be merged back into `master` after review) or your own custom mods,
|
||
no matter how large or small.
|
||
|
||
For starters, simply naming functions or global variables to reflect their
|
||
actual intent will already be helpful. *Any* name is better than
|
||
`sub_<something>`, and can always be fixed or improved later.
|
||
|
||
# Contribution guidelines
|
||
|
||
## Rule #1
|
||
|
||
**`master` must never introduce code changes that change the decompressed
|
||
program image, or the unordered set of relocations, of any original game
|
||
binary, as compared using [mzdiff].** The only allowed exceptions are:
|
||
1) different encodings of identical x86 instructions within code segments
|
||
2) padding with `00` bytes at the end of the file.
|
||
|
||
## Taste issues
|
||
|
||
* Use tabs for indentation.
|
||
|
||
* Spaces for alignment are allowed, especially if they end up giving the code
|
||
a nice visual structure, e.g. with multiple calls to the same function with
|
||
varying pixel coordinates.
|
||
|
||
* Don't indent `extern "C"` blocks that span the entire file.
|
||
|
||
* Always use `{ brackets }`, even around single-statement conditional
|
||
branches.
|
||
* Add spaces around binary operators. `for(i = 0; i < 12; i++)`
|
||
|
||
* Variables should be *signed* in the absence of any ASM instruction
|
||
(conditional jump, arithmetic, etc.) or further context (e.g. parameters
|
||
with a common source) that defines their signedness. If a variable is used
|
||
in both signed and unsigned contexts, declare it as the more common one.
|
||
|
||
## Code organization
|
||
|
||
* Try to avoid repeating numeric constants – after all, easy moddability
|
||
should be one of the goals of this project. For local arrays, use `sizeof()`
|
||
if the size can be expressed in terms of another array or type. Otherwise,
|
||
`#define` a macro if there is a clear intent behind a number.
|
||
(Counterexample: Small, insignificant amounts of pixels in e.g. entity
|
||
movement code.)
|
||
|
||
* Try rewriting padding instructions in ASM land into TASM directives:
|
||
|
||
* `db 0` / `NOP` → `even` / `align 2`
|
||
* `db ?` → `evendata`
|
||
|
||
This makes mzdiffs a bit shorter in common cases where a single byte was
|
||
erroneously added somewhere, by providing a chance for the code to catch up
|
||
to its original byte positions.
|
||
|
||
* Documenting function comments exclusively go into C/C++ header files, right
|
||
above the corresponding function prototype, *not* into ASM slices.
|
||
|
||
* Newly named symbols in ASM land (functions, global variables, `struc`ts, and
|
||
"sequence of numeric equate" enums) should immediately be reflected in C/C++
|
||
land, with the correct types and calling conventions. Typically, these
|
||
definitions would go into header files, but they can stay in .c/.cpp files
|
||
if they aren't part of a public interface, i.e., not used by unrelated
|
||
functions.
|
||
|
||
* Compress calls to *known* functions in ASM land to use TASM's one-line,
|
||
interfaced call syntax, whenever all parameters are passed via consecutive
|
||
`PUSH` instructions:
|
||
|
||
* `pascal`:
|
||
<table>
|
||
<tr>
|
||
<td>
|
||
<code>push param1</code><br />
|
||
<code>push param2</code><br />
|
||
<code>call foo</code>
|
||
</td>
|
||
<td>→</td>
|
||
<td>
|
||
<code>call foo pascal, param1, param2</code>
|
||
</td>
|
||
</tr>
|
||
</table>
|
||
|
||
* `__cdecl`, single call, single parameter:
|
||
<table>
|
||
<tr>
|
||
<td>
|
||
<code>push param1</code><br />
|
||
<code>call foo</code><br />
|
||
<code>pop cx</code>
|
||
</td>
|
||
<td>→</td>
|
||
<td>
|
||
<code>call foo stdcall, param1</code><br />
|
||
<code>pop cx</code>
|
||
</td>
|
||
</tr>
|
||
</table>
|
||
|
||
* `__cdecl`, single call, multiple parameters:
|
||
<table>
|
||
<tr>
|
||
<td>
|
||
<code>push param2</code><br />
|
||
<code>push param1</code><br />
|
||
<code>call foo</code><br />
|
||
<code>add sp, 4</code>
|
||
</td>
|
||
<td>→</td>
|
||
<td>
|
||
<code>call foo c, param1, param2</code>
|
||
</td>
|
||
</tr>
|
||
</table>
|
||
|
||
* `__cdecl`, single call, 32-bit parameters (Note that you have to use
|
||
`large` whenever a parameter happens to be 32-bit, even if the disassembly
|
||
didn't need it):
|
||
<table>
|
||
<tr>
|
||
<td>
|
||
<code>push 012345678h</code><br />
|
