- 📝 Posted:
- 🚚 Summary of:
- P0138
- ⌨ Commits:
8d953dc...864e864- 💰 Funded by:
- [Anonymous], Blue Bolt
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Technical debt, part 9… and as it turns out, it's highly impractical to repay 100% of it at this point in development. 😕
The reason: graph_putsa_fx(), ZUN's function for rendering
optionally boldfaced text to VRAM using the font ROM glyphs, in its
ridiculously micro-optimized TH04 and TH05 version. This one sets the
"callback function" for applying the boldface effect by self-modifying
the target of two CALL rel16 instructions… because
there really wasn't any free register left for an indirect
CALL, eh? The necessary distance, from the call site to the
function itself, has to be calculated at assembly time, by subtracting the
target function label from the call site label.
This usually wouldn't be a problem… if ZUN didn't store the resulting
lookup tables in the .DATA segment. With code segments, we
can easily split them at pretty much any point between functions because
there are multiple of them. But there's only a single .DATA
segment, with all ZUN and master.lib data sandwiched between Borland C++'s
crt0 at the
top, and Borland C++'s library functions at the bottom of the segment.
Adding another split point would require all data after that point to be
moved to its own translation unit, which in turn requires
EXTERN references in the big .ASM file to all that moved
data… in short, it would turn the codebase into an even greater
mess.
Declaring the labels as EXTERN wouldn't work either, since
the linker can't do fancy arithmetic and is limited to simply replacing
address placeholders with one single address. So, we're now stuck with
this function at the bottom of the SHARED segment, for the
foreseeable future.
We can still continue to separate functions off the top of that segment,
though. Pretty much the only thing noteworthy there, so far: TH04's code
for loading stage tile images from .MPN files, which we hadn't
reverse-engineered so far, and which nicely fit into one of
Blue Bolt's pending ⅓ RE contributions. Yup, we finally moved
the RE% bars again! If only for a tiny bit.
Both TH02 and TH05 simply store one pointer to one dynamically allocated
memory block for all tile images, as well as the number of images, in the
data segment. TH04, on the other hand, reserves memory for 8 .MPN slots,
complete with their color palettes, even though it only ever uses the
first one of these. There goes another 458 bytes of conventional RAM… I
should start summing up all the waste we've seen so far. Let's put the
next website contribution towards a tagging system for these blog posts.
At 86% of technical debt in the SHARED segment repaid, we
aren't quite done yet, but the rest is mostly just TH04 needing to catch
up with functions we've already separated. Next up: Getting to that
practical 98.5% point. Since this is very likely to not require a full
push, I'll also decompile some more actual TH04 and TH05 game code I
previously reverse-engineered – and after that, reopen the store!