Huc6280 cpu fixes - #136
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Three HuC6280 accuracy fixes that SF2's boot sequence depends on: - BSR (opcode $44): relative branch-to-subroutine. Pushes the return address like JSR, then branches PC-relative. Fixed 8 cycles. - Block transfers (TII/TDD/TIN/TIA/TAI) save Y/A/X on the stack for the duration of the copy and restore them as X/A/Y. A transfer whose destination overlaps the stack page therefore clobbers the registers exactly as hardware does (SF2's RNG seed relies on this). - IRQ recognition samples the I-flag from *before* the instruction ran, giving CLI/SEI/PLP the real one-instruction delay. check_interrupts / is_irq_triggered now take the pre-execution irq_enabled flag.
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I think this change is uncontroversial as it's just a bunch of bugfixes that I found while testing on an actual game. So I'll merge it right away and publish 0.10.1 to unblock the TurboGrafx release. |
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So when I tested my TurboGrafx emulator, I noticed that I forgot a few HuC6280 CPU quirks in the 0.10.0 release. Street Fighter II booted to a black screen, and tracking it down turned up three things I forgot to push.
BSR was just missing. Opcode
$44, the HuC6280's relative branch-to-subroutine, wasn't decoded at all. It now pushes the return address the same way JSR/RTS do and branches PC-relative, with the fixed 8-cycle cost.Block transfers clobber A/X/Y when they hit the stack. The TII/TDD/TIN/TIA/TAI microcode saves Y, A and X on the stack while it runs and restores them (as X, A, Y) when it's done. That's normally invisible, but if the destination range overwrites the stack page, the values read back are the copied data, not the original registers. SF2's boot RNG uses exactly this: a
TII $2000,$2001,$1FFFsmears work RAM across the stack and zeroes the loop counter in X. Without it, the seed is wrong, and it doesn't boot. :(CLI/SEI/PLP now have the one-instruction delay. The IRQ check was reading the I-flag after the instruction ran. Real hardware recognizes an interrupt at the end of an instruction using the flag value from before it executed, so an enable/disable lags by one instruction. SF2 depends on this when it does a CLI and then jumps straight into the handler's resume point. The check now samples the mask up front and passes it through.