||
<code>pushd param1</code><br />
|
||
<code>call foo</code><br />
|
||
<code>add sp, 8</code>
|
||
</td>
|
||
<td>→</td>
|
||
<td>
|
||
<code>call foo c, large param1, large 012345678h</code>
|
||
</td>
|
||
</tr>
|
||
</table>
|
||
|
||
* `__cdecl`, multiple calls with a single `add sp` instruction for their
|
||
combined parameter size at the end:
|
||
<table>
|
||
<tr>
|
||
<td>
|
||
<code>push param2</code><br />
|
||
<code>push param1</code><br />
|
||
<code>call foo</code><br />
|
||
<code>[…]</code><br />
|
||
<code>push param2</code><br />
|
||
<code>pushd param1</code><br />
|
||
<code>call bar</code><br />
|
||
<code>add sp, 0Ah</code>
|
||
</td>
|
||
<td>→</td>
|
||
<td>
|
||
<code>call foo stdcall, param1, param2</code><br />
|
||
<code>[…]</code><br />
|
||
<code>call bar stdcall, large param1, param2</code><br />
|
||
<code>add sp, 10</code>
|
||
</td>
|
||
</tr>
|
||
</table>
|
||
|
||
* Try moving repeated sections of code into a separate `inline` function
|
||
before grabbing the `#define` hammer. Turbo C++ will generally inline
|
||
everything declared as `inline` that doesn't contain `do`, `for`, `while`,
|
||
`goto`, `switch`, `break`, `continue`, or `case`.
|
||
|
||
* These inlining rules also apply to C++ class methods, so feel free to
|
||
declare classes if you keep thinking "overloaded operators would be nice
|
||
here" or "this code would read really nicely if this functionality was
|
||
encapsulated in a method". (Sometimes, you will have little choice, in
|
||
fact!) Despite Turbo C++'s notoriously outdated C++ implementation, [there
|
||
are quite a lot of possibilites for abstractions that inline perfectly][1].
|
||
Subpixels, as seen in 9d121c7, are the prime example here. Don't overdo it,
|
||
though – use classes where they meaningfully enhance the original procedural
|
||
code, not to replace it with an overly nested, "enterprise-y" class
|
||
hierarchy.
|
||
|
||
## Decompilation
|
||
|
||
* Don't try to decompile self-modifying code. Yes, it may be *possible* by
|
||
calculating addresses relative to the start of the function, but as soon as
|
||
someone starts modding or porting that function, things *will* crash at
|
||
runtime. Inline ASM in C/C++ source files is fine, that will trip up future
|
||
port developers at compile time. Self-modifying code can only do the same if
|
||
it's kept in separate ASM files.
|
||
|
||
* Don't use TCC's `-a` command-line option to force a particular code or data
|
||
alignment. Instead, directly spell out the alignment by adding padding
|
||
members to structures, and additional global variables. It's simply not
|
||
worth requiring every structure to work around it. For functions with
|
||
`switch` tables that originally were word-alignment, put a single
|
||
`#pragma option -a2` at the top of the translation unit, after all header
|
||
inclusions.
|
||
|
||
## Naming conventions
|
||
|
||
* ASM file extensions: `.asm` if they emit code, `.inc` if they don't
|
||
* Macros defining the number of instances of an entity: `<ENTITY>_COUNT`
|
||
* Macros defining the number of distinct sprites in an animation: `*_CELS`
|
||
* Frame variables counting from a frame count to 0: `*_time`
|
||
* Frame variables and other counters starting from 0: `*_frames`
|
||
* Functionally identical reimplementations or micro-optimizations of
|
||
master.lib functions: `z_<master.lib function name>`
|
||
|
||
## Identifiers from ZUN's original code
|
||
|
||
On some occasions, ZUN leaked pieces of the actual PC-98 Touhou source code
|
||
during interviews. From these, we can derive ZUN's original names for certain
|
||
variables, functions, or macros. To indicate one of those and protect them
|
||
from being renamed, put a `/* ZUN symbol [reference] */` comment next to the
|
||
declaration of the identifier in question.
|
||
|
||
Currently, we know about the following [references]:
|
||
|
||
* `[MAGNet2010]`: Interview with ZUN for the NHK BS2 TV program MAG・ネット
|
||
(MAG.Net), originally broadcast 2010-05-02. At 09m36s, ZUN's monitor briefly
|
||
displays a piece of TH04's `MAIN.EXE`, handling demo recording and the setup
|
||
of the game's EMS area.
|
||
|
||
[mzdiff]: https://github.com/nmlgc/mzdiff
|
||
[1]: Research/Borland%20C++%20decompilation.md#c
|
||
[2]: https://github.com/nmlgc/ReC98/invitations
|