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Old 19 December 2018, 11:12   #1001
JimDrew
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Join Date: Dec 2013
Location: Lake Havasu City, AZ
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Quote:
...Over time full-software virtualization was available, but a ROM image was still necessary. Example virtualization software include ShapeShifter (not to be confused with the third party preference pane ShapeShifter), later superseded by Basilisk II (both by the same programmer who conceived SheepShaver, Christian Bauer), Fusion and iFusion (the latter ran classic Mac OS by using a PowerPC "coprocessor" accelerator card).
Virtual machines provide equal or faster speed than a Macintosh with the same processor, especially with respect to the m68k series due to real Macs running in MMU trap mode, hampering performance...
Wow... whoever wrote this has no clue what they are talking about... FUSION is a lot faster than the equiv. speed Mac because of me replacing MacOS traps with specialized code, and letting the Amiga handle certain things. The Mac is not some victim held hostage by the MMU. In fact, FUSION is much faster with the MMU enabled because memory used for video storage can be mapped (4K pages) so a dirty bit is set when the Mac changes the video memory. Checking a bit for every 4K page (and updating as necessary) is way faster than blindly updating all of the video memory every refresh cycle! The MMU can remap the lower (CHIP) memory into fast memory as well to increase performance.

I happen to know the x86 and 68K extremely well, having written a full 68040 emulator for the x86, and a x86 emulator for the 68K - both of which were in written in assembly for their native CPUs. I also wrote iFUSION for the PPC. You can certainly kill a x86 program by having an INT function change a segment register. This is how some of the first virus programs worked. Calling a segment register a "MMU" is like calling an Atari ST an Amiga.

Litwr - It seems that you are a proponent of ARM and DOS, and are a bit blind to the facts that people (like Meynaf) are stating here. This is the wrong crowd to be arguing with on this particular subject - there are those here that have extensive backgrounds in CPU design and implementation. My expertise is in microcode level emulation of CPUs, with a history of working on CPU core projects for Motorola - besides all of the various software based emulations I have done for different computer systems. Maybe you are getting ARM and PPC confused, because that is one CPU where you can do bit manipulations (like invoke a shift) as an extended instruction operand. My 2 cents...

Last edited by JimDrew; 20 December 2018 at 00:31. Reason: Lots of typos... smart device... sheesh!
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