[SeaBIOS] insn prefixes?

Kevin O'Connor kevin at koconnor.net
Thu Sep 30 05:51:38 CEST 2010

On Wed, Sep 29, 2010 at 08:19:33AM -0700, H. Peter Anvin wrote:
> Yeah... big real mode is rather expensive in terms of the 66 and 67
> prefixes you end up littering the code with just to be in the mode you
> actually want.  Better then to load a GDT, flip CR0.PE, and go to a
> 32-bit code segment anyway.

That brings up an interesting question.  Do you know if instruction
prefixes result in slower cpu execution?  I know they bloat the code,
but it's not been clear to me if there is a speed impact (besides a
small cost to insn fetching).  Real mode execution is documented to be
slow, but it's unclear if using regular 32bit operations (via
prefixes) would be even slower, the same, or a little faster.

BTW, I calculate that prefixes represent 12% of the seabios 16bit code
size (4720 of 38920 bytes).  I've found the code size with gcc (using
prefixes) was smaller than the code size was with bcc - largely due to
the optimizations and improved code structure that gcc enabled.


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