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Kevin
3cbe6c1cde
The BIOS exception handler ran interpreted for the life of every process,
1.22 billion instructions and 44.6% of all interpreted work in Breath of
Fire III, even though it is compiled and even though the stub that
unblessed it sat at exactly the address bios/SCPH1001.toml already
declared as an install slot. Two mechanisms existed and did not compose:
the emitter hooked the declared slot PC, while the runtime's bless
verifier memcmp'd the whole body including that slot, so the body was
MISMATCH forever and native code never reached the hook.
Three parts, none of which pays off alone:
1. Publish the declared ranges to the runtime. The emitter emits a
PsxKernelPatchRange table next to the kernel body table and routes it
through the backend descriptor; the bless verifier compares a body in
segments that skip them. CLEAN now means "every byte outside every
declared range still matches ROM", which is the claim the native code
actually depends on.
2. Generalise the slot model to what the patches are. The old model
assumed a 4-word lui/addiu/jalr/nop stub written over ROM zeros and
fired on `word != 0`. Measured against both images, only one of five
observed shapes fits: patchers also plant a `jr` over real ROM
instructions, rewrite an 11-word prologue and insert an `mfc0 Cause`,
and NOP out a driver routine. A slot is now a range with a length and
an explicit `resume` (jalr / fallthrough / none), detected by
comparing every word against its ROM-baked value.
3. Hand back from the interpreter at the range end. Kernel page 0 is
permanently dirty, because the handler saves registers there on every
exception, so straight-line interpretation would run the whole
function after the patch. The emitter registers the resume PC as a
continuation key and dirty_ram_interp surfaces there.
A PC inside a declared range must never be a native dispatch key: the
compiled bytes there are not what executes. Seven pre-existing
jal-return continuations violated that and wedged the boot at frame 0 in
an infinite dispatch loop (OpenBIOS RAM 0x357C). The emitter now drops
continuation keys inside a range so dispatch misses through to the
interpreter, and refuses to emit if a range covers a function entry,
which would silently remove a callable function instead.
Both profiles declare their measured ranges, and SCPH1001.toml finally
pins [program.image] sha256 to v2.2 (CRC32 37157331) so a v2.0 dump
refuses to generate rather than producing phantom discovery gaps.
Measured on Breath of Fire III (SLPS-00990, Psy-Q SDK 3.70), headless,
matched frames, PSX_KERNEL_PATCH_RANGES=0 for the OFF side:
OpenBIOS SCPH-1001 v2.2
bless mismatch 21 -> 0 74 -> 0
kernel bodies -87.7%/frame -97.8%/frame
all interp insns -42.6%/frame -94.2%/frame
The residual kernel-body work is exactly declared-words x entries on
every range, so nothing leaks past the hook. The A0/B0/C0 vector
trampolines are unchanged (+0.2%, noise): the profile excludes them by
design and they are a separate job. Card read traces are byte-identical
across the A/B on both images, all 32 entries, which exercises the two
ranges that replace the kernel's card handler.
Rule 18 holds throughout: the guest's patched instructions still execute
as written on the dirty-RAM interpreter. Only the code around them
changes backend. Nothing is synthesised.
Tests: runtime/tests/test_kernel_patch_ranges.c (new, the verifier's
accept/reject behaviour and the hand-back predicate) and the install-slot
range invariants in recompiler/tests/bios_address_model_test.cpp.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>