Author Topic: Split traps  (Read 87 times)

Offline OS923

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Split traps
« on: October 10, 2018, 06:31:53 AM »
I search the complete list of split traps.

From Inside Macintosh - PowerPC System Software page 1-68:
Note also that the system software includes a small number of split traps, system software routines that are implemented with 680x0 code (usually in ROM) and as PowerPC code in an import library. Because the PowerPC code is contained directly in the import library, you cannot patch the PowerPC portion of a split trap. In general, however, only those routines are implemented as split traps that are not likely candidates for patching. For example, a number of very small utility routines like AddPt and SetRect are implemented as split traps.

Finally, there are two problems which even fat patches can’t solve: “split traps” and selector-based traps. “Split” traps are minor utility routines (such as AddPt) which would be overwhelmed by the overhead of the trap dispatching mechanisms and which never really need to be patched. These routines are implemented as 68K code in the ROM and as PowerPC code in the Interface Library, which allows 68K code to access the 68K copy efficiently, and the PowerPC code to use the function without going through interface glue. Since the ROM-based versions of these routines are never called from PowerPC code, a patch on one of the “split traps” will only apply to 68K code calling the trap.

The hardest news about working with traps is that you never truly know whether you are calling an “accelerated” (PowerPC) trap, an emulated trap, or a “split trap.” Inside Macintosh doesn’t say which traps are which. A clever programmer could figure out which traps are which in a given machine with a given version of the system, but, as in the rest of the system software, these details are subject to change over time, and the presence of split traps make an otherwise easy task (of telling an emulated trap from an accelerated one) extremely difficult; a “split trap” looks just like any other 68K trap in the ROM.