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Re: 32-64 bit switches
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Re: 32-64 bit switches

On Sunday, Jun 29, 2003, at 11:01 US/Eastern, Brian Barnes wrote:

Just a quick question that's been on my mind ... for a while we'll probably be dealing with 32-bit libraries in our shiny new 64-bit apps. Is there any kind of context switch in software or is it all handled transparently on the chip (not sure how that would work if it would be possible.)

The only switch is a bit in the MSR (machine state register), Read the PPC documentation for more info on this switch.
The main thing is that the registers are always 64bit (even in 32 bit mode). Only a few instructions are changed by this switch:
From PEM for 32bit PPC by Mot.:
8.1.4 Computation Modes
64-bit implementations
The mode controls how the effective address is interpreted, how condition bits are set,
and how the count register (CTR) is test by branch conditional instructions.

So all of the dot instructions are changed (but only for setting the condition bits)
and the instructions that set the carry bit are changed also (but only for changing the carry bit).
So these instructions are just ignore the top 32 and assume that the register is 32 bit wide when
setting either the carry bit or the condition bits.

I assume it's something very small (no where near going into a 68K emulator :) ) but just curious if any engineer wants to reveal this.

It is just a matter of going to supervisor mode and switching the bit.

Unlike most other 64/32 bit processors out there is no penalty for being in 32bit mode (unlike ia64).



Thanks,
Andrew Pinski
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 >32-64 bit switches (From: Brian Barnes <email@hidden>)



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