Last use of grab_return was removed in f204caa9079aa94d90e1a8ef650b845283c1d46a.
grab_breakpoint was added in 2bd717e2931cba5be3152f92b3cca5e82e446759 but
never used.
There's no reason for it to be a global, we always set it and use it
in instruction implementations, and we never read it directly.
Perhaps the compiler could optimize this away, but it's better to be
simpler (and also be easier to read).
They happen surprisingly often, and flushing the TLB is expensive
because we need to walk over all entries.
Takes booting 10.2 on a Beige G3 from binary start to "Welcome to Macintosh"
from 58s to 38s on my machine.
- Rename DEC to DEC_S and add DEC_U.
- MQ, RTCL_U, RTCU_U, and DEC_U should cause an illegal instruction program exception for non-MPC601 CPUs. The exception handler of classic Mac OS uses this to emulate the instruction.
- For mtspr, the SPRs RTCL_U, RTCU_U, and DEC_U are treated as no-op on MPC601.
- For debugging, use the supervisor instead of the user SPR number as the index for storing the values for RTC, TB, and DEC.
- For debugging, RTC, TB, and DEC should be updated after each access. Previously, mfspr and mtspr would only update the half of RTC and TB that was being accessed instead of both halves.
Accessing an SPR with bit 4 set (> 15) requires supervisor privilege and should cause a supervisor-level instruction exception (privileged instruction type program exception).
Use "logical" since the functions deal with multiple bits instead of a single boolean value and because the 601 manual calls them Logical Instructions.
Use "ppc" for the enums because logical_and is defined elsewhere and because the original DPPC code used these names for those functions.
Add MPC601 variants. Variants that decrement and test the ctr are invalid bon't don't appear to trigger an exception. The manual says MPC601 can decrement the counter. Other CPUs do not decrement the counter but will branch based on the value.
