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Mini vMac 3.2.1 Alpha

[0] @ Poniedziałek, 13 Września 2010 07:39CET

Mini vMac 3.2.1 Alpha
Mini vMac jest emulatorem klasycznych komputerów Apple z procesorem motoroli, produkowanym w okresie '84 - '96. Bardzo wczesna wersja mini jest pisana z myślą o notebookach i słabszych komputerach, na razie należy do niej podchodzić jak do każdej alfy - z rezerwą;).
  • The biggest changes since the last snapshot are not related to the netbook, but about improved accuracy of timing. Mini vMac now measures time in cycles rather than instructions executed. In the simplest form, all instructions are assumed to take the same number of cycles, and this closely matches the results of previous versions of Mini vMac. (Mini vMac actually counts sixty fourth cycles, not just integer number of cycles, so that average times of instructions can be more accurate.)
  • Mini vMac now assigns an average number of cycles for each of the 65536 primary opcodes. The larger table now used, as of the last snapshot, for the primary opcodes provides a convenient place to store this, allowing the more accurate timing to be done fairly cheaply.
  • As a compile time option, in addition to using the table, Mini vMac can try to compute more accurate cycles for certain instructions, depending on the current data. This is slower, and only implemented in the C version of the 68000 emulation, making it slower still. By the way, I believe the MESS emulator takes this approach.
  • The build system option "-ta 0" selects the least accurate of these three methods, the default is "-ta 1", and "-ta 2" selects the most accurate.
  • Completely accurate timing would be exceedingly difficult. The CPU and video output conflict for accesses to RAM, and that would seem very complex to model.
  • The greater accuracy is so far mostly theoretical. The timings were entered from Motorola documentation. It needs to be tested and calibrated by comparing to real hardware
  • Currently 68000 timings are used even in the 68020 emulation. More accurate timing for 68020 should be added in a future version. Truly accurate timing for 68020 would be much more difficult than for the 68000 because of pipelining and caching, probably to the point of being unfeasible for Mini vMac. But more accurate averages should be possible.
wstecz13/09/2010 07:39
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