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Old 11 November 2006, 16:44   #1
pjhutch
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New features in winuae

Would it be possible to document in a readme file, what new features in Winuae do?

For example, 'More Accurate FPU' on cPU page? What does this really mean?
Also, what does 'Faster RTG' on Chipset page mean?
There are a couple of items on Misc tab that are not self - evident either...
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Old 11 November 2006, 17:13   #2
Toni Wilen
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"Faster RTG" = disable non-needed emulation features while RTG (P96) screen is active for small speed boost. (like internal bitplane emulation, copper is replaced with "instant copper" etc..)

"Accurate FPU" = slower but more accurate (mainly improves accuracy of rounding. x86 and 68k FPUs have slightly different rounding modes, in non-accurate mode x86 rounding is used. Normally the difference is insignificant.)
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Old 13 November 2006, 13:55   #3
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Quote:
Originally Posted by Toni Wilen
"Accurate FPU" = slower but more accurate (mainly improves accuracy of rounding. x86 and 68k FPUs have slightly different rounding modes, in non-accurate mode x86 rounding is used. Normally the difference is insignificant.)
Not really

Sorry Toni, but "Accurate FPU" is not necessarily more accurate in terms of
the rounding precision and the rounding modes and precisions of the x86
and the m68k are exactly the same, just in a different bit-field order.

So, what's the real intention behind this FPU option in WinUAEs GUI?

I would prefer to call this switch "strict FPU emulation", because it can
be used to force the JIT/FPU to do exactly what Motorola says in the
68000 Programmers Manuals about how some special FPU instructions
should behave. There are two commands for the 68881 and a bunch of new
special FPU instructions introduced with the 68040/60 FPU. Unlike all other
normal commands these instructions have a fixed implicit rounding precision
that has to be used to round the operations result independently of the
actual global rounding precision, which is stored in an extra FPU register.

It's the 2. time now, that this stupid editor logs me out while I'm still
writing and the complete text disappeared again.

For the 68881/2 the two special instructions are FSGLMUL and FSGLDIV.
Both have to be executed completely with SINGLE PRECISION rounding, no
matter what the actual setting for the global precision in the FPU register is.
WinUAEs default behaviour is not to take care of the implicit fixed mode
of these special instructions and keep the global rounding all the time, and
it's of course a bit faster than switching the precision for a single command.

For the 040/60 FPU there are a lot more new commands with a fixed
rounding precision like FSMOVE, FSABS, FSNEG, FSADD, FSSUB, FSMUL,
FSDIV and FSSQRT and with DOUBLE PRECISION it's FDMOVE, FDABS,
FDNEG, FDADD, FDSUB, FDMUL, FDDIV and FDSQRT.

Usually, WinUAE won't take any care of the desired rounding for the result.
Without enabling this "More accurate FPU" option you will get the same as if
your program had just used FMOVE, FABS, FNEG, FADD, FSUB etc instead.
But, if you switch this option on, then WinUAEs FPU emulation will do exactly
what Motorola wants the command to do and the result should be equal to
the result of the same instruction performed with an FPU of a real Amiga.

Last edited by PeterK; 13 November 2006 at 14:50.
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Old 13 November 2006, 14:09   #4
Toni Wilen
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Quote:
Originally Posted by PeterK
Not really
Ok, but thats how I understood the meaning of the update (it was your update?) Maybe I confused it with something else..

JIT, Picasso96, FPU, MIDI (for example) are things I am not too interested which can explain the mistake
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Old 13 November 2006, 15:09   #5
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Quote:
Originally Posted by Toni Wilen
JIT, Picasso96, FPU, MIDI (for example) are things I am not too interested which can explain the mistake
No problem, I just wanted to give an explanation of the details behind this
option to answer the question of pjhutch. I hope he can understand it
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Old 13 November 2006, 16:40   #6
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I’m due to update my guide site soon. Just want to check a few things before I get around to updating the site. (Sorry Peter, I’m hijacking your thread )

The ‘Automatic Switching’ tick box under Sound Properties - I take it, this disables sound when not in use during emulation, which reduces CPU load/use, and then auto enables sound when required. Should I recommend users to tick this box?

Should I also recommend users to also tick the 'Faster RTG' on Chipset Properties?

So, with the 'Accurate FPU' tick box, I will put this as optional and should I tell users to: Enable if they experience problems. Or should I say, enable if the program requires more accurate CPU emulation?
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Old 13 November 2006, 18:06   #7
Toni Wilen
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Generally options that are recommended are enabled by default (includes both new options)
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Old 13 November 2006, 21:21   #8
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Quote:
Originally Posted by Paul
So, with the 'Accurate FPU' tick box, I will put this as optional and should I tell users to: Enable if they experience problems. Or should I say, enable if the program requires more accurate CPU emulation?
The stricter FPU emulation should only be enabled if this setting can solve
any problems, because on some systems the more complex FPU routines
may cause a small speedloss. On my PC it was only 5%, but it can be more.
